AN938 Datasheet, PDF (14/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

Power supply, UVLO and voltage reference
AN938
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
V_OVP
=
1.08
⋅
V_f
B
⋅
(_---R_-----a_----+-----R_-----b_----)
Rb
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 R_DS(on) of 150 mΩ (measured at the device pins).
The low R_DS(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)
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Doc ID 5410 Rev 10
AM03500v1