8.1.1 Input Capacitor (C_IN)

Fast switching of large currents in the buck converter places a heavy demand on the voltage source supplying PVIN. The input capacitor, C_IN, supplies extra charge when the switcher needs to draw a burst of current from the supply. The RMS current rating and the voltage rating of the C_IN capacitor are therefore important in the selection of C_IN. The RMS current specification can be approximated by:

Equation 1.

where D is the duty cycle, V_OUT/V_IN. C_IN also provides filtering of the supply. Trace resistance and inductance degrade the benefits of the input capacitor, so C_IN should be placed very close to PVIN in the layout. A 22 µF or 47 µF ceramic capacitor is typically sufficient for C_IN. In parallel with the large input capacitance a smaller capacitor should be added such as a 1 µF ceramic for higher frequency filtering. Ceramic capacitors with high quality dielectrics such as X5R or X7R should be used to provide a constant capacitance across temperature and line variations. For improved load regulation and transient performance, the use of a small 1 µF ceramic capacitor is also recommended as a local bypass for the AVIN pin.

8.1.2 Soft-Start Capacitor (C_SS)

The DAC that sets the reference voltage of the error amplifier sources a current through a resistor to set the reference voltage. The reference voltage is one half of the output voltage of the switcher due to the 200 kΩ divider connected to the SNS pin. Upon start-up, the output voltage of the switcher tracks the reference voltage with a two to one ratio as the DAC current charges the capacitance connected to the reference voltage node. Internal capacitance of 20 pF is permanently attached to the reference voltage node which is also connected to the soft start pin, SS. Adding a soft-start capacitor externally increases the time it takes for the output voltage to reach its final level. The charging time required for the reference voltage can be estimated using the RC time constant of the DAC resistor and the capacitance connected to the SS pin. Three RC time constant periods are needed for the reference voltage to reach 95% of its final value. The actual start up time will vary with differences in the DAC resistance and higher-order effects.

If little or no soft-start capacitance is connected, then the start up time may be determined by the time required for the current limit current to charge the output filter capacitance. The capacitor charging equation I = CΔV/Δt can be used to estimate the start-up time in this case. For example, a part with a 3 V output, a 100 μF output capacitance and a 5A current limit threshold would require a time of 60 µs:

Equation 2.

Since it is undesirable for the power supply to start up in current limit, a soft-start capacitor must be chosen to force the LM2853 to start up in a more controlled fashion based on the charging of the soft-start capacitance. In this example, suppose a 3 ms start time is desired. Three time constants are required for charging the soft-start capacitor to 95% of the final reference voltage. So in this case RC = 1 ms. The DAC resistor, R, is 450 kΩ so C can be calculated to be 2.2 nF. A 2.2 nF ceramic capacitor can be chosen to yield approximately a 3 ms start-up time.

8.1.3 Soft-Start Capacitor (C_SS) and Fault Conditions

Various fault conditions such as short circuit and UVLO of the LM2853 activate internal circuitry designed to control the voltage on the soft-start capacitor. For example, during a short circuit current limit event, the output voltage typically falls to a low voltage. During this time, the soft-start voltage is forced to track the output so that once the short is removed, the LM2853 can restart gracefully from whatever voltage the output reached during the short circuit event. The range of soft-start capacitors is therefore restricted to values 1 nF to 50 nF.

8.1.4 Compensation

The LM2853 provides a highly integrated solution to power supply design. The compensation of the LM2853, which is type-three, is included on-chip. The benefit of integrated compensation is straight-forward, simple power supply design. Since the output filter capacitor and inductor values impact the compensation of the control loop, the range of L_O, C_O and C_ESR values is restricted in order to ensure stability.

8.1.5 Output Filter Values

Table 1 details the recommended inductor and capacitor ranges for the LM2853 that are suggested for various typical output voltages. Values slightly different than those recommended may be used, however the phase margin of the power supply may be degraded. For best performance when output voltage ripple is a concern, ESR values near the minimum of the recommended range should be paired with capacitance values near the maximum. If a minimum output voltage ripple solution from a 5 V input voltage is desired, a 6.8 μH inductor can be paired with a 220 μF (50 mΩ) capacitor without degraded phase margin.

8.1.6 Choosing an Inductance Value

The current ripple present in the output filter inductor is determined by the input voltage, output voltage, switching frequency and inductance according to Equation 3.

Equation 3.

where ΔI_L is the peak to peak current ripple, D is the duty cycle V_OUT/V_IN, V_IN is the input voltage applied to the output stage, V_OUT is the output voltage of the switcher, f is the switching frequency and L_O is the inductance of the output filter inductor. Knowing the current ripple is important for inductor selection since the peak current through the inductor is the load current plus one half the ripple current. Care must be taken to ensure the peak inductor current does not reach a level high enough to trip the current limit circuitry of the LM2853. As an example, consider a 5 V to 1.2 V conversion and a 550 kHz switching frequency. According to Table 1, a 4.7 µH inductor may be used. Calculating the expected peak-to-peak ripple,

Equation 4.

The maximum inductor current for a 3A load would therefore be 3A plus 177 mA, 3.177A. As shown in the ripple equation (Equation 4), the current ripple is inversely proportional to inductance.

8.1.7 Output Filter Inductors

Once the inductance value is chosen, the key parameter for selecting the output filter inductor is its saturation current (I_SAT) specification. Typically I_SAT is given by the manufacturer as the current at which the inductance of the coil falls to a certain percentage of the nominal inductance. The I_SAT of an inductor used in an application should be greater than the maximum expected inductor current to avoid saturation. Table 2 lists inductors that are suitable in LM2853 applications.

8.1.8 Output Filter Capacitors

The recommended capacitors that may be used in the output filter with the LM2853 are limited in value and ESR range according to Table 1.

Table 3 shows some examples of capacitors that can typically be used in a LM2853 application.

8.1.9 Split-Rail Operation

The LM2853 can be powered using two separate voltages for AVIN and PVIN. AVIN is the supply for the control logic; PVIN is the supply for the power FETs. The output filter components need to be chosen based on the value of PVIN. For PVIN levels lower than 3.3 V, use output filter component values recommended for 3.3 V. PVIN must always be equal to or less than AVIN.

Figure 9. Split-Rail Operation Example Circuit

8.1.10 Switch Node Protection

The LM2853 includes protection circuitry that monitors the voltage on the switch pin. Under certain fault conditions, switching is disabled in order to protect the switching devices. One side effect of the protection circuitry may be observed when power to the LM2853 is applied with no or light load on the output. The output will regulate to the rated voltage, but no switching may be observed. As soon as the output is loaded, the LM2853 will begin normal switching operation.