Data Sheet

LT8350S

analog.com

Rev. A

32 of 36

stability. The inductor and output capacitor are chosen based on performance, size and cost. The compensation

resistor and capacitor on the VC pin are set to optimize control loop response and stability. For a typical

application, a 2.2nF compensation capacitor on the VC pin is adequate, and a series resistor should always be used

to increase the slew rate on the VC pin to maintain tighter output voltage regulation during fast transients on the

input supply of the converter.

Efficiency Considerations

The power efficiency of a switching regulator is equal to the output power divided by the input power times 100%.

It is useful to analyze individual losses to determine what limits the efficiency and which change would produce the

most improvement. Although all dissipative elements in circuits produce losses, four main sources account for

most of the losses in the LT8350S circuit:

1) DC I²R losses. These arise from the resistances of the MOSFETs, sensing resistor, inductor, and PC board

traces and cause the efficiency to drop at high output currents.

2) Transition loss. This loss arises from the brief amount of time switch A or switch C spends in the saturated

region during switch node transitions. It depends upon the input voltage, load current, driver strength and

MOSFET capacitance, among other factors.

3) INTVCC current. This is the sum of the MOSFET driver and control currents.

4) CIN and COUT loss. The input capacitor has the difficult job of filtering the large Root Mean Square (RMS)

input current to the regulator in buck region. The output capacitor has the difficult job of filtering the large

RMS output current in boost region. Both CIN and COUT are required to have low Equivalent series resistance

(ESR) to minimize the AC I²R loss and sufficient capacitance to prevent the RMS current from causing

additional upstream losses in fuses or batteries.