AN938 Datasheet, PDF (16/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

Power supply, UVLO and voltage reference
Figure 21. Re-start output voltage when inhibit goes low (Css = 56 nF)
AN938
2.9
2.9.1
2.9.2
AM03503v1
Turn-off
The turn-off behavior is shown in Figure 18.
Figure 19 shows the details of the internal power stage and driver. The request for turn-off
of the power switch S occurs at Q2.
Synchronization function
The device is able to synchronize other L4973 devices (up to 6).
Moreover, this function has been implemented to allow the device to operate as a master or
slave.
As a master, it’s able to source a current of 3 mA min at 4.5 V min. As a slave, it requires a
maximum current of 0.45 mA and a minimum pulse width of about 350 ns.
Figure 20 shows the typical synchronization waveforms when the device is used as slave
and as master.
Inhibit function
The inhibit pin is active high. An internal current source ties this pin to V_CC in case of
disconnection, disabling the device.
In disabled state, Vinh = 5 V, the device quickly discharges the soft-start capacitor, also
switches off the reference voltage which limits the power consumption to a leakage value
only (at 24 V, about 100 µA).
Figure 21 shows the device behavior when the inhibit pin goes low. The soft-start capacitor
is linearly recharged, and the output voltage rises until the nominal value.
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Doc ID 5410 Rev 10