SNVS026O March 2000 – June 2016 LM2679
PRODUCTION DATA.
NOTE
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
Power supply design using the LM2679 is greatly simplified by using recommended external components. A wide range of inductors, capacitors and Schottky diodes from several manufacturers have been evaluated for use in designs that cover the full range of capabilities (input voltage, output voltage, and load current) of the LM2679. A simple design procedure using nomographs and component tables provided in this data sheet leads to a working design with very little effort.
The individual components from the various manufacturers called out for use are still just a small sample of the vast array of components available in the industry. While these components are recommended, they are not exclusively the only components for use in a design. After a close comparison of component specifications, equivalent devices from other manufacturers could be substituted for use in an application.
Important considerations for each external component and an explanation of how the nomographs and selection tables were developed follows.
The inductor is the key component in a switching regulator. For efficiency the inductor stores energy during the switch ON time and then transfers energy to the load while the switch is OFF.
Nomographs are used to select the inductance value required for a given set of operating conditions. The nomographs assume that the circuit is operating in continuous mode (the current flowing through the inductor never falls to zero). The magnitude of inductance is selected to maintain a maximum ripple current of 30% of the maximum load current. If the ripple current exceeds this 30% limit the next larger value is selected.
The inductors offered have been specifically manufactured to provide proper operation under all operating conditions of input and output voltage and load current. Several part types are offered for a given amount of inductance. Both surface mount and through-hole devices are available. The inductors from each of the three manufacturers have unique characteristics:
The output capacitor acts to smooth the DC output voltage and also provides energy storage. Selection of an output capacitor, with an associated equivalent series resistance (ESR), impacts both the amount of output ripple voltage and stability of the control loop.
The output ripple voltage of the power supply is the product of the capacitor ESR and the inductor ripple current. The capacitor types recommended in the tables were selected for having low ESR ratings.
In addition, both surface mount tantalum capacitors and through-hole aluminum electrolytic capacitors are offered as solutions.
Impacting frequency stability of the overall control loop, the output capacitance, in conjunction with the inductor, creates a double pole inside the feedback loop. In addition the capacitance and the ESR value create a zero. These frequency response effects together with the internal frequency compensation circuitry of the LM2679 modify the gain and phase shift of the closed-loop system.
As a general rule for stable switching regulator circuits it is desired to have the unity gain bandwidth of the circuit to be limited to no more than one-sixth of the controller switching frequency. With the fixed 26-kHz switching frequency of the LM2679, the output capacitor is selected to provide a unity gain bandwidth of 40 kHz maximum. Each recommended capacitor value has been chosen to achieve this result.
In some cases multiple capacitors are required either to reduce the ESR of the output capacitor, to minimize output ripple (a ripple voltage of 1% of V_{OUT} or less is the assumed performance condition), or to increase the output capacitance to reduce the closed loop unity gain bandwidth (to less than 40 kHz). When parallel combinations of capacitors are required it has been assumed that each capacitor is the exact same part type.
The RMS current and working voltage (WV) ratings of the output capacitor are also important considerations. In a typical step-down switching regulator, the inductor ripple current (set to be no more than 30% of the maximum load current by the inductor selection) is the current that flows through the output capacitor. The capacitor RMS current rating must be greater than this ripple current. The voltage rating of the output capacitor must be greater than 1.3 times the maximum output voltage of the power supply. If operation of the system at elevated temperatures is required, the capacitor voltage rating may be de-rated to less than the nominal room temperature rating. Careful inspection of the manufacturer's specification for de-rating of working voltage with temperature is important.
Fast changing currents in high current switching regulators place a significant dynamic load on the unregulated power source. An input capacitor helps to provide additional current to the power supply as well as smooth out input voltage variations.
Like the output capacitor, the key specifications for the input capacitor are RMS current rating and working voltage. The RMS current flowing through the input capacitor is equal to one-half of the maximum DC load current so the capacitor must be rated to handle this. Paralleling multiple capacitors proportionally increases the current rating of the total capacitance. The voltage rating must also be selected to be 1.3 times the maximum input voltage. Depending on the unregulated input power source, under light load conditions the maximum input voltage could be significantly higher than normal operation. Consider this when selecting an input capacitor.
The input capacitor must be placed very close to the input pin of the LM2679. Due to relative high current operation with fast transient changes, the series inductance of input connecting wires or PCB traces can create ringing signals at the input terminal which could possibly propagate to the output or other parts of the circuitry. It may be necessary in some designs to add a small valued (0.1-μF to 0.47-μF) ceramic type capacitor in parallel with the input capacitor to prevent or minimize any ringing.
When the power switch in the LM2679 turns OFF, the current through the inductor continues to flow. The path for this current is through the diode connected between the switch output and ground. This forward biased diode clamps the switch output to a voltage less than ground. This negative voltage must be greater than −1 V so a low voltage drop (particularly at high current levels) Schottky diode is recommended. Total efficiency of the entire power supply is significantly impacted by the power lost in the output catch diode. The average current through the catch diode is dependent on the switch duty cycle (D) and is equal to the load current times (1-D). Use of a diode rated for much higher current than is required by the actual application helps to minimize the voltage drop and power loss in the diode.
