AN938 datasheet(16/33 Pages) STMICROELECTRONICS

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Power supply, UVLO and voltage reference

AN938

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Doc ID 5410 Rev 10

Figure 21.

Re-start output voltage when inhibit goes low (Css = 56 nF)

2.9

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.

2.9.1

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.

2.9.2

Inhibit function

The inhibit pin is active high. An internal current source ties this pin to VCC 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.

AM03503v1

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