WIDE INPUT 2A STEP DOWN CONVERTER
FSP3126
FUNCTIONAL BLOCK DIAGRAM
IN
ENABLE
REGULATOR
&
REFERENCE
EN
BS
CURRENT SENSE
AMPLIFIER
COMP
ERROR
AMPLIFIER
+
–
1.298V
FB
+
+
–
PWM
COMPARATOR
–
+
–
0.2O
HIGH-SIDE
POWER
SWITCH
SW
FOLDBACK
CONTROL
OSCILLATOR
&
RAMP
LOGIC
4.7O LOW-SIDE
POWER SWITCH
THERMAL
SHUTDOWN
G
FUNCTIONAL DESCRIPTION
As seen in the above Figure, Functional Block Diagram, the FSP3126 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 the 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 output. This state continues until the cycle starts again.
The High-Side Power Switch is driven by logic using BS 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 FB input and the internal 1.293V 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 2.55V.
The Oscillator normally switches at 410kHz. However, if FB voltage is less than 0.7V, then the switching frequency
decreases until it reaches a minimum of 50kHz at V
FB
= 0.5V.
Shutdown control
The FSP3126 has an enable input EN for turning the IC on or off. When EN is less than 0.7V, the IC is in 8µA low
current shutdown mode and 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 unconnected
for always-on operation. Note that EN is a low voltage input with a maximum voltage of 6V; it should never be directly
connected to IN.
Thermal Shutdown
The FSP3126 automatically turns off when its junction temperature exceeds 160°C.
3/9
2007-5-28
FOSLINK [ FOSLINK SEMICONDUCTOR CO.,LTD ]
