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Enable and Start-Up Sequence

1.6

6

EN

COMP

V

OUT

T - Time - ms

Configure

ILIM

Threshold

Soft Start

Soft Start

REFIN

SS

V

t

6.0ms

1.5 V

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TPS40042

SLUS777 – NOVEMBER 2007

The EN pin of the TPS40042 is internally pulled to VDD. When power is applied to VDD, the EN pin is allowed to

float high, and the TPS40042 remains OFF. Only when the EN pin is externally pulled below the threshold

cycle where the short circuit current threshold is determined. The TPS40042 monitors the current out of the

COMP pin and selects a threshold based on the sensed value of the current. See Selecting the Short Circuit

Current Limit Threshold section for for details. When this calibration time is completed, the soft-start cycle is

allowed to begin. See Figure 15 below.

DESIGN

If the enable function is not used, the EN pin should be connected to ground

HINT:

(GND).

DESIGN

When designing the feedback loop compensation, ensure the capacitors used

HINT:

are not so large that they distort the COMP pin calibration waveform.

At the end of a calibration cycle, the TPS40042 slowly increases the voltage to the non-inverting input of the

error amplifier. In this way, the output voltage slowly ramps up until the voltage on the non-inverting input to the

to the error amplifier remains at the applied reference voltage.

During the soft-start interval, pulse-by-pulse current limiting is active. If seven consecutive current limit pulses are

detected, overcurrent is declared and a timeout period equivalent to seven calibration/soft-start cycles goes into

effect. See Output Short Circuit Protection section for details.

Since the rate of rise of the output voltage is constant with different REFIN voltage levels, the actual soft start

time is directly proportional to the value of the external reference voltage. The rate of rise at the non-inverting

input of the error amplifier is 0.25 V/ms. The rate of rise measured at the output terminals of the DC/DC

converter will be increased by the output voltage-to-reference voltage ratio.

For example, if a 1-V external reference is applied for a 1.5-V output DC/DC converter, the soft-start

time-to-output voltage regulation is 4 ms.

Copyright © 2007, Texas Instruments Incorporated

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Product Folder Link(s): TPS40042