TPS40056

SLVS612 − APRIL 2006

11

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APPLICATION INFORMATION

PROGRAMMING SOFT START

TPS40056 uses a closed-loop approach to ensure a controlled ramp on the output during start-up. Soft-start

rises above the external reference voltage, regulation is based on the external reference. To ensure a controlled

in equation (10).

t

L

C

O

(seconds)

the higher the input current required during start-up. This relationship is describe in more detail in the section

equation (11).

necessary to increase the soft-start time to between approximately 2 ms and 5 ms to prevent nuisance UVLO

C

2.3

mA

0.7 V

t

START

(Farads)

PROGRAMMING CURRENT LIMIT

The TPS40056 uses a two-tier approach for overcurrent protection. The first tier is a pulse-by-pulse protection

scheme. Current limit is implemented on the high-side MOSFET by sensing the voltage drop across the

MOSFET when the gate is driven high. The MOSFET voltage is compared to the voltage dropped across a

resistor connected from VIN pin to the ILIM pin when driven by a constant current sink. If the voltage drop across

the MOSFET exceeds the voltage drop across the ILIM resistor, the switching pulse is immediately terminated.

The MOSFET remains off until the next switching cycle is initiated.

The second tier consists of a fault counter. The fault counter is incremented on an overcurrent pulse and

decremented on a clock cycle without an overcurrent pulse. When the counter reaches seven (7) a restart is

issued and seven soft-start cycles are initiated. Both the upper and lower MOSFETs are turned off during this

period. The counter is decremented on each soft-start cycle. When the counter is decremented to zero, the

PWM is re-enabled. If the fault has been removed the output starts up normally. If the output is still present the

counter counts seven overcurrent pulses and re-enters the second-tier fault mode. See Figure 3 for typical

overcurrent protection waveforms.

I

C

O

V

O

t

START

) I

L

(Amperes)

(10)

(11)

(12)