L6725_07 STMICROELECTRONICS [STMicroelectronics], L6725_07 Datasheet - Page 17

• Manufacturer Part Number: L6725_07
• Description: Voltage mode PWM controller with bootstrap anti-discharging system
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

L6725 - L6725A
5.12
5.12.1 Fan's power supply
Figure 12. 100% duty cycle operation
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 R
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:
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 L6725
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 behaviour when input voltage is 5V and 100% is set by the external
reference.
DSON
. In some cases the OCP can be also triggered. The
Device description
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