During the switch ON time the diode is reversed biased by the input voltage. The reverse voltage rating of the diode must be at least 1.3 times greater than the maximum input voltage.
The boost capacitor creates a voltage used to overdrive the gate of the internal power MOSFET. This improves efficiency by minimizing the ON-resistance of the switch and associated power loss. For all applications TI recommends a 0.01-μF, 50-V ceramic capacitor.
A key feature of the LM2679 is the ability to control the peak switch current. Without this feature the peak switch current would be internally set to 7 A or higher to accommodate 5-A load current designs. This requires that both the inductor (which could saturate with excessively high currents) and the catch diode be able to safely handle up to 7 A which would be conducted under load fault conditions.
If an application only requires a load current of 3 A or 4 A the peak switch current can be set to a limit just over the maximum load current with the addition of a single programming resistor. This allows the use of less powerful and more cost-effective inductors and diodes.
The peak switch current is equal to a factor of 37,125 divided by R_{ADJ}. A resistance of 5.6 kΩ sets the current limit to typically 6.3 A and an R_{ADJ} of 8.25 kΩ reduces the maximum current to approximately 4.4 A. For predictable control of the current limit, TI recommends keeping the peak switch current greater than 3 A. For lower current applications a 3-A switching regulator with adjustable current limit, the LM2673, is available.
When the power switch reaches the current limit threshold it is immediately turned OFF and the internal switching frequency is reduced. This extends the OFF time of the switch to prevent a steady-state high current condition. As the switch current falls below the current limit threshold, the switch turns back ON. If a load fault continues, the switch again exceeds the threshold and switch back OFF. This results in a low duty cycle pulsing of the power switch to minimize the overall fault condition power dissipation.
This optional capacitor controls the rate at which the LM2679 starts up at power on. The capacitor is charged linearly by an internal current source. This voltage ramp gradually increases the duty cycle of the power switch until it reaches the normal operating duty cycle defined primarily by the ratio of the output voltage to the input voltage. The soft-start turnon time is programmable by the selection of C_{SS}.
The formula for selecting a soft-start capacitor is Equation 1.
where
If this feature is not desired, leave the soft-start pin (pin 7) open circuited.
With certain soft-start capacitor values and operating conditions, the LM2679 can exhibit an overshoot on the output voltage during turnon. Especially when starting up into no load or low load, the soft-start function may not be effective in preventing a larger voltage overshoot on the output. With larger loads or lower input voltages during start-up this effect is minimized. In particular, avoid using soft-start capacitors between 0.033 µF and 1 µF.
When the output voltage is greater than approximately 6 V, and the duty cycle at minimum input voltage is greater than approximately 50%, the designer must exercise caution in selection of the output filter components. When an application designed to these specific operating conditions is subjected to a current limit fault condition, it may be possible to observe a large hysteresis in the current limit. This can affect the output voltage of the device until the load current is reduced sufficiently to allow the current limit protection circuit to reset itself.
Under current limiting conditions, the LM267x is designed to respond in the following manner:
If the output capacitance is sufficiently ‘arge, it may be possible that as the output tries to recover, the output capacitor charging current is large enough to repeatedly re-trigger the current limit circuit before the output has fully settled. This condition is exacerbated with higher output voltage settings because the energy requirement of the output capacitor varies as the square of the output voltage (½ CV^{2}), thus requiring an increased charging current.
A simple test to determine if this condition might exist for a suspect application is to apply a short circuit across the output of the converter, and then remove the shorted output condition. In an application with properly selected external components, the output recovers smoothly.
Practical values of external components that have been experimentally found to work well under these specific operating conditions are C_{OUT} = 47 µF, L = 22 µH. It must be noted that even with these components, for a device’s current limit of I_{CLIM}, the maximum load current under which the possibility of the large current limit hysteresis can be minimized is I_{CLIM}/2. For example, if the input is 24 V and the set output voltage is 18 V, then for a desired maximum current of 1.5 A, the current limit of the chosen switcher must be confirmed to be at least 3 A.
Under extreme overcurrent or short-circuit conditions, the LM267X employs frequency foldback in addition to the current limit. If the cycle-by-cycle inductor current increases above the current limit threshold (due to short circuit or inductor saturation for example) the switching frequency is automatically reduced to protect the IC. Frequency below 100 kHz is typical for an extreme short-circuit condition.
Select the power supply operating conditions and the maximum output current. Then follow the procedure below to find external components for LM2679.
Using the nomographs and tables in this data sheet (or use the available design software at http://www.ti.com) a complete step-down regulator can be designed in a few simple steps.
Step 1: Define the power supply operating conditions:
Step 2: Set the output voltage by selecting a fixed output LM2679 (3.3-V, 5-V, or 12-V applications) or determine the required feedback resistors for use with the adjustable LM2679−ADJ
Step 3: Determine the inductor required by using one of the four nomographs, Figure 14 through Figure 17. Table 3 provides a specific manufacturer and part number for the inductor.
