OUT

SS

SLEW

SLEW

V

t

C

I

=

´

TPS51363

www.ti.com

SLUSBB5A – FEBRUARY 2013 – REVISED JUNE 2013

Power Sequences: Soft-Start and Power Good

Prior to asserting EN high, the power stage conversion voltage and V5IN voltage should be up and running.

When EN is asserted high, TPS51363 provides soft start operation to suppress in-rush current during startup.

The soft start action is achieved by an internal SLEW current of 10 µA (typ) sourcing into a small external MLCC

capacitor connected from SLEW pin to GND.

Use Equation 4 to determine the soft-start timing.

where

•

•

•

(4)

The TPS51363 includes a PGOOD open drain output. During the startup, once the output voltage is slewing up

within -8% of the final setpoint target, the PGOOD becomes asserted after 1.5 ms of delay from the end of the

soft-start period. During the operation, if the output voltage rises beyond 120% (typ) of the setpoint, the PGOOD

pin becomes immediately de-asserted without hysteresis. Re-asserting the PGOOD pin requires either resetting

either the V5IN pin or the EN pin. If the output voltage falls below 68% (typ) of the setpoint, the PGOOD pin

becomes immediately de-asserted without hysteresis. Re-asserting the PGOOD pin requires resetting either the

V5IN pin or the EN pin.

Fault Protection

Overcurrent Limit

TPS51363 integrates both high side and low side FETs to support a maximum DC current of 8-A or 10-A

operation. The current sensing method employed for over current limit is to monitor the SW node during the “ON”

state of the low side FET for each switching cycle. TRIP pin is used to program one of the two current limits for

TPS51363 operation (see Table 4). When the overcurrent limit is detected, the converter does not allow the next

“ON” cycle for the high side FET until the overcurrent limit is no longer reached. This ensures the safe operation

of the converter. And when the overcurrent limit condition persists, the current to the load exceeds the current to

the output capacitors, the output voltage tends to fall. When the output voltage falls below the undervoltage

protection threshold, the converter latch shut down.

Table 4. TRIP Pin Settings

TYPICAL OCL

TRIP

GND

8

5V

12

Negative Overcurrent Limit

TPS51363 is also protected by the negative over current limit. Both positive and negative current limit is

programmed by the TRIP pin. Negative current limit level is the same as that of positive current limit level (see

Table 4). During the normal converter operation, negative current is not allowed due to the mandatory light load

operation for this device (Auto Skip). It is during the OOB or OVP operation, negative overcurrent might be

engaged.

Copyright © 2013, Texas Instruments Incorporated

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