Data Sheet

LT8350S

analog.com

Rev. A

31 of 36

In addition, the FB pin also sets the output overvoltage threshold, output power good thresholds, and output short

threshold. For an application with small output capacitors, the output voltage may exhibit considerable overshoots

during a load transient event. Once the FB pin hits its overvoltage threshold of 1.1V, the LT8350S stops switching by

turning off all four power switches and turns off LOADTG to disconnect the output load for protection.

The output overvoltage threshold can be set as follows:

VOUT(OVP) = 1.1V x

R3 + R4

R4

To provide the output short-circuit detection and protection, when the FB pin hits VFB < 0.25V, the output short

threshold can be set as follows:

VOUT(SHORT) = 0.25V x

R3 + R4

R4

Power Good (PGOOD) Pin

The LT8350S provides an open-drain status pin, PGOOD, which is pulled low when VFB is within ±10% of the 1V

regulation voltage. The PGOOD pin is allowed to be pulled up by an external resistor to INTVCC or an external

voltage source of up to 5V.

Soft-Start and Fault Protection

As shown in Figure 35 and explained in the Operation section, the SS pin can be used to program 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 and transitions into its final voltage regulation. The soft-start time can be

calculated as:

tSS = 1V x

CSS

12.5µA

Ensure the CSS is at least five to ten times larger than the compensation capacitor on the VC pin for a well-controlled

output voltage soft-start. A 22nF ceramic capacitor is a good starting point.

The SS pin is also used as a fault timer. Once a short-circuit fault is detected, a 1.25µA pull-down current source is

activated. Using a single resistor from the SS pin to the VREF pin, the LT8350S can be set to three different fault

protection modes: hiccup (no resistor), latch-off (499kΩ), and keep-running (100kΩ).

With a 100kΩ resistor in keep-running mode, the LT8350S continues switching normally and regulates the current

into ground. With a 499kΩ resistor in latch-off mode, the LT8350S stops switching until the EN/UVLO pin is pulled

low and high to restart. With no resistor in hiccup mode, the LT8350S enters low-duty cycle auto-retry operation.

The 1.25µA pull-down current discharges the SS pin to 0.2V, and then the 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 LT8350S uses an internal transconductance error amplifier, the output of which, VC, compensates the control

loop. The external inductor, output capacitor, and compensation resistor and capacitor determine the loop