THEORY OF OPERATION
as simple as an RC circuit to Ground. If a more
complex compensation is required, then the
amplifier has enough bandwidth (45° at 4 MHz)
andenoughgain(60dB)torunTypeIIIcompen-
sation schemes with adequate gain and phase
margins at crossover frequencies greater than
50kHz.
V
B
S
T
G
H
Volta
g
e
V
S
WN
V(V
C
C)
GL
Volta
g
e
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
ensureproper0%to100%dutycyclecapability.
The voltage loop also includes two other very
important features. One is non-synchronous
startup mode. Basically, the synchronous recti-
fier cannot turn on unless the high side switch
has attempted to turn on or the SS pin has
exceeded 1.7V. This feature prevents the con-
troller from “dragging down” the output voltage
during startup or in fault modes. The second
feature is a 100% duty cycle timeout that en-
sures synchronized refreshing of the BST ca-
pacitor 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. The boost capacitor is used to
generate a high voltage drive supply for the high
side switch, which is 5V above VIN.
0
V
V(V )
IN
S
WN
Volta
g
e
-0
V
-V(Dio
d
e)V
V(V )+V(V
)
IN
C
C
B
S
T
Volta
g
e
V(V
)
C
C
TIME
Setting Output Voltages
The SP7655 can be set to different output volt-
ages. The relationship in the following formula
is based on a voltage divider from the output to
thefeedbackpinVFB, whichissettoaninternal
reference voltage of 0.80V. Standard 1% metal
film resistors of surface mount size 0603 are
recommended.
Power MOSFETs
The SP7655 contains a pair of integrated low
resistanceN-channelswitchesdesignedