SNVS346F November 2007 – November 2014 LM3481 , LM3481-Q1
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.
The LM3481 may be operated in either continuous or discontinuous conduction mode. The following applications are designed for continuous conduction operation. This mode of operation has higher efficiency and lower EMI characteristics than the discontinuous mode.
The most common topology for the LM3481 is the boost or step-up topology. The boost converter converts a low input voltage into a higher output voltage. The basic configuration for a boost regulator is shown in Figure 31. In continuous conduction mode (when the inductor current never reaches zero at steady state), the boost regulator operates in two cycles. In the first cycle of operation, MOSFET Q is turned on and energy is stored in the inductor. During this cycle, diode D1 is reverse biased and load current is supplied by the output capacitor, C_{OUT}.
In the second cycle, MOSFET Q is off and the diode is forward biased. The energy stored in the inductor is transferred to the load and output capacitor. The ratio of these two cycles determines the output voltage. The output voltage is defined as:
(ignoring the voltage drop across the MOSFET and the diode), or
where D is the duty cycle of the switch, V_{D1} is the forward voltage drop of the diode, and V_{Q} is the drop across the MOSFET when it is on. The following sections describe selection of components for a boost converter.
To properly size the components for the application, the designer needs the following parameters: Input voltage range, output voltage, output current range and required switching frequency. These four main parameters will affect the choices of component available to achieve a proper system behavior.
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The inductor is one of the two energy storage elements in a boost converter. Figure 32 shows how the inductor current varies during a switching cycle. The current through an inductor is quantified as:
If V_{L}(t) is constant, di_{L}(t)/dt must be constant. Hence, for a given input voltage and output voltage, the current in the inductor changes at a constant rate.
The important quantities in determining a proper inductance value are I_{L} (the average inductor current) and Δi_{L} (the inductor current ripple difference between the peak inductor current and the average inductor current). If Δi_{L} is larger than I_{L}, the inductor current drops to zero for a portion of the cycle and the converter operates in discontinuous conduction mode. If Δi_{L} is smaller than I_{L}, the inductor current stays above zero and the converter operates in continuous conduction mode. All the analysis in this data sheet assumes operation in continuous conduction mode. To operate in continuous conduction mode, the following conditions must be met:
Choose the minimum I_{OUT} to determine the minimum L. A common choice is to set (2 x Δi_{L}) to 30% of I_{L}. Choosing an appropriate core size for the inductor involves calculating the average and peak currents expected through the inductor. In a boost converter,
A core size with ratings higher than these values should be chosen. If the core is not properly rated, saturation will dramatically reduce overall efficiency.
The LM3481 can be set to switch at very high frequencies. When the switching frequency is high, the converter can operate with very small inductor values. With a small inductor value, the peak inductor current can be extremely higher than the output currents, especially under light load conditions.
The LM3481 senses the peak current through the switch. The peak current through the switch is the same as the peak current calculated above.
The output voltage can be programmed using a resistor divider between the output and the feedback pins, as shown in Figure 33. The resistors are selected such that the voltage at the feedback pin is 1.275 V. R_{F1} and R_{F2} can be selected using the equation,
A 100-pF capacitor may be connected between the feedback and ground pins to reduce noise.
The maximum amount of current that can be delivered at the output can be controlled by the sense resistor, R_{SEN}. Current limit occurs when the voltage that is generated across the sense resistor equals the current sense threshold voltage, V_{SENSE}. Limits for V_{SENSE} have been specified in the Electrical Characteristics section. This can be expressed as:
The peak current through the switch is equal to the peak inductor current.
Therefore for a boost converter,
Combining the two equations yields an expression for R_{SEN},
Evaluate R_{SEN} at the maximum and minimum V_{IN} values and choose the smallest R_{SEN} calculated.
If an external slope compensation resistor is used (see Figure 23) the internal control signal will be modified and this will have an effect on the current limit.
If R_{SL} is used, then this will add to the existing slope compensation. The command voltage, V_{CS}, will then be given by:
Where V_{SENSE} is a defined parameter in the Electrical Characteristics section and ΔV_{SL} is the additional slope compensation generated as discussed in the Slope Compensation Ramp section. This changes the equation for R_{SEN} to:
Note that because ΔV_{SL} = R_{SL} x K as defined earlier, R_{SL}can be used to provide an additional method for setting the current limit. In some designs R_{SL} can also be used to help filter noise to keep the I_{SEN} pin quiet.
Observation of the boost converter circuit shows that the average current through the diode is the average load current, and the peak current through the diode is the peak current through the inductor. The diode should be rated to handle more than the inductor peak current. The peak diode current can be calculated using the formula:
In Equation 35, I_{OUT} is the output current and Δi_{L} has been defined in Figure 32.