Step 4: Using Table 1 and Table 6 (fixed output voltage) or Table 9 and Table 10 (adjustable output voltage), determine the output capacitance required for stable operation. Table 1 and Table 2 provide the specific capacitor type from the manufacturer of choice.
Step 5: Determine an input capacitor from Table 7 and Table 8 for fixed output voltage applications. Use Table 1 and Table 2 to find the specific capacitor type. For adjustable output circuits select a capacitor from Table 1 and Table 2 with a sufficient working voltage (WV) rating greater than V_{IN} max, and an RMS current rating greater than one-half the maximum load current (2 or more capacitors in parallel may be required).
Step 6: Select a diode from Table 4. The current rating of the diode must be greater than I_{LOAD} max and the reverse voltage rating must be greater than V_{IN} max.
Step 7: Include a 0.01-μF, 50-V capacitor for C_{BOOST} in the design and then determine the value of a soft-start capacitor if desired.
Step 8: Define a value for R_{ADJ} to set the peak switch current limit to be at least 20% greater than I_{OUT} max to allow for at least 30% inductor ripple current (±15% of I_{OUT}). For designs that must operate over the full temperature range the switch current limit must be set to at least 50% greater than I_{OUT} max (1.5 × I_{OUT} max).
CAPACITOR REFERENCE CODE | SURFACE MOUNT | ||||||||
---|---|---|---|---|---|---|---|---|---|
AVX TPS SERIES | SPRAGUE 594D SERIES | KEMET T495 SERIES | |||||||
C (µF) | WV (V) | Irms (A) | C (µF) | WV (V) | Irms (A) | C (µF) | WV (V) | Irms (A) | |
C1 | 330 | 6.3 | 1.15 | 120 | 6.3 | 1.1 | 100 | 6.3 | 0.82 |
C2 | 100 | 10 | 1.1 | 220 | 6.3 | 1.4 | 220 | 6.3 | 1.1 |
C3 | 220 | 10 | 1.15 | 68 | 10 | 1.05 | 330 | 6.3 | 1.1 |
C4 | 47 | 16 | 0.89 | 150 | 10 | 1.35 | 100 | 10 | 1.1 |
C5 | 100 | 16 | 1.15 | 47 | 16 | 1 | 150 | 10 | 1.1 |
C6 | 33 | 20 | 0.77 | 100 | 16 | 1.3 | 220 | 10 | 1.1 |
C7 | 68 | 20 | 0.94 | 180 | 16 | 1.95 | 33 | 20 | 0.78 |
C8 | 22 | 25 | 0.77 | 47 | 20 | 1.15 | 47 | 20 | 0.94 |
C9 | 10 | 35 | 0.63 | 33 | 25 | 1.05 | 68 | 20 | 0.94 |
C10 | 22 | 35 | 0.66 | 68 | 25 | 1.6 | 10 | 35 | 0.63 |
C11 | — | — | — | 15 | 35 | 0.75 | 22 | 35 | 0.63 |
C12 | — | — | — | 33 | 35 | 1 | 4.7 | 50 | 0.66 |
C13 | — | — | — | 15 | 50 | 0.9 | — | — | — |
CAPACITOR REFERENCE CODE | THROUGH HOLE | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
SANYO OS-CON SA SERIES | SANYO MV-GX SERIES | NICHICON PL SERIES | PANASONIC HFQ SERIES | |||||||||
C (µF) | WV (V) | Irms (A) | C (µF) | WV (V) | Irms (A) | C (µF) | WV (V) | Irms (A) | C (µF) | WV (V) | Irms (A) | |
C1 | 47 | 6.3 | 1 | 1000 | 6.3 | 0.8 | 680 | 10 | 0.8 | 82 | 35 | 0.4 |
C2 | 150 | 6.3 | 1.95 | 270 | 16 | 0.6 | 820 | 10 | 0.98 | 120 | 35 | 0.44 |
C3 | 330 | 6.3 | 2.45 | 470 | 16 | 0.75 | 1000 | 10 | 1.06 | 220 | 35 | 0.76 |
C4 | 100 | 10 | 1.87 | 560 | 16 | 0.95 | 1200 | 10 | 1.28 | 330 | 35 | 1.01 |
C5 | 220 | 10 | 2.36 | 820 | 16 | 1.25 | 2200 | 10 | 1.71 | 560 | 35 | 1.4 |
C6 | 33 | 16 | 0.96 | 1000 | 16 | 1.3 | 3300 | 10 | 2.18 | 820 | 35 | 1.62 |
C7 | 100 | 16 | 1.92 | 150 | 35 | 0.65 | 3900 | 10 | 2.36 | 1000 | 35 | 1.73 |
C8 | 150 | 16 | 2.28 | 470 | 35 | 1.3 | 6800 | 10 | 2.68 | 2200 | 35 | 2.8 |
C9 | 100 | 20 | 2.25 | 680 | 35 | 1.4 | 180 | 16 | 0.41 | 56 | 50 | 0.36 |
