TPS64200,TPS64201

TPS64202,TPS64203

SLVS485 − AUGUST 2003

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11

Soft Start

The TPS6420x has an internal soft start circuit that limits the inrush current during start up. This prevents

possible voltage drops of the input voltage in case a battery or a high impedance power source is connected

to the input of the TPS6420x. During soft start the current limit is increased from 25% of its maximum to the

maximum within about 250

µs.

100% Duty Cycle Low Dropout Operation

The TPS6420x offers the lowest possible input to output voltage difference while still maintaining regulation with

the use of the 100% duty cycle mode. In this mode the P-channel switch is constantly turned on. This is

particularly useful in battery powered applications to achieve longest operation time by taking full advantage

of the whole battery voltage range.

Enable

A voltage higher than the EN trip point of 1.3 V up to the input voltage forces the TPS64200 into shutdown. In

shutdown, the power switch, drivers, voltage reference, oscillator, and all other functions are turned off. The

supply current is reduced to less than 1

µA in shutdown. Pulling enable low starts up the TPS64200 with the

softstart as described under the chapter softstart.

Undervoltage Lockout

The undervoltage lockout circuit prevents the device from misoperation at low input voltages. Basically, it

prevents the converter from turning on the external PMOS under undefined conditions.

Current Limit

The ISENSE input is used to set the current limit for the external PMOS. The sense resistor must be connected

PMOS. The maximum current is calculated by:

I

(cur lim) +

V

(ISENSE)

R

S

(1)

the ISENSE pin is connected to the drain of the PMOS. The current in the PMOS is automatically sampled by

the TPS6420x some 10 ns after the PMOS is turned on. The ISENSE pin should always be connected to either

the source of the PMOS or the drain if an additional sense resistor is used. Otherwise there is no working

sense resistor is calculated by:

I

(cur lim) +

V

(ISENSE)

r

DS(on)

(2)

Short-Circuit Protection

With a controller only limited short circuit protection is possible because the temperature of the external

components is not supervised. In an overload condition, the current in the external diode may exceed the

maximum rating. To protect the diode against overcurrent, the off-time of the TPS6420x is increased when the

voltage at the feedback pin is lower than its nominal value. The off-time when the output is shorted (feedback

voltage is zero) is about 4

µs. This allows the current in the external diode to drop until the PMOS is turned on

again and the overcurrent protection switches off the PMOS again. The off-time is directly proportional to the

voltage at feedback.