L6730C datasheet(28/50 Pages) STMICROELECTRONICS

28 / 50 page

Device description

L6730C - L6730D

28/50

5.14

Bootstrap anti-discharging system

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

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

keeping it charged, eventually turning-on the low-side MOSFET for approximately 200ns. If the

bootstrap capacitor is not enough charged 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. It’s

possible to mention at least two application conditions during which the bootstrap capacitor can

be discharged:

5.14.1 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.

Obviously the current has to be limited in case of overload or short-circuit but without stopping

the MOTOR. With the L6730D the current can be limited without shutting down the system

because a constant-current-protection is provided. In order to vary the MOTOR speed the

output voltage of the converter must be varied. Both L6730C and L6730D have 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 L6730X can work at 100% without any problem. The following picture

shows the device behaviour when input voltage is 5V and 100% is set by the external

reference.

Figure 23. 100% duty cycle operation

Vout=5V

Vin=5V

LGate

Fsw?6.3KHz

TOFF

≈ 200ns

Vout=5V

Vin=5V

LGate

Fsw?6.3KHz

TOFF

≈ 200ns

VOUT = 5V

VIN = 5V

LGate

FSW

≈ 6.3KHz

L6730C 数据表(PDF) 28 Page - STMicroelectronics

L6730C 数据表(HTML) 28 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

数据表下载

链接网址

L6730C 数据表(PDF) 28 Page - STMicroelectronics

L6730C 数据表(HTML) 28 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

数据表 下载

链接网址

数据表下载