WIDE INPUT 3A STEP-DOWN CONVERTER
FSP3128
FUNCTIONAL BLOCK DIAGRAM
FUNCTIONAL DESCROPTION
As seen in the Functional Block Diagram, the FSP3128 is a current mode pulse width modulation (PWM) converter.
The converter operates as follows :
A switching cycle starts when the rising edge of the Oscillator clock output causes the High-Side Power Switch to turn
on and the Low-Side Power Switch to turn off. With the SW side of the inductor now connected to IN, the inductor
current ramps up to store energy in its magnetic field. The inductor current level is measured by the Current Sense
Amplifier and added to the Oscillator ramp signal. If the resulting summation is higher than the COMP voltage, the
output of the PWM Comparator goes high. When this happens or when Oscillator clock output goes low, the
High-Side Power Switch turns off and the Low-Side Power Switch turns on. At this point, the SW side of the inductor
swings to a diode voltage below ground, causing the inductor current to decrease and magnetic energy to be
transferred to the output. This state continues until the cycle starts again.
The High-Side Power Switch is driven by logic using the BS bootstrap pin as the positive rail. This pin is charged to
V
SW
+ 6V when the Low-Side Power Switch turns on.
The COMP voltage is the integration of the error between the FB input and the internal 1.222V reference. If FB is
lower than the reference voltage, COMP tends to go higher to increase current to the output. Current limit happens
when COMP reaches its maximum clamp value of 2.65V.
The Oscillator normally switches at 400kHz. However, if the FB voltage is less than 0.7V, then the switching frequency
decreases until it reaches a typical value of 40kHz at V
FB
= 0V.
Shutdown Control
The FSP3128 has an enable input EN for turning the IC on or off. When EN is less than 0.7V, the IC is in 6µA low
current shutdown mode and the output is discharged through the Low-Side Power Switch. When EN is higher than
1.3V, the IC is in normal operation mode. EN is internally pulled up with a 2µA current source and can be left uncon-
nected for always-on operation.
Thermal Shutdown
The FSP3128 automatically turns off when its junction temperature exceeds 160°C and then restarts once the
temperature falls to 150°C.
3/10
2007-1-4
FOSLINK [ FOSLINK SEMICONDUCTOR CO.,LTD ]
