1.5 MHZ, 600MA SYNCHRONOUS STEP-DOWN CONVERTER
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
current is above the error voltage. The current comparator, ICOMP, limits the peak inductor current. When the main
switch is off, the synchronous rectifier will be turned on immediately and stay on until either the inductor current
starts to reverse, as indicated by the current reversal comparator, IZERO, or the beginning of the next clock cycle. The
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.
The bottom MOSFET is turned off by the current reversal comparator, IZERO, and the switch voltage will ring. This is
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:
V
D=TON ×fOSC ×100% ≈ OUT ×100%
V
IN
Where TON is the main switch on time and fOSC is the oscillator frequency (1.5MHz).
The output voltage then is the input voltage minus the voltage drop across the main switch and the inductor. At low
input supply voltage, the RDS(ON) of the P-Channel MOSFET increase, and the efficiency of the converter decreases.
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.
8/11
2007-1-29
FOSLINK [ FOSLINK SEMICONDUCTOR CO.,LTD ]
