(

)

(

)

(

)

-

´

=

+

=

+

´

´ ´

´

´

IND(ripple)

IN

OUT

OUT

TRIP

TRIP

OCP

SW

IN

DS(on)

DS(on)

I

V

V

V

V

V

1

I

2

2 L f

V

32 R

32 R

(

)

( )

( )

=

W ´

m

TRIP

TRIP

TRIP

V

mV

R

k

I

A

TPS53353

SLUSAK2B – AUGUST 2011 – REVISED FEBRUARY 2015

www.ti.com

7.3.5 Power-Good

The TPS53353 has power-good output that indicates high when switcher output is within the target. The power-

good function is activated after soft-start has finished. If the output voltage becomes within +10% and –5% of the

target value, internal comparators detect power-good state and the power-good signal becomes high after a 1-

ms internal delay. If the output voltage goes outside of +15% or –10% of the target value, the power-good signal

becomes low after two microsecond (2-

μs) internal delay. The power-good output is an open drain output and

must be pulled up externally.

good logic. It is recommended to pull PGOOD up to VREG (or a voltage divided from VREG) so that the power-

good logic is still valid even without VDD supply.

7.3.6 Current Sense, Overcurrent and Short Circuit Protection

TPS53353 has cycle-by-cycle overcurrent limiting control. The inductor current is monitored during the OFF state

and the controller maintains the OFF state during the period in that the inductor current is larger than the

overcurrent trip level. In order to provide both good accuracy and cost effective solution, TPS53353 supports

(1)

The inductor current is monitored by the LL pin. The GND pin is used as the positive current sensing node and

(2)

In an overcurrent or short circuit condition, the current to the load exceeds the current to the output capacitor

thus the output voltage tends to decrease. Eventually, it crosses the undervoltage protection threshold and shuts

down. After a hiccup delay (16 ms with 0.7 ms sort-start), the controller restarts. If the overcurrent condition

remains, the procedure is repeated and the device enters hiccup mode.

Hiccup time calculation:

where

•

N = 8, 9, 10, or 11 depending on soft start time selection

(3)

(4)

Table 2. Hiccup Time Calculation

SELECTED SOFT-START TIME

n

0.7

8

2.052

14.364

1.4

9

3.076

21.532

2.8

10

5.124

35.868

5.6

11

9.22

64.54

7.3.7 Overvoltage and Undervoltage Protection

TPS53353 monitors a resistor divided feedback voltage to detect over and under voltage. When the feedback

voltage becomes lower than 70% of the target voltage, the UVP comparator output goes high and an internal

UVP delay counter begins counting. After 1ms, TPS53353 latches OFF both high-side and low-side MOSFETs

drivers. The controller restarts after a hiccup delay (16 ms with 0.7 ms soft-start). This function is enabled 1.5-ms

after the soft-start is completed.

16

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