L6730C datasheet(29/50 Pages) STMICROELECTRONICS

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L6730C - L6730D

Device description

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5.14.2 No-sink at zero current operation

The L6730C can work in no-sink mode. If output current is zero the converter skip some pulses

and works with a lower switching frequency. Between two pulses can pass a relatively long time

(say 200-300µs) during which there’s no switching activity and the current into the inductor is

zero. In this condition the phase node is at the output voltage and in some cases this is not

enough to keep the bootstrap cap charged. For example, if Vout is 3.3V the voltage across the

bootstrap cap is only 1.7V. The high-side MOSFET cannot be effectively turned-on and the

regulation can be lost. Thanks to the “bootstrap anti-discharging system” the bootstrap cap is

always kept charged. The following picture shows the behaviour of the device in the following

conditions: 12V

◊3.3V@0A.

It can be observed that between two pulses trains the low-side is turned-on in order to keep the

bootstrap cap charged.

Figure 24. 12V -> 3.3V@0A in no-sink

Pulse train

Minimum Bootstrap Voltage

VPHASE

L6730C 数据表(PDF) 29 Page - STMicroelectronics