LT8350RVPBF datasheet(20/24 Pages) AD

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LT8350

Rev. 0

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from all internal loads, and turn off LOADTG. The LOADEN

pin should not be open (tie to INTVCC or VREF if not used).

High Side PMOS Load Switch Selection

A high side PMOS load switch is recommended in some

LT8350 applications requiring load switch control. The

high side PMOS load switch is typically selected for drain-

source voltage VDS, gate-source threshold voltage VGS(TH),

and continuous drain current ID. For proper operations,

VDS rating should exceed the maximum output voltage

set by the FB pin, the absolute value of VGS(TH) should

be less than 3V, and ID rating should be above IOUT(MAX).

Programming Output Voltage and Thresholds

The LT8350 has a voltage feedback pin FB that can be

used to program a constant-voltage output. The output

voltage can be set by selecting the values of R3 and R4

(Figure 13) according to the following equation:

VOUT = 1V •

R3 + R4

VOUT

8350 F13

GND

LT8350

Figure 13. Feedback Resistor Connection

In addition, the FB pin also sets output overvoltage thresh-

old, output power good thresholds, and output short

threshold. For an application with small output capacitors,

the output voltage may exhibit considerable overshoots

during load transient event. Once the FB pin hits its over-

voltage threshold 1.1V, the LT8350 stops switching by

turning off all four power switches, and also turns off

LOADTG to disconnect the output load for protection. The

output overvoltage threshold can be set as:

VOUT(OVP) = 1.1V •

R3 + R4

APPLICATIONS INFORMATION

To provide the output short-circuit detection and protec-

tion, the output short threshold can be set as:

VOUT(SHORT) = 0.25V •

R3 + R4

Power Good (PGOOD) Pin

The LT8350 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 8 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 transi-

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

can be calculated as:

tss = 1V •

Css

12.5µA

Make sure the CSS is at least five to ten times larger than

the compensation capacitor on the VC pin for a well-con-

trolled 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-cir-

cuit 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 LT8350 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 LT8350

continues switching normally and regulates the current

into ground. With a 499kΩ resistor in latch-off mode, the

LT8350 stops switching until the EN/UVLO pin is pulled

low and high to restart. With no resistor in hiccup mode,

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