AN557 datasheet(19/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

The criterium is to define Z1, Z2 close to the resonant pole of the output LC filter. The Gm/2

ΠC1 pole must

be placed at a frequency at which open loop gain is below 0dB axis (fig. 22).

CURRENT LIMITING

A complete regulation system will be equipped with a good current limiter able to protect from load break-

ing and operator error controlls. The current limiting function is totally integrated and does not require any

external component. The output current is sensed by an internal low-value resistor, in series with the drain

of the DMOS vertical power transistor..

A precision current limitation of ± 10% relative at the peak current is guaranted. During overcurrent situa-

tion the pulse by pulse current limitation produce an output stage switching frequency reduction. The block

diagram of the current limiting is shown in fig. 23.

Figure 23. Current protection circuit.

In overcurrent situation the comparator send a signal at the flip-flop set input, an inhibit pulse is immediatly

generated from it and sended at the output stage switching off the power mos.

A reset pulse input in generated from an 40KHz internal oscillator.

After the first reset pulse the control loop will start to regulate the system an the output current will increase

following the principal oscillator frequency. If overcurrent condition is still present the current limiting will

be activate again.

This type of current limiting ensure a constant output current in overload or short circuit condition and allow

a good reliability at high frequency (500KHz) reducing the problems relative at the internal signal delay

through the protection blocks. The inductor current in overload condition in shown in fig. 24.

Figure 24. Overload inductance current.

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