1.2MHZ MICROPOWER SYNCHRONOUS STEP-UP CONVERTER

2007-4-19

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FSP3603

FUNCTIONAL BLOCK DIAGRAM

FUNCTION DESCROPTION

Operation

The FSP3603 is a synchronous step-up DC-DC converter. It utilizes internal MOSFETs to achieve high efficiency

over the full load current range. It operates at a fixed switching frequency of 1.2MHz, and uses the slope

compensated current mode architecture. The device can operate with input voltage even below 1 V and the typical

start-up voltage is 0.85V.

Synchronous Rectification

The FSP3603 integrates a synchronous rectifier to improve efficiency as well as to eliminate the external Schottky

diode. The synchronous rectifier is used to reduce the conduction loss contributed by the forward voltage of

Schottky diode. The synchronous rectifier is realized by a P-CH MOSFET with gate control circuitry that

incorporates relatively complicated timing concerns.

Low Voltage Start-Up

The FSP3603 can start up at supply voltage down to 0.85V. During start-up, the internal low voltage start-up circuitry

Current Mode PWM Control

The FSP3603 is based on a slope compensated current mode control topology. It operates at a fixed frequency of

1.2MHz. At the beginning of each clock cycle, the main switch (NMOS) is turned on and the inductor current starts

to ramp. After the maximum duty cycle or the sense current signal equals to the error amplifier(EA) output, the main

switch is turned off and the synchronous switch (PMOS) is turn on. This control topology features cycle by cycle

current limiting which can prevent the main switch from overstress and prevent external inductor from saturation.

Pulse Skipping Mode

At very light load, the FSP3603 automatically switches into Pulse Skipping Mode to improve efficiency. During this

mode, the PWM control will skip some pulses to maintain regulation. If the load increases and the output voltage

drops, the device will automatically switch back to normal PWM mode and maintain regulation.

Antiringing Control

An antiringing circuitry is included to remove the high frequency ringing that appears on the SW pin when the

inductor current goes to zero. In this case, a ringing on the SW pin is induced due to remaining energy stored in

parasitic components of switch and inductor. The antiringing circuitry clamps the voltage internally to battery voltage