AN938 Datasheet, PDF (6/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

Power supply, UVLO and voltage reference
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Power supply, UVLO and voltage reference
AN938
The device is provided with an internal stabilized power supply (of about 12 V typ.) that
powers the analog and digital control blocks and the bootstrap section.
Moreover, a safe turn-on sequence is guaranteed by an internal UVLO.
When the supply voltage reaches about 3.8 V, the band gap goes into regulation, while the
internal 5.1 V reference starts to increase from zero to its nominal value.
Figure 4. Turn-on sequence
AM03486v1
At 6.5 V of V_cc, the device begins charging the soft-start capacitor and the power stage
generates the first PWM pulses.
The output voltage increases with a slope controlled by the soft-start time. Figure 4 shows
the turn-on sequence of the signals.
From the 12 V pre-regulator, a stable 5.1 V ±2% reference voltage, externally available, is
generated, with 10 mA of current capability.
This reference is available on both devices, while the feedback reference is 5.1 V for the
L4973V5.1 and 3.3 V for the L4973V3.3.
Figure 5.
Oscillator internal circuit
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Doc ID 5410 Rev 10