C10 | 47 | 25 | 2.09 | 1000 | 35 | 1.7 | 270 | 16 | 0.55 | 100 | 50 | 0.5 |
C11 | — | — | — | 220 | 63 | 0.76 | 470 | 16 | 0.77 | 220 | 50 | 0.92 |
C12 | — | — | — | 470 | 63 | 1.2 | 680 | 16 | 1.02 | 470 | 50 | 1.44 |
C13 | — | — | — | 680 | 63 | 1.5 | 820 | 16 | 1.22 | 560 | 50 | 1.68 |
C14 | — | — | — | 1000 | 63 | 1.75 | 1800 | 16 | 1.88 | 1200 | 50 | 2.22 |
C15 | — | — | — | — | — | — | 220 | 25 | 0.63 | 330 | 63 | 1.42 |
C16 | — | — | — | — | — | — | 220 | 35 | 0.79 | 1500 | 63 | 2.51 |
C17 | — | — | — | — | — | — | 560 | 35 | 1.43 | — | — | — |
C18 | — | — | — | — | — | — | 2200 | 35 | 2.68 | — | — | — |
C19 | — | — | — | — | — | — | 150 | 50 | 0.82 | — | — | — |
C20 | — | — | — | — | — | — | 220 | 50 | 1.04 | — | — | — |
C21 | — | — | — | — | — | — | 330 | 50 | 1.3 | — | — | — |
C22 | — | — | — | — | — | — | 100 | 63 | 0.75 | — | — | — |
C23 | — | — | — | — | — | — | 390 | 63 | 1.62 | — | — | — |
C24 | — | — | — | — | — | — | 820 | 63 | 2.22 | — | — | — |
C25 | — | — | — | — | — | — | 1200 | 63 | 2.51 | — | — | — |
INDUCTOR REFERENCE NUMBER |
INDUCTANCE (µH) |
CURRENT (A) |
RENCO | PULSE ENGINEERING | COILCRAFT | ||
---|---|---|---|---|---|---|---|
THROUGH HOLE | SURFACE MOUNT | THROUGH HOLE | SURFACE MOUNT | SURFACE MOUNT | |||
L23 | 33 | 1.35 | RL-5471-7 | RL1500-33 | PE-53823 | PE-53823S | DO3316-333 |
L24 | 22 | 1.65 | RL-1283-22-43 | RL1500-22 | PE-53824 | PE-53824S | DO3316-223 |
L25 | 15 | 2 | RL-1283-15-43 | RL1500-15 | PE-53825 | PE-53825S | DO3316-153 |
L29 | 100 | 1.41 | RL-5471-4 | RL-6050-100 | PE-53829 | PE-53829S | DO5022P-104 |
L30 | 68 | 1.71 | RL-5471-5 | RL6050-68 | PE-53830 | PE-53830S | DO5022P-683 |
L31 | 47 | 2.06 | RL-5471-6 | RL6050-47 | PE-53831 | PE-53831S | DO5022P-473 |
L32 | 33 | 2.46 | RL-5471-7 | RL6050-33 | PE-53932 | PE-53932S | DO5022P-333 |
L33 | 22 | 3.02 | RL-1283-22-43 | RL6050-22 | PE-53933 | PE-53933S | DO5022P-223 |
L34 | 15 | 3.65 | RL-1283-15-43 | — | PE-53934 | PE-53934S | DO5022P-153 |
L38 | 68 | 2.97 | RL-5472-2 | — | PE-54038 | PE-54038S | — |
L39 | 47 | 3.57 | RL-5472-3 | — | PE-54039 | PE-54039S | — |
L40 | 33 | 4.26 | RL-1283-33-43 | — | PE-54040 | PE-54040S | — |
L41 | 22 | 5.22 | RL-1283-22-43 | — | PE-54041 | P0841 | — |
L44 | 68 | 3.45 | RL-5473-3 | — | PE-54044 | — | — |
L45 | 10 | 4.47 | RL-1283-10-43 | — | — | P0845 | DO5022P-103HC |
L46 | 15 | 5.6 | RL-1283-15-43 | — | — | P0846 | DO5022P-153HC |
L47 | 10 | 5.66 | RL-1283-10-43 | — | — | P0847 | DO5022P-103HC |
L48 | 47 | 5.61 | RL-1282-47-43 | — | — | P0848 | — |
L49 | 33 | 5.61 | RL-1282-33-43 | — | — | P0849 | — |
REVERSE VOLTAGE (V) | SURFACE MOUNT | THROUGH HOLE | ||
---|---|---|---|---|
3 A | 5 A OR MORE | 3 A | 5 A OR MORE | |
20 | SK32 | — | 1N5820 | — |
— | — | SR302 | — | |
30 | SK33 | MBRD835L | 1N5821 | — |
30WQ03F | — | 31DQ03 | — | |
40 | SK34 | MBRD1545CT | 1N5822 | 1N5825 |
30BQ040 | 6TQ045S | MBR340 | MBR745 | |
30WQ04F | — | 31DQ04 | 80SQ045 | |
MBRS340 | — | SR403 | 6TQ045 | |
MBRD340 | — | — | — | |
50 or more | SK35 | — | MBR350 | — |
30WQ05F | — | 31DQ05 | — | |
— | — | SR305 | — |
For continuous mode operation
A system logic power supply bus of 3.3 V is to be generated from a wall adapter which provides an unregulated DC voltage of 13 V to 16 V. The maximum load current is 4 A. A soft-start delay time of 50 ms is desired. Through-hole components are preferred.
