TPS40020

TPS40021

SLUS535C − MARCH 2003 − REVISED SEPTEMBER 2004

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APPLICATION INFORMATION

Bootstrap/Charge Pump

The TPS4002X series includes a charge pump to boost the drive voltage to the power MOSFET’s to higher

levels when the input supply is low. A capacitor connected from PVDD to PGND is the storage cap for the pump.

A capacitor connected from SW to BOOT2 gets charged every switching cycle while LDRV is high and its charge

is dumped on the PVDD capacitor when HDRV goes high. An internal switch disables the charge pump when

the voltage on PVDD reaches approximately 4.8 V and enables pumping when PVDD falls to approximately

4.6 V. The high-side driver uses the capacitor from SW to BOOT1 as its power supply. When SW is low, this

capacitor charges from the PVDD capacitor. When the SW pin goes high, this capacitor provides above-rail

drive for the high-side N-channel FET.

PVDD, BOOT1 and BOOT2 are pre-charged to the VDD voltage during a shutdown condition. For low-input

voltage converters, utilizing higher gate threshold voltage MOSFETs, it may be necessary to add an Schottky

diode from VDD (anode) to BOOT1 to guarantee sufficient voltage for initial start up. Once switching starts the

charge pump reverses bias on the Schottky diode.

When operating the TPS40020 under no load or extremely light-load conditions the controller will be operating

in discontinuous Mode (DCM); reverse current is prevented from flowing in the synchronous rectifier. In DCM

the on times for both the HDRV and LDRV pulses can become too narrow to provide adequate charging of

PVDD and BOOT1 outputs, causing their voltages to collapse. Insufficient PVDD and BOOT1 voltages prevent

the external MOSFETS from becomming fully enhanced, causing loss of converter output regulation. Schottky

diodes from VIN (anode) to PVDD, and VIN (anode) to BOOT1, as well as a pre-load can be added to maintain

PVDD and BOOT1 at voltage levels sufficient enough to fully enhance the external MOSFETs. The amount of

pre-load typically ranges from 50 mA to 100 mA depending on operating conditions and external MOSFET

selection.

Drivers

The HDRV and LDRV MOSFET drivers are capable of driving gate-to-source voltages up to 5.0 V. Using

appropriate MOSFETs, a 25-A converter can be achieved. The LDRV driver switches between VDD and

ground, while the HDRV driver is referenced to SW and switches between BOOT1 and SW. The maximum

voltage between BOOT1 and SW is 5.0 V when PVDD is in regulation.