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Date: 08/12/04
SP7652 Wide Input Voltage Range 6A, 600kHz, Buck Regulator Copyright 2004 Sipex Corporation
THEORY OF OPERATION
control the COMP pin externally. If a simple,
single pole, single zero response is desired, then
compensation can be as simple as an RC to
ground. If a more complex compensation is
required, then the amplifier has enough band-
width (45
°
at 4 MHz) and enough gain (60dB) to
run Type III compensation schemes with ad-
equate gain and phase margins at cross over
frequencies greater than 50kHz.
The common mode output of the error amplifier
is 0.9V to 2.2V. Therefore, the PWM voltage
ramp has been set between 1.1V and 2.2V to
ensure proper 0% to 100% duty cycle capability.
The voltage loop also includes two other very
important features. One is an asynchronous start
up mode. Basically, the synchronous rectifier
can not turn on unless the high side NFET has
attempted to turn on or the SS pin has exceeded
1.7V. This feature prevents the controller from
“dragging down” the output voltage during
startup or in fault modes. The second feature is
a 100% duty cycle timeout that ensures synchro-
nized refreshing of the BST capacitor at very
high duty ratios. In the event that the high side
NFET is on for 20 continuous clock cycles, a
reset is given to the PWM flip flop half way
through the 21st cycle. This forces GL to rise for
the cycle, in turn refreshing the BST capacitor.
Power MOSFETs
The SP7652 contains a pair of integrated low
resistance N MOSFETs designed to drive up to
6A of output current. Maximum output current
could be limited by thermal limitations of a
particular application. The SP7652 incorpo-
rates a built-in over-temperature protection to
prevent internal overheating.
GH
Voltage
GL
Voltage
V(VIN)
0V
-0V
-V(Diode) V
V(VIN)+V(VCC)
BST
Voltage
V(VCC)
TIME
SWN
Voltage
VBST
VSWN
V(VCC)
The SP7652 can be set to different output
voltages. The relationship in the following
formula is based on a voltage divider from the
output to the feedback pin VFB, which is set
to an internal reference voltage of 0.80V.
Standard 1% metal film resistors of surface
mount size 0603 are recommended.
Vout = 0.80V ( R1 / R2 + 1 ) => R2 = R1 / [ (
Vout / 0.80V ) – 1 ]
Where R1 = 68.1K
and for Vout = 0.80V
setting, simply remove R2 from the board.
Furthermore, one could select the value of R1
and R2 combination to meet the exact output
voltage setting by restricting R1 resistance
range such that 50K
< R1 < 100K
for
overall system loop stability.
Setting Output Voltages