Step 1: Operating conditions are:
Step 2: Select an LM2679T-3.3. The output voltage has a tolerance of ±2% at room temperature and ±3% over the full operating temperature range.
Step 3: Use the nomograph for the 3.3-V device, Figure 14. The intersection of the 16 V horizontal line (V_{IN} max) and the 4 A vertical line (I_{load} max) indicates that L46, a 15-μH inductor, is required.
From Table 3, L46 in a through-hole component is available from Renco with part number RL-1283-15-43.
Step 4: Use Table 5 and Table 6 to determine an output capacitor. With a 3.3-V output and a 15-μH inductor there are four through-hole output capacitor solutions with the number of same type capacitors to be paralleled and an identifying capacitor code given. Table 1 and Table 2 provide the actual capacitor characteristics. Any of the following choices works in the circuit:
Step 5: Use Table 7 and Table 8 to select an input capacitor. With 3.3-V output and 15 μH there are three through-hole solutions. These capacitors provide a sufficient voltage rating and an RMS current rating greater than 2 A (1/2 I_{LOAD} max). Again using Table 1 and Table 2 for specific component characteristics the following choices are suitable:
Step 6: From Table 4, a 5-A or more Schottky diode must be selected. For through-hole components only 40-V rated diodes are indicated and 4 part types are suitable:
Step 7: A 0.01-μF capacitor is used for C_{BOOST}. For the 50-ms soft-start delay the following parameters are to be used:
Using V_{IN} max ensures that the soft-start delay time is at least the desired 50 ms.
Using the formula for C_{SS} a value of 0.148 μF is determined to be required. Use of a standard value 0.22-μF capacitor produces more than sufficient soft-start delay.
Step 8: Determine a value for R_{ADJ} with Equation 2 to provide a peak switch current limit of at least 4 A plus 50% or 6 A.
Use a value of 6.2 kΩ.
OUTPUT VOLTAGE (V) | INDUCTANCE (µH) | SURFACE MOUNT | |||||
---|---|---|---|---|---|---|---|
AVX TPS SERIES | SPRAGUE 594D SERIES | KEMET T495 SERIES | |||||
NO. | C CODE | NO. | C CODE | NO. | C CODE | ||
3.3 | 10 | 5 | C1 | 5 | C1 | 5 | C2 |
15 | 4 | C1 | 4 | C1 | 4 | C3 | |
22 | 3 | C2 | 2 | C7 | 3 | C4 | |
33 | 1 | C1 | 2 | C7 | 3 | C4 | |
5 | 10 | 4 | C2 | 4 | C6 | 4 | C4 |
15 | 3 | C3 | 2 | C7 | 3 | C5 | |
22 | 3 | C2 | 2 | C7 | 3 | C4 | |
33 | 2 | C2 | 2 | C3 | 2 | C4 | |
47 | 2 | C2 | 1 | C7 | 2 | C4 | |
12 | 10 | 4 | C5 | 3 | C6 | 5 | C9 |
15 | 3 | C5 | 2 | C7 | 4 | C9 | |
22 | 2 | C5 | 2 | C6 | 3 | C8 | |
33 | 2 | C5 | 1 | C7 | 3 | C8 | |
47 | 2 | C4 | 1 | C6 | 2 | C8 | |
68 | 1 | C5 | 1 | C5 | 2 | C7 | |
100 | 1 | C4 | 1 | C5 | 1 | C8 |
OUTPUT VOLTAGE (V) | INDUCTANCE (µH) | THROUGH HOLE | |||||||
---|---|---|---|---|---|---|---|---|---|
SANYO OS-CON SA SERIES | SANYO MV-GX SERIES | NICHICON PL SERIES | PANASONIC HFQ SERIES | ||||||
NO. | C CODE | NO. | C CODE | NO. | C CODE | NO. | C CODE | ||
3.3 | 10 | 2 | C5 | 2 | C6 | 1 | C8 | 2 | C6 |
15 | 2 | C5 | 2 | C5 | 1 | C7 | 2 | C5 | |
22 | 1 | C5 | 1 | C10 | 1 | C5 | 1 | C7 | |
33 | 1 | C5 | 1 | C10 | 1 | C5 | 1 | C7 | |
5 | 10 | 2 | C4 | 2 | C5 | 1 | C6 | 2 | C5 |
15 | 1 | C5 | 1 | C10 | 1 | C5 | 1 | C7 | |
22 | 1 | C5 | 1 | C9 | 1 | C5 | 1 | C5 | |
33 | 1 | C4 | 1 | C5 | 1 | C4 | 1 | C4 | |
47 | 1 | C4 | 1 | C4 | 1 | C2 | 2 | C4 | |
