LT8331 datasheet(15/34 Pages) AD

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LT8357

Rev. 0

For more information www.analog.com

Power GOOD (PGOOD) Pin

The LT8357 provides an open-drain status pin, PGOOD,

which is pulled high when the FB pin voltage is within ±8%

(typical) of the 1V regulation voltage. The PGOOD pin is

pulled up by an external resistor to INTVCC or an external

voltage source up to 6V.

Programming the SENSE Pin

For control and protection, the LT8357 measures the

power MOSFET current by using a sense resistor (RSENSE)

from the MOSFET source to GND. Figure 6 shows a typical

waveform of the sense voltage (VSENSE) across the sense

resistor. It is important to use Kelvin traces between the

SENSE pin and RSENSE, and place the IC GND as close

as possible to the GND terminal of RSENSE for proper

operation.

8357 F06

VSENSE(PEAK)

ΔVSENSE = χ • VSENSE(MAX)

VSENSE

DTS

VSENSE(MAX)

Figure 6. The Sense Voltage Waveform

Due to the current limit function of the SENSE pin, RSENSE

should be selected to guarantee that the peak switch cur-

rent sense voltage VSENSE(PEAK) in the steady state of

normal operation does not exceed the SENSE maximum

current threshold (60mV typical). Then, the maximum

switch current ripple percentage can be calculated using

the following equation:

χ =

ΔVSENSE

60mV−0.5•ΔVSENSE

χ will be used in the subsequent design examples to cal-

culate inductor value. ΔVSENSE is the ripple voltage across

RSENSE. The final RSENSE value should be lower than the

calculated RSENSE. A 20% to 30% margin is usually recom-

mended. Always choose a low ESL current sense resistor.

APPLICATIONS INFORMATION

The LT8357 uses a time interval close to the minimum

on-time to blank the ringing on the switch current sense

signal immediately after the power MOSFET is turned on.

This ringing is caused by the parasitic inductance and

capacitance of the PCB trace, the sense resistor, the diode,

and the MOSFET. The blanking time interval increases

along with the minimum on-time as the switching fre-

quency decreases. In applications that have very large and

long ringing on the current sense signal, a small RC filter

can be added to filter out the excess ringing.

Soft-Start

As explained in the Operation section, the SS pin can be

used to program the output voltage soft-start by con-

necting an external capacitor from the SS pin to GND.

The internal 15μA (typical) 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:

TSS =1V•

CSS

15µA

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 switch

over current fault is detected, the LT8357 enters a low

duty cycle auto-retry hiccup mode. In this scenario, the

switching is disabled immediately and a 1.5μA (typical)

pull-down current is activated to discharge CSS. Once the

SS pin voltage is discharged below 0.2V, the 15μA pull-up

current charges up the SS pin again. If the switch over

current fault condition has not been removed, then a new

hiccup cycle will be initiated. This will continue until the

fault is removed. Once the switch over current condition

is removed, the output will have a smooth recovery due

to soft-start.

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