The peak reverse voltage for a boost converter is equal to the regulator output voltage. The diode must be capable of handling this peak reverse voltage. To improve efficiency, a low forward drop Schottky diode is recommended.
The drive pin, DR, of the LM3481 must be connected to the gate of an external MOSFET. In a boost topology, the drain of the external N-Channel MOSFET is connected to the inductor and the source is connected to the ground. The drive pin voltage, V_{DR}, depends on the input voltage (see Typical Characteristics). In most applications, a logic level MOSFET can be used. For very low input voltages, a sub-logic level MOSFET should be used.
The selected MOSFET directly controls the efficiency. The critical parameters for selection of a MOSFET are:
The off-state voltage of the MOSFET is approximately equal to the output voltage. V_{DS(MAX)} of the MOSFET must be greater than the output voltage. The power losses in the MOSFET can be categorized into conduction losses and ac switching or transition losses. R_{DS(ON)} is needed to estimate the conduction losses. The conduction loss, P_{COND}, is the I^{2}R loss across the MOSFET. The maximum conduction loss is given by:
where D_{MAX} is the maximum duty cycle.
At high switching frequencies the switching losses may be the largest portion of the total losses.
The switching losses are very difficult to calculate due to changing parasitics of a given MOSFET in operation. Often, the individual MOSFET datasheet does not give enough information to yield a useful result. Equation 38 and Equation 39 give a rough idea how the switching losses are calculated:
Due to the presence of an inductor at the input of a boost converter, the input current waveform is continuous and triangular, as shown in Figure 32. The inductor ensures that the input capacitor sees fairly low ripple currents. However, as the input capacitor gets smaller, the input ripple goes up. The rms current in the input capacitor is given by:
The input capacitor should be capable of handling the rms current. Although the input capacitor is not as critical in a boost application, low values can cause impedance interactions. Therefore a good quality capacitor should be chosen in the range of 100 µF to 200 µF. If a value lower than 100 µF is used, then problems with impedance interactions or switching noise can affect the LM3481. To improve performance, especially with V_{IN} below 8 V, it is recommended to use a 20Ω resistor at the input to provide a RC filter. This resistor is placed in series with the V_{IN} pin with only a bypass capacitor attached to the V_{IN} pin directly (see Figure 34). A 0.1-µF or 1-µF ceramic capacitor is necessary in this configuration. The bulk input capacitor and inductor will connect on the other side of the resistor with the input power supply.
The output capacitor in a boost converter provides all the output current when the inductor is charging. As a result it sees very large ripple currents. The output capacitor should be capable of handling the maximum rms current. The rms current in the output capacitor is:
Where
and D, the duty cycle is equal to (V_{OUT} − V_{IN})/V_{OUT}.
The ESR and ESL of the output capacitor directly control the output ripple. Use capacitors with low ESR and ESL at the output for high efficiency and low ripple voltage. Surface mount tantalums, surface mount polymer electrolytic and polymer tantalum, Sanyo- OSCON, or multi-layer ceramic capacitors are recommended at the output.
A good quality ceramic bypass capacitor must be connected from the V_{CC} pin to the PGND pin for proper operation. This capacitor supplies the transient current required by the internal MOSFET driver, as well as filtering the internal supply voltage for the controller. A value of between 0.47 µF and 4.7 µF is recommended.
For detailed explanation on how to select the right compensation components to attach to the compensation pin for a boost topology please see AN-1286 Compensation for the LM3478 Boost Controller (SNVA067). When calculating the Error Amplifier DC gain, A_{EA}, R_{OUT} = 152 kΩ for the LM3481.
Because the LM3481 controls a low-side N-Channel MOSFET, it can also be used in SEPIC (Single Ended Primary Inductance Converter) applications. An example of SEPIC using the LM3481 is shown in Figure 36. As shown in Figure 36, the output voltage can be higher or lower than the input voltage. The SEPIC uses two inductors to step-up or step-down the input voltage. The inductors L1 and L2 can be two discrete inductors or two windings of a coupled transformer because equal voltages are applied across the inductor throughout the switching cycle. Using two discrete inductors allows use of catalog magnetics, as opposed to a custom transformer. The input ripple can be reduced along with size by using the coupled windings of transformer for L1 and L2.
Due to the presence of the inductor L1 at the input, the SEPIC inherits all the benefits of a boost converter. One main advantage of SEPIC over a boost converter is the inherent input to output isolation. The capacitor C_{S} isolates the input from the output and provides protection against shorted or malfunctioning load. Hence, the SEPIC is useful for replacing boost circuits when true shutdown is required. This means that the output voltage falls to 0V when the switch is turned off. In a boost converter, the output can only fall to the input voltage minus a diode drop.
The duty cycle of a SEPIC is given by:
In Equation 43, V_{Q } is the on-state voltage of the MOSFET, Q1, and V_{DIODE} is the forward voltage drop of the diode.