12 | 10 | 2 | C7 | 1 | C10 | 1 | C14 | 2 | C4 |
15 | 1 | C8 | 1 | C6 | 1 | C17 | 1 | C5 | |
22 | 1 | C7 | 1 | C5 | 1 | C13 | 1 | C5 | |
33 | 1 | C7 | 1 | C4 | 1 | C12 | 1 | C4 | |
47 | 1 | C7 | 1 | C3 | 1 | C11 | 1 | C3 | |
68 | 1 | C6 | 1 | C2 | 1 | C10 | 1 | C3 | |
100 | 1 | C6 | 1 | C2 | 1 | C9 | 1 | C1 |
OUTPUT VOLTAGE (V) | INDUCTANCE (µH) | SURFACE MOUNT | |||||
---|---|---|---|---|---|---|---|
AVX TPS SERIES | SPRAGUE 594D SERIES | KEMET T495 SERIES | |||||
NO. | C CODE | NO. | C CODE | NO. | C CODE | ||
3.3 | 10 | 3 | C7 | 2 | C10 | 3 | C9 |
15 | See^{(3)} | See^{(3)} | 3 | C13 | 4 | C12 | |
22 | See^{(3)} | See^{(3)} | 2 | C13 | 3 | C12 | |
33 | See^{(3)} | See^{(3)} | 2 | C13 | 3 | C12 | |
5 | 10 | 3 | C4 | 2 | C6 | 3 | C9 |
15 | 4 | C9 | 3 | C12 | 4 | C10 | |
22 | See^{(3)} | See^{(3)} | 3 | C13 | 4 | C12 | |
33 | See^{(3)} | See^{(3)} | 2 | C13 | 3 | C12 | |
47 | See^{(3)} | See^{(3)} | 1 | C13 | 2 | C12 | |
12 | 10 | 4 | C9 | 2 | C10 | 4 | C10 |
15 | 4 | C8 | 2 | C10 | 4 | C10 | |
22 | 4 | C9 | 3 | C12 | 4 | C10 | |
33 | See^{(3)} | See^{(3)} | 3 | C13 | 4 | C12 | |
47 | See^{(3)} | See^{(3)} | 2 | C13 | 3 | C12 | |
68 | See^{(3)} | See^{(3)} | 2 | C13 | 2 | C12 | |
100 | See^{(3)} | See^{(3)} | 1 | C13 | 2 | C12 |
OUTPUT VOLTAGE (V) | INDUCTANCE (µH) | THROUGH HOLE | |||||||
---|---|---|---|---|---|---|---|---|---|
SANYO OS-CON SA SERIES | SANYO MV-GX SERIES | NICHICON PL SERIES | PANASONIC HFQ SERIES | ||||||
NO. | C CODE | NO. | C CODE | NO. | C CODE | NO. | C CODE | ||
3.3 | 10 | 2 | C9 | 2 | C8 | 1 | C18 | 1 | C8 |
15 | See^{(3)} | See^{(3)} | 2 | C13 | 1 | C25 | 1 | C16 | |
22 | See^{(3)} | See^{(3)} | 1 | C14 | 1 | C24 | 1 | C16 | |
33 | See^{(3)} | See^{(3)} | 1 | C14 | 1 | C24 | 1 | C16 | |
5 | 10 | 2 | C7 | 2 | C8 | 1 | C25 | 1 | C8 |
15 | See^{(3)} | See^{(3)} | 2 | C8 | 1 | C25 | 1 | C8 | |
22 | See^{(3)} | See^{(3)} | 2 | C13 | 1 | C25 | 1 | C16 | |
33 | See^{(3)} | See^{(3)} | 1 | C14 | 1 | C23 | 1 | C13 | |
47 | See^{(3)} | See^{(3)} | 1 | C12 | 1 | C19 | 1 | C11 | |
12 | 10 | 2 | C10 | 2 | C8 | 1 | C18 | 1 | C8 |
15 | 2 | C10 | 2 | C8 | 1 | C18 | 1 | C8 | |
22 | See^{(3)} | See^{(3)} | 2 | C8 | 1 | C18 | 1 | C8 | |
33 | See^{(3)} | See^{(3)} | 2 | C12 | 1 | C24 | 1 | C14 | |
47 | See^{(3)} | See^{(3)} | 1 | C14 | 1 | C23 | 1 | C13 | |
68 | See^{(3)} | See^{(3)} | 1 | C13 | 1 | C21 | 1 | C15 | |
100 | See^{(3)} | See^{(3)} | 1 | C11 | 1 | C22 | 1 | C11 |
In this example it is desired to convert the voltage from a two battery automotive power supply (voltage range of 20 V to 28 V, typical in large truck applications) to the 14.8-VDC alternator supply typically used to power electronic equipment from single battery 12-V vehicle systems. The load current required is 3.5 A maximum. It is also desired to implement the power supply with all surface mount components. Soft start is not required.
Step 1: Operating conditions are:
Step 2: Select an LM2679S-ADJ. To set the output voltage to 14.9 V, two resistors need to be chosen (R1 and R2 in Figure 19). For the adjustable device, the output voltage is set by Equation 3.
where
A recommended value to use for R1 is 1 k. In this example, R2 is determined with Equation 4.
R2 = 11.23 kΩ
The closest standard 1% tolerance value to use is 11.3 kΩ
This sets the nominal output voltage to 14.88 V which is within 0.5% of the target value.
