LT8350RVPBF datasheet(21/24 Pages) AD

21 / 24 page

LT8350

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

the LT8350 enters low duty cycle auto-retry operation.

The 1.25µA pull-down current discharges the SS pin to

0.2V and then 12.5µA pull-up current charges the SS pin

up. If the short-circuit condition has not been removed

when the SS pin reaches 1.75V, the 1.25µA pull-down

current turns on again, initiating a new hiccup cycle. This

will continue until the fault is removed. Once the output

short-circuit condition is removed, the output will have a

smooth short-circuit recovery due to soft-start.

Loop Compensation

The LT8350 uses an internal transconductance error

amplifier, the output of which, VC, compensates the con-

trol loop. The external inductor, output capacitor, and the

compensation resistor and capacitor determine the loop

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 volt-

age 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 often useful to analyze individual losses to determine

what is limiting 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 LT8350 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 dif-

ficult job of filtering the large 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 ESR to minimize the AC I2R loss and sufficient

capacitance to prevent the RMS current from causing

additional upstream losses in fuses or batteries.

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