L6730 datasheet(32/52 Pages) STMICROELECTRONICS

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Device description

L6730 - L6730B

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5.14.2 No-Sink at zero current operation

The L6730 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 26. 12V -> 3.3V@0A in no-sink

Pulse train

Minimum Bootstrap Voltage

VPHASE

VBOOT

L6730 数据表(PDF) 32 Page - STMicroelectronics