Step 3: To use the nomograph for the adjustable device, Figure 17, requires a calculation of the inductor Volt • microsecond constant (E • T expressed in V • μS) from Equation 5.
where
In this example, this is typically 0.12 Ω × 3.5 A or 0.42 V and V_{D} is the voltage drop across the forward biased Schottky diode, typically 0.5 V. The switching frequency of 260 kHz is the nominal value to use to estimate the ON time of the switch during which energy is stored in the inductor.
For this example E • T is found with Equation 6 and Equation 7.
Using Figure 17, the intersection of 27 V • μS horizontally and the 3.5 A vertical line (I_{LOAD} max) indicates that L48 , a 47-μH inductor, or L49, a 33-μH inductor could be used. Either inductor is suitable, but for this example selecting the larger inductance results in lower ripple current.
From Table 3, L48 in a surface mount component is available from Pulse Engineering with part number P0848.
Step 4: Use Table 9 and Table 10 to determine an output capacitor. With a 14.8-V output the 12.5 to 15 V row is used and with a 47-μH inductor there are three surface mount output capacitor solutions. Table 1 and Table 2 provide the actual capacitor characteristics based on the C Code number. Any of the following choices can be used:
NOTE
When using the adjustable device in low voltage applications (less than 3-V output), if the nomograph, Figure 17, selects an inductance of 22 μH or less, Table 9 and Table 10 do not provide an output capacitor solution. With these conditions the number of output capacitors required for stable operation becomes impractical. It is recommended to use either a 33-μH or 47-μH inductor and the output capacitors from Table 9 and Table 10.
Step 5: An input capacitor for this example requires at least a 35-V WV rating with an RMS current rating of 1.75 A (1/2 I_{OUT} max). From Table 1 and Table 2, it can be seen that C12, a 33-μF, 35-V capacitor from Sprague, has the highest voltage and current rating of the surface mount components and that two of these capacitor in parallel is adequate.
Step 6: From Table 4, a 5-A or more Schottky diode must be selected. For surface mount diodes with a margin of safety on the voltage rating one of two diodes can be used:
Step 7: A 0.01-μF capacitor is used for C_{BOOST}.
The soft-start pin is left open circuited.
Step 8: Determine a value for R_{ADJ} with Equation 8 to provide a peak switch current limit of at least 3.5 A plus 50% or 5.25 A.
Use a value of 7.15 kΩ.
OUTPUT VOLTAGE (V) | INDUCTANCE (µH) | SURFACE MOUNT | |||||
---|---|---|---|---|---|---|---|
AVX TPS SERIES | SPRAGUE 594D SERIES | KEMET T495 SERIES | |||||
NO. | C CODE | NO. | C CODE | NO. | C CODE | ||
1.21 to 2.5 | 33^{(3)} | 7 | C1 | 6 | C2 | 7 | C3 |
47^{(3)} | 5 | C1 | 4 | C2 | 5 | C3 | |
2.5 to 3.75 | 33^{(3)} | 4 | C1 | 3 | C2 | 4 | C3 |
47^{(3)} | 3 | C1 | 2 | C2 | 3 | C3 | |
3.75 to 5 | 22 | 4 | C1 | 3 | C2 | 4 | C3 |
33 | 3 | C1 | 2 | C2 | 3 | C3 | |
47 | 2 | C1 | 2 | C2 | 2 | C3 | |
5 to 6.25 | 22 | 3 | C2 | 3 | C3 | 3 | C4 |
33 | 2 | C2 | 2 | C3 | 2 | C4 | |
47 | 2 | C2 | 2 | C3 | 2 | C4 | |
68 | 1 | C2 | 1 | C3 | 1 | C4 | |
6.25 to 7.5 | 22 | 3 | C2 | 1 | C4 | 3 | C4 |
33 | 2 | C2 | 1 | C3 | 2 | C4 | |
