Datasheet

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Bootstrap for N-Channel MOSFET Drive

Bootstrap Capacitor Series Resistor

External Schottky Diode for Low Input Voltage

VDD Bypass and Filtering

TPS40040 , TPS40041

SLUS700D – MARCH 2006 – REVISED DECEMBER 2007

The PWM duty cycle is limited to a maximum of 95%, allowing the bootstrap capacitor to charge during every

cycle. During each PWM OFF period, the voltage on VDD charges the bootstrap capacitor. When the PWM

switch is next commanded to turn ON, the voltage used to drive the MOSFET is derived from the voltage on this

capacitor. Since this is a charge transfer circuit, the value of the bootstrap capacitor must be sized such that the

energy stored in the capacitor on a per cycle basis is greater then the gate charge requirement of the MOSFET

being used. See the Design Example section for details.

Since resistors should not be placed in series with the high-side gate, it may be necessary to place a small 1- Ω

to 3- Ω resistor in series with the bootstrap capacitor to control the turn-on of the main switching MOSFET and

reduce the dV/dt rate of rise of the switch node voltage. A resistor placed between the BOOT pin and the

bootstrap capacitor increases the series resistance during the turn-on of the high-side MOSFET, and has no

effect during the high-side MOSFET ’ s turn-off period. This prevents the TPS40040/1 from sensing the upper

switch MOSFET ’ s turn-off too early and reducing the upper switch MOSFET turn-off to the SR MOSFET turn-on

delay timing too far.

DESIGN To reduce EMI, place a small 1- Ω to 3- Ω resistor in series with the boost

HINT: capacitor to control the turn-on of the main switching FET.

The TPS40040/1 uses an internal P-channel MOSFET switch between VDD and BOOT to charge the bootstrap

capacitor during synchronous rectifier conduction time. At low input voltages, a MOSFET can not be turned on

hard enough to rapidly replenish the charge required to turn on an (high gate charge) external high-side

MOSFET. For this situation, an external Schottky diode between the VDD and BOOT pins may be added. While

the diode carries very small average current (Q

x F

) it may be required to carry several hundred mA of peak

surge current. The diode should be rated for at least 500 mA of surge current. For higher input voltage

applications, if a resistor is used in series with the boost capacitor, connect the diode to the junction of the

resistor and capacitor to remove the added resistance from the capacitor ’ s charge path.

For low input voltages, and a high gate charge upper switch MOSFET, a small

DESIGN

Schottky diode should be placed from VDD to BOOT. Do not use a resistor in

HINT:

series with the boost capacitor.

To prevent switching noise from being injected into the TPS40040/1 control circuitry, a ceramic capacitor (1 µ F

minimum) must be placed as close to the VDD pin and GND pad as possible.

Product Folder Link(s): TPS40040 TPS40041