LT8253/LT8253A

17

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Soft-Start and Short-Circuit Protection

As shown in Figure 6 and explained in the Operation sec-

tion, the SS pin can be used to program the output volt-

age soft-start by connecting an external capacitor from

the SS pin to ground. The internal 12.5µA pull-up current

charges up the capacitor, creating a voltage ramp on the

SS pin. As the SS pin voltage rises linearly from 0.25V

to 1V (and beyond), the output voltage rises smoothly

into its final voltage regulation. The soft-start time can

be calculated as:

12.5µA

trolled output voltage soft-start.

The SS pin is also used as a fault timer. Once an output

short-circuit fault is detected, a 1.25µA pull-down current

source is activated. Using a single resistor from the SS

three different fault protection modes: hiccup (no resis-

tor), latch-off (499k), and keep-running (100k).

With a 100k resistor in keep-running mode, the LT8253/

LT8253A continue switching normally and regulates the

current into ground. With a 499k resistor in latch-off

mode, the LT8253/LT8253A stop switching until the EN/

UVLO pin is pulled low and high to restart. With no resis-

tor in hiccup mode, the LT8253/LT8253A enter 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 output short-circuit

condition has not been removed when the SS pin reaches

1.75V, the 1.25µA pull-down current turns on again, ini-

tiating 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 LT8253/LT8253A use an internal transconductance

control 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

loop response and stability.

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 LT8253/

LT8253A circuits:

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.

and control currents.

cult 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

capacitance to prevent the RMS current from causing

additional upstream losses in fuses or batteries.