47 | 1 | C3 | 1 | C4 | 1 | C6 | |
68 | 1 | C2 | 1 | C3 | 1 | C4 | |
7.5 to 10 | 33 | 2 | C5 | 1 | C6 | 2 | C8 |
47 | 1 | C5 | 1 | C6 | 2 | C8 | |
68 | 1 | C5 | 1 | C6 | 1 | C8 | |
100 | 1 | C4 | 1 | C5 | 1 | C8 | |
10 to 12.5 | 33 | 1 | C5 | 1 | C6 | 2 | C8 |
47 | 1 | C5 | 1 | C6 | 2 | C8 | |
68 | 1 | C5 | 1 | C6 | 1 | C8 | |
100 | 1 | C5 | 1 | C6 | 1 | C8 | |
12.5 to 15 | 33 | 1 | C6 | 1 | C8 | 1 | C8 |
47 | 1 | C6 | 1 | C8 | 1 | C8 | |
68 | 1 | C6 | 1 | C8 | 1 | C8 | |
100 | 1 | C6 | 1 | C8 | 1 | C8 | |
15 to 20 | 33 | 1 | C8 | 1 | C10 | 2 | C10 |
47 | 1 | C8 | 1 | C9 | 2 | C10 | |
68 | 1 | C8 | 1 | C9 | 2 | C10 | |
100 | 1 | C8 | 1 | C9 | 1 | C10 | |
20 to 30 | 33 | 2 | C9 | 2 | C11 | 2 | C11 |
47 | 1 | C10 | 1 | C12 | 1 | C11 | |
68 | 1 | C9 | 1 | C12 | 1 | C11 | |
100 | 1 | C9 | 1 | C12 | 1 | C11 | |
30 to 37 | 10 | No values available | 4 | C13 | 8 | C12 | |
15 | 3 | C13 | 5 | C12 | |||
22 | 2 | C13 | 4 | C12 | |||
33 | 1 | C13 | 3 | C12 | |||
47 | 1 | C13 | 2 | C12 | |||
68 | 1 | C13 | 2 | C12 |
OUTPUT VOLTAGE (V) | INDUCTANCE (µH) | THROUGH HOLE | |||||||
---|---|---|---|---|---|---|---|---|---|
SANYO OS-CON SA SERIES | SANYO MV-GX SERIES | NICHICON PL SERIES | PANASONIC HFQ SERIES | ||||||
NO. | C CODE | NO. | C CODE | NO. | C CODE | NO. | C CODE | ||
1.21 to 2.5 | 33^{(3)} | 2 | C3 | 5 | C1 | 5 | C3 | 3 | C |
47^{(3)} | 2 | C2 | 4 | C1 | 3 | C3 | 2 | C5 | |
2.5 to 3.75 | 33^{(3)} | 1 | C3 | 3 | C1 | 3 | C1 | 2 | C5 |
47^{(3)} | 1 | C2 | 2 | C1 | 2 | C3 | 1 | C5 | |
3.75 to 5 | 22 | 1 | C3 | 3 | C1 | 3 | C1 | 2 | C5 |
33 | 1 | C2 | 2 | C1 | 2 | C1 | 1 | C5 | |
47 | 1 | C2 | 2 | C1 | 1 | C3 | 1 | C5 | |
5 to 6.25 | 22 | 1 | C5 | 2 | C6 | 2 | C3 | 2 | C5 |
33 | 1 | C4 | 1 | C6 | 2 | C1 | 1 | C5 | |
47 | 1 | C4 | 1 | C6 | 1 | C3 | 1 | C5 | |
68 | 1 | C4 | 1 | C6 | 1 | C1 | 1 | C5 | |
6.25 to 7.5 | 22 | 1 | C5 | 1 | C6 | 2 | C1 | 1 | C5 |
33 | 1 | C4 | 1 | C6 | 1 | C3 | 1 | C5 | |
47 | 1 | C4 | 1 | C6 | 1 | C1 | 1 | C5 | |
68 | 1 | C4 | 1 | C2 | 1 | C1 | 1 | C5 | |
7.5 to 10 | 33 | 1 | C7 | 1 | C6 | 1 | C14 | 1 | C5 |
47 | 1 | C7 | 1 | C6 | 1 | C14 | 1 | C5 | |
68 | 1 | C7 | 1 | C2 | 1 | C14 | 1 | C2 | |
100 | 1 | C7 | 1 | C2 | 1 | C14 | 1 | C2 | |
10 to 12.5 | 33 | 1 | C7 | 1 | C6 | 1 | C14 | 1 | C5 |
47 | 1 | C7 | 1 | C2 | 1 | C14 | 1 | C5 | |
68 | 1 | C7 | 1 | C2 | 1 | C9 | 1 | C2 | |
100 | 1 | C7 | 1 | C2 | 1 | C9 | 1 | C2 | |
12.5 to 15 | 33 | 1 | C9 | 1 | C10 | 1 | C15 | 1 | C2 |
47 | 1 | C9 | 1 | C10 | 1 | C15 | 1 | C2 | |
68 | 1 | C9 | 1 | C10 | 1 | C15 | 1 | C2 | |
100 | 1 | C9 | 1 | C10 | 1 | C15 | 1 | C2 | |
15 to 20 | 33 | 1 | C10 | 1 | C7 | 1 | C15 | 1 | C2 |
47 | 1 | C10 | 1 | C7 | 1 | C15 | 1 | C2 | |
68 | 1 | C10 | 1 | C7 | 1 | C15 | 1 | C2 | |
100 | 1 | C10 | 1 | C7 | 1 | C15 | 1 | C2 | |
20 to 30 | 33 | No values available | 1 | C7 | 1 | C16 | 1 | C2 | |
47 | 1 | C7 | 1 | C16 | 1 | C2 | |||
68 | 1 | C7 | 1 | C16 | 1 | C2 | |||
100 | 1 | C7 | 1 | C16 | 1 | C2 | |||
30 to 37 | 10 | No values available | 1 | C12 | 1 | C20 | 1 | C10 | |
15 | 1 | C11 | 1 | C20 | 1 | C11 | |||
22 | 1 | C11 | 1 | C20 | 1 | C10 | |||
33 | 1 | C11 | 1 | C20 | 1 | C10 | |||
47 | 1 | C11 | 1 | C20 | 1 | C10 | |||
68 | 1 | C11 | 1 | C20 | 1 | C10 |