L6732

Device description

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5.12

Bootstrap anti-discharging system

This built-in system avoids that the voltage across the bootstrap capacitor becomes less

than 3.3 V. An internal comparator senses the voltage across the external bootstrap

capacitor keeping it charged, eventually turning-on the low-side MOSFET for approximately

200 ns. If the bootstrap capacitor is not enough charged the high-side MOSFET cannot be

also triggered. The bootstrap capacitor can be discharged during the soft-start in case of

very long soft-start time and light loads. It's also possible to mention one application

condition during which the bootstrap capacitor can be discharged:

5.12.1

Fan's power supply

In many applications the FAN is a DC MOTOR driven by a voltage-mode DC/DC converter.

Often only the speed of the MOTOR is controlled by varying the voltage applied to the input

terminal and there's no control on the torque because the current is not directly controlled.

In order to vary the MOTOR speed the output voltage of the converter must be varied. The

L6732 has a dedicated pin called EAREF (see the related section) that allows providing an

external reference to the non-inverting input of the error-amplifier.

In these applications the duty cycle depends on the MOTOR's speed and sometimes 100 %

has to be set in order to go at the maximum speed. Unfortunately in these conditions the

bootstrap capacitor can not be recharged and the system cannot work properly. Some PWM

controller limits the maximum duty-cycle to 80-90 % in order to keep the bootstrap cap

charged but this make worse the performance during the load transient. Thanks to the

“bootstrap anti-discharging system” the L6732 can work at 100 % without any problem. The

following picture shows the device behavior when input voltage is 5 V and 100 % is set by

the external reference.

Figure 16.

100 % duty cycle operation