L6730 datasheet(30/52 Pages) STMICROELECTRONICS

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Device description

L6730 - L6730B

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5.14

Bootstrap anti-discharging system

This built-in anti-discharging system keeps the voltage going across the bootstrap capacitor

from going below 3.3V. An internal comparator senses the voltage across the external

bootstrap capacitor and helps to keep it charged, eventually turning on the low-side MOSFET

for approximately 200ns. If the bootstrap capacitor is not charged up enough, the high-side

MOSFET cannot be effectively turned on and it will present a higher RDS(on) . In some cases,

the OCP can be also triggered. There are up to two conditions during which the bootstrap

capacitor can be discharged:

●

fan power supply failure, and

●

no sink at zero current operation.

5.14.1 Fan power supply failure

In many applications the fan is driven by a DC motor that uses a DC/DC converter. Often only

the speed of the motor is controlled by varying the voltage applied to the input terminal and

there is no control on the torque because the current is not directly controlled. The current has

to be limited in case of overload or short-circuit, but without stopping the motor.

With the L6730B, the current can be limited without shutting down the system because

constant current protection is provided. In order to vary the motor speed, the output voltage of

the converter must be varied. Both L6730 and L6730B have a dedicated EAREF pin (see Table

3.) which provides 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 a 100% duty

cycle setting has to be used to attain the maximum speed. In these conditions, the bootstrap

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

limit the maximum duty cycle to 80 or 90% in order to keep the bootstrap capacitor charged, but

this makes performance during the load transient worse. The “bootstrap anti-discharging

system” allows the L6730x to work at 100% without any problem.

Figure 25.: 100% Duty Cycle Operation on page 31 shows The following picture illustrates the

device behavior when the input voltage is 5V and a 100% duty cycle is set by an external

reference.

L6730 bảng dữ liệu(PDF) 30 Page - STMicroelectronics