SLVS638C January 2006 – November 2014
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.
For stability concerns, an input bypass capacitor (electrolytic, C_{IN} ≥ 47 μF) needs to be located as close as possible to the regulator. For operating temperatures below –25°C, C_{IN} may need to be larger in value. In addition, since most electrolytic capacitors have decreasing capacitances and increasing ESR as temperature drops, adding a ceramic or solid tantalum capacitor in parallel increases the stability in cold temperatures.
To extend the capacitor operating lifetime, the capacitor RMS ripple current rating should be calculated as shown in Equation 1.
where
For both loop stability and filtering of ripple voltage, an output capacitor is required, again in close proximity to the regulator. For best performance, lowESR aluminum electrolytics are recommended, although standard aluminum electrolytics may be adequate for some applications as shown in Equation 2.
Output ripple of 50 mV to 150 mV typically can be achieved with capacitor values of 220 μF to 680 μF. Larger C_{OUT} can reduce the ripple 20 mV to 50 mV peak to peak. To improve further on output ripple, paralleling of standard electrolytic capacitors may be used. Alternatively, highergrade capacitors such as high frequency, low inductance, or low ESR can be used.
The following should be taken into account when selecting C_{OUT}:
As with other external components, the catch diode should be placed close to the output to minimize unwanted noise. Schottky diodes have fast switching speeds and low forward voltage drops and, thus, offer the best performance, especially for switching regulators with low output voltages (V_{OUT} < 5 V). If a highefficiency, fastrecovery, or ultrafastrecovery diode is used in place of a Schottky, it should have a soft recovery (versus abrupt turnoff characteristics) to avoid the chance of causing instability and EMI. Standard 50 to 60Hz diodes, such as the 1N4001 or 1N5400 series, are not suitable.
Proper inductor selection is key to the performanceswitching powersupply designs. One important factor to consider is whether the regulator is used in continuous mode (inductor current flows continuously and never drops to zero) or in discontinuous mode (inductor current goes to zero during the normal switching cycle). Each mode has distinctively different operating characteristics and, therefore, can affect the regulator performance and requirements. In many applications, the continuous mode is the preferred mode of operation, since it offers greater output power with lower peak currents, and also can result in lower output ripple voltage. The advantages of continuous mode of operation come at the expense of a larger inductor required to keep inductor current continuous, especially at low output currents and/or high input voltages.
The TL2575 and TL2575HV devices can operate in either continuous or discontinuous mode. With heavy load currents, the inductor current flows continuously and the regulator operates in continuous mode. Under light load, the inductor fully discharges and the regulator is forced into the discontinuous mode of operation. For light loads (approximately 200 mA or less), this discontinuous mode of operation is perfectly acceptable and may be desirable solely to keep the inductor value and size small. Any buck regulator eventually operates in discontinuous mode when the load current is light enough.
The type of inductor chosen can have advantages and disadvantages. If high performance or high quality is a concern, then moreexpensive toroid core inductors are the best choice, as the magnetic flux is contained completely within the core, resulting in less EMI and noise in nearby sensitive circuits. Inexpensive bobbin core inductors, however, generate more EMI as the open core does not confine the flux within the core. Multiple switching regulators located in proximity to each other are particularly susceptible to mutual coupling of magnetic fluxes from each other’s open cores. In these situations, closed magnetic structures (such as a toroid, pot core, or Ecore) are more appropriate.
Regardless of the type and value of inductor used, the inductor never should carry more than its rated current. Doing so may cause the inductor to saturate, in which case the inductance quickly drops, and the inductor looks like a lowvalue resistor (from the dc resistance of the windings). As a result, switching current rises dramatically (until limited by the currentbycurrent limiting feature of the TL2575 and TL2575HV devices) and can result in overheating of the inductor and the IC itself.
NOTE
Different types of inductors have different saturation characteristics.
As with any switching power supply, the output of the TL2575 and TL2575HV devices have a sawtooth ripple voltage at the switching frequency. Typically about 1% of the output voltage, this ripple is due mainly to the inductor sawtooth ripple current and the ESR of the output capacitor (see Output Capacitor (COUT)). Furthermore, the output also may contain small voltage spikes at the peaks of the sawtooth waveform. This is due to the fast switching of the output switch and the parasitic inductance of C_{OUT}. These voltage spikes can be minimized through the use of lowinductance capacitors.
There are several ways to reduce the output ripple voltage: a larger inductor, a larger C_{OUT}, or both. Another method is to use a small LC filter (20 μH and 100 μF) at the output. This filter can reduce the output ripple voltage by a factor of 10 (see Figure 11).
The power and ground connections of the TL2575 and TL2575HV devices must be low impedance to help maintain output stability. For the 5pin packages, both pin 3 and tab are ground, and either connection can be used as they are both part of the same lead frame. With the 16pin package, all the ground pins (including signal and power grounds) should be soldered directly to wide PCB copper traces to ensure lowinductance connections and good thermal dissipation.
There is an internal diode from the output to VIN. Therefore, the device does not protect against reverse current and care must be taken to limit current in this scenario.
PROCEDURE (Fixed Output)  EXAMPLE (Fixed Output) 

