Datasheet

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Enable and Start-Up Sequence

Configure ILIM Threshold

Soft Start

4 ms

1.5 ms

OUT

ENB

COMP

Soft Start

TPS40040 , TPS40041

SLUS700D – MARCH 2006 – REVISED DECEMBER 2007

The EN pin of the TPS40040/1s internally pulled to VDD. When power is applied to VDD, the EN pin is allowed

to float high, and the TPS40040/1 remains OFF. Only when the EN pin is externally pulled below the threshold

voltage of VDD - 1.2 V is the TPS40040/1 allowed to start. When enabled, the TPS40040/1 enters a calibration

cycle where the short circuit current threshold is determined. The TPS40040/1 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 19 below.

Figure 19. Startup

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 TPS40040/1 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

error amplifier reaches the internal reference voltage. At that time, the voltage at the non-inverting input to the

error amplifier remains at the 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.

Product Folder Link(s): TPS40040 TPS40041