To properly size the components for the application, the designer needs the following parameters: Input voltage range, output voltage, output current range and required switching frequency. These four main parameters will affect the choices of component available to achieve a proper system behavior.
As in a boost converter, the parameters governing the selection of the MOSFET are the minimum threshold voltage, V_{TH(MIN)}, the on-resistance, R_{DS(ON)}, the total gate charge, Q_{g}, the reverse transfer capacitance, C_{RSS}, and the maximum drain to source voltage, V_{DS(MAX)}. The peak switch voltage in a SEPIC is given by:
The selected MOSFET should satisfy the condition:
The peak switch current is given by:
Where ΔI_{L1} and ΔI_{L2} are the peak-to-peak inductor ripple currents of inductors L1 and L2 respectively.
The rms current through the switch is given by:
The Power diode must be selected to handle the peak current and the peak reverse voltage. In a SEPIC, the diode peak current is the same as the switch peak current. The off-state voltage or peak reverse voltage of the diode is V_{IN} + V_{OUT}. Similar to the boost converter, the average diode current is equal to the output current. Schottky diodes are recommended.
Proper selection of the inductors L1 and L2 to maintain constant current mode requires calculations of the following parameters.
Average current in the inductors:
Peak-to-peak ripple current, to calculate core loss if necessary:
Maintaining the condition I_{L} > ΔI_{L}/2 to ensure continuous conduction mode yields the following minimum values for L1 and L2:
Peak current in the inductor, to ensure the inductor does not saturate:
I_{L1PK} must be lower than the maximum current rating set by the current sense resistor.
The value of L1 can be increased above the minimum recommended value to reduce input ripple and output ripple. However, once ΔI_{L1} is less than 20% of I_{L1AVE}, the benefit to output ripple is minimal.
By increasing the value of L2 above the minimum recommendation, ΔI_{L2} can be reduced, which in turn will reduce the output ripple voltage:
where ESR is the effective series resistance of the output capacitor.
If L1 and L2 are wound on the same core, then L1 = L2 = L. All the equations above will hold true if the inductance is replaced by 2L. A good choice for transformer with equal turns is Coiltronics CTX series Octopack.
The peak current through the switch, I_{SWPEAK}, can be adjusted using the current sense resistor, R_{SEN}, to provide a certain output current. Resistor R_{SEN} can be selected using the formula:
The selection of SEPIC capacitor, C_{S}, depends on the rms current. The rms current of the SEPIC capacitor is given by:
The SEPIC capacitor must be rated for a large ACrms current relative to the output power. This property makes the SEPIC much better suited to lower power applications where the rms current through the capacitor is small (relative to capacitor technology). The voltage rating of the SEPIC capacitor must be greater than the maximum input voltage. Tantalum capacitors are the best choice for SMT, having high rms current ratings relative to size. Ceramic capacitors could be used, but the low C values will tend to cause larger changes in voltage across the capacitor due to the large currents, and high C value ceramics are expensive. Electrolytics work well for through hole applications where the size required to meet the rms current rating can be accommodated. There is an energy balance between C_{S} and L1, which can be used to determine the value of the capacitor. The basic energy balance equation is:
Where
is the ripple voltage across the SEPIC capacitor, and
is the ripple current through the inductor L1. The energy balance equation can be solved to provide a minimum value for C_{S}:
Similar to a boost converter, the SEPIC has an inductor at the input. Hence, the input current waveform is continuous and triangular. The inductor ensures that the input capacitor sees fairly low ripple currents. However, as the input capacitor gets smaller, the input ripple goes up. The rms current in the input capacitor is given by:
The input capacitor should be capable of handling the rms current. Although the input capacitor is not as critical in a SEPIC application, low values can cause impedance interactions. Therefore a good quality capacitor should be chosen in the range of 100 µF to 200 µF. If a value lower than 100 µF is used, then problems with impedance interactions or switching noise can affect the LM3481. To improve performance, especially with V_{IN} below 8 V, it is recommended to use a 20Ω resistor at the input to provide a RC filter. This resistor is placed in series with the V_{IN} pin with only a bypass capacitor attached to the V_{IN} pin directly (see Figure 34). A 0.1 µF or 1 µF ceramic capacitor is necessary in this configuration. The bulk input capacitor and inductor will connect on the other side of the resistor with the input power supply.
The output capacitor of the SEPIC sees very large ripple currents similar to the output capacitor of a boost converter. The rms current through the output capacitor is given by:
The ESR and ESL of the output capacitor directly control the output ripple. Use capacitors with low ESR and ESL at the output for high efficiency and low ripple voltage. Surface mount tantalums, surface mount polymer electrolytic and polymer tantalum, Sanyo- OSCON, or multi-layer ceramic capacitors are recommended at the output for low ripple.