Known: V_{OUT} = 3.3 V, 5 V, 12 V, or 15 V V_{IN(Max)} = Maximum input voltage I_{LOAD(Max)} = Maximum load current 
Known: V_{OUT} = 5 V V_{IN(Max)} = 20 V I_{LOAD(Max)} = 1 A 
1. Inductor Selection (L1) 
1. Inductor Selection (L1) 
A. From Figure 13 through Figure 16, select the appropriate inductor code based on the intersection of V_{IN(Max)} and I_{LOAD(Max)}. 
A. From Figure 14 (TL257505), the intersection of 20V line and 1A line gives an inductor code of L330. 
B. The inductor chosen should be rated for operation at 52kHz and have a current rating of at least 1.15 × I_{LOAD(Max)} to allow for the ripple current. The actual peak current in L1 (in normal operation) can be calculated as follows: I_{L1(pk)} = I_{LOAD(Max)} + (V_{IN} – V_{OUT}) × t_{on }/ 2L1 Where t_{on} = V_{OUT }/ V_{IN} × (1 / f_{osc}) 
B. L330 → L1 = 330 μH Choose from: 34042 (Schott) PE52627 (Pulse Engineering) RL1952 (Renco) 
2. Output Capacitor Selection (C_{OUT}) 
2. Output Capacitor Selection (C_{OUT}) 
A. The TL2575 control loop has a twopole twozero frequency response. The dominant polezero pair is established by C_{OUT} and L1. To meet stability requirements while maintaining an acceptable output ripple voltage (V_{ripple} ≉ 0.01 × V_{OUT}), the recommended range for a standard aluminum electrolytic C_{OUT} is between 100 μF and 470 μF. 
A. C_{OUT} = 100μF to 470μF, standard aluminum electrolytic 
B. C_{OUT} should have a voltage rating of at least 1.5 × V_{OUT}. But if a low output ripple voltage is desired, choose capacitors with a highervoltage ratings than the minimum required, due to their typically lower ESRs. 
B. Although a C_{OUT} rated at 8 V is sufficient for V_{OUT} = 5 V, a highervoltage capacitor is chosen for its typically lower ESR (and hence lower output ripple voltage) → Capacitor voltage rating = 20 V. 
3. Catch Diode Selection (D1) (see Table 1) 
3. Catch Diode Selection (D1) (see Table 1) 
A. In normal operation, the catch diode requires a current rating of at least 1.2 × I_{LOAD(Max)}. For the most robust design, D1 should be rated to handle a current equal to the TL2575 maximum switch peak current; this represents the worstcase scenario of a continuous short at V_{OUT}. 
A. Pick a diode with 3A rating. 
B. The diode requires a reverse voltage rating of at least 1.25 × V_{IN(Max)}. 
B. Pick 30V rated Schottky diode (1N5821, MBR330, 31QD03, or SR303) or 100V rated Fast Recovery diode (31DF1, MURD310, or HER302). 
4. Input Capacitor (C_{IN}) An aluminum electrolytic or tantalum capacitor is needed for input bypassing. Locate C_{IN} as close to the V_{IN} and GND pins as possible. 
4. Input Capacitor (C_{IN}) C_{IN} = 100 μF, 25 V, aluminum electrolytic 
PROCEDURE (Adjustable Output)  EXAMPLE (Adjustable Output) 

