1.5 MHZ, 600MA SYNCHRONOUS STEP-DOWN CONVERTER

2007-1-29

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FSP3100

OPERATION

FSP3100 is a monolithic switching mode Step-Down DC-DC converter. It utilizes internal MOSFETs to achieve high

efficiency and can generate very low output voltage by using internal reference at 0.6V. It operates at a fixed

switching frequency, and uses the slope compensated current mode architecture. This Step-Down DC-DC

Converter suppliers 600mA output current at VIN=3V with input voltage range from 2.5V to 5.5V.

CURRENT MODE PWM CONTROL

Slope compensated current mode PWM control provides stable switching and cycle-by-cycle current limit for

excellent load and line responses and protection of the internal main switch (P-Ch MOSFET) and synchronous

rectifier (N-Ch MOSFET). During normal operation, the internal P-Ch MOSFET is turned on for a certain time to

ramp the inductor current at each rising edge of the internal oscillator, and switched off when the peak inductor

switch is off, the synchronous rectifier will be turned on immediately and stay on until either the inductor current

OVDET comparator controls output transient overshoots by turning the main switch off and keeping it off until the

faults is no longer present.

IDLE MODE OPERATION

At very light loads, the FSP3100 automatically enters Idle Mode. In the Idle mode, the inductor current may reach

zero or reverse on each pulse. The PWM control loop will automatically skip pulses to maintain output regulation.

discontinuous mode operation, and is normal behavior for the switching regulator.

DROPOUT OPERATION

When the input voltage decreases toward the value of the output voltage, the FSP3100 allows the main switch to

remain on for more than one switching cycle and increases the duty cycle until it reaches 100%.

The duty cycle D of a step-down converter is defined as:

OUT

ON

OSC

IN

V

D=T

f

100%

100%

V

××

≈

×

The output voltage then is the input voltage minus the voltage drop across the main switch and the inductor. At low

Caution must be exercised to ensure the heat dissipated not to exceed the maximum junction temperature of the IC.

MAXIMUM LOAD CURRENT

The FSP3100 will operate with input supply voltages as low as 2.5V, however, the maximum load current decreases

at lower input due to large IR drop on the main switch and synchronous rectifier. The slope compensation signal

reduces the peak inductor current as a function of the duty cycle to prevent sub harmonic oscillations at duty cycles

greater than 50%. Conversely the current limit increases as the duty cycle decreases.