AN938 datasheet(14/33 Pages) STMICROELECTRONICS

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Power supply, UVLO and voltage reference

AN938

14/33

Doc ID 5410 Rev 10

2.6

Output overvoltage protection (OVP)

The overvoltage protection uses an embedded comparator that monitors the voltage of the

FB voltage.

As the reference of the OVP comparator is 8% higher than that of the error amplifier, the

protection is triggered when the output voltage is:

Equation 7

where Ra, Rb represent the voltage divider to set the nominal output voltage.

2.7

Power stage

The embedded power element is an N-channel DMOS transistor with a Vdss in excess of

60 V and typ RDS(on) of 150 mΩ (measured at the device pins).

The low RDS(on) value of the power power element minimizes the conduction losses.

2.8

Turn-on

The gate driver circuitry of the L4973D implements solutions to minimize the switching

losses.

This is typically done by increasing the rise time of the switching node in order to reduce the

crossing losses.

The reverse recovery charge of the external diode (Qrr) which is proportional to the current

slew rate does not make it convenient to increase the di/dt in the embedded power element

over 100 A/µsec. In fact the extra current at turn-on decreases system efficiency/reliability

and increases EMI.

Figure 18.

Turn-on and turn-off (pin 2, 3)

OVP

1.08 V

()

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AM03500v1

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