Known: V_{OUT(Nom)} V_{IN(Max)} = Maximum input voltage I_{LOAD(Max)} = Maximum load current 
Known: V_{OUT} = 10 V V_{IN(Max)} = 25 V I_{LOAD(Max)} = 1 A 
1. Programming Output Voltage (Selecting R1 and R2) Referring to Figure 2, V_{OUT} is defined by: Choose a value for R1 between 1 kΩ and 5 kΩ (use 1% metalfilm resistors for best temperature coefficient and stability over time). 
1. Programming Output Voltage (Selecting R1 and R2) Select R1 = 1 kΩ R2 = 1 (10 / 1.23 – 1) = 7.13 kΩ Select R2 = 7.15 kΩ (closest 1% value) 
2. Inductor Selection (L1) 
2. Inductor Selection (L1) 
A. Calculate the "set" voltssecond (E × T) across L1: E × T = (V_{IN} – V_{OUT}) × t_{on} E × T = (V_{IN} – V_{OUT}) × (V_{OUT }/ V_{IN}) × \{1000 / f_{osc}(in kHz)\} [V × μs] NOTE: Along with I_{LOAD}, the "set" voltssecond (E × T) constant establishes the minimum energy storage requirement for the inductor. 
A. Calculate the "set" voltssecond (E × T) across L1: E × T = (25 – 10) × (10 / 25) × (1000 / 52) [V × μs] E × T = 115 V × μs 
B. Using Figure 17, select the appropriate inductor code based on the intersection of E × T value and I_{LOAD(Max)}. 
B. Using Figure 17, the intersection of 115 V • μs and 1 A corresponds to an inductor code of H470. 
C. The inductor chosen should be rated for operation at 52kHz and have a current rating of at least 1.15 x I_{LOAD(Max)} to allow for the ripple current. The actual peak current in L1 (in normal operation) can be calculated as follows: I_{L1(pk)} = I_{LOAD(Max)} + (V_{IN} – V_{OUT}) × t_{on} / 2L1 Where t_{on} = V_{OUT }/ V_{IN} × (1 / f_{osc}) 
C. H470 → L1 = 470 μH Choose from: 34048 (Schott) PE53118 (Pulse Engineering) RL1961 (Renco) 
3. Output Capacitor Selection (C_{OUT}) 
3. Output Capacitor Selection (C_{OUT}) 
A. The TL2575 control loop has a twopole twozero frequency response. The dominant polezero pair is established by C_{OUT} and L1. To meet stability requirements, C_{OUT} must meet the following requirement: However, C_{OUT} may need to be several times larger than the calculated value above in order to achieve an acceptable output ripple voltage of ~0.01 × V_{OUT}. 
A. C_{OUT} ≥ 7785 × 25 / (10 × 470) [μF] C_{OUT} ≥ 41.4 μF To obtain an acceptable output voltage ripple → C_{OUT} = 220 μF electrolytic 
B. C_{OUT} should have a voltage rating of at least 1.5 × V_{OUT}. But if a low output ripple voltage is desired, choose capacitors with a higher voltage ratings than the minimum required due to their typically lower ESRs. 

4. Catch Diode Selection (D1) (see Table 1) 
4. Catch Diode Selection (D1) (see Table 1) 
A. In normal operation, the catch diode requires a current rating of at least 1.2 × I_{LOAD(Max)}. For the most robust design, D1 should be rated for a current equal to the TL2575 maximum switch peak current; this represents the worstcase scenario of a continuous short at V_{OUT}. 
A. Pick a diode with a 3A rating. 
B. The diode requires a reverse voltage rating of at least 1.25 × V_{IN(Max)}. 
B. Pick a 40V rated Schottky diode (1N5822, MBR340, 31QD04, or SR304) or 100V rated Fast Recovery diode (31DF1, MURD310, or HER302) 
5. Input Capacitor (C_{IN}) An aluminum electrolytic or tantalum capacitor is needed for input bypassing. Locate C_{IN} as close to V_{IN} and GND pins as possible. 
5. Input Capacitor (C_{IN}) C_{IN} = 100 μF, 35 V, aluminum electrolytic 
V_{R}  SCHOTTKY  FAST RECOVERY  

1A  3A  1A  3A  
20 V  1N5817 MBR120P SR102 
1N5820 MBR320 SR302 
The following diodes are all rated to 100 V: 11DF1 MUR110 HER102 
The following diodes are all rated to 100 V: 31DF1 MURD310 HER302 
30 V  1N5818 MBR130P 11DQ03 SR103 
1N5821 MBR330 31DQ03 SR303 

40 V  1N5819 MBR140P 11DQ04 SR104 
IN5822 MBR340 31DQ04 SR304 

50 V  MBR150 11DQ05 SR105 
MBR350 31DQ05 SR305 

60 V  MBR160 11DQ06 SR106 
MBR360 31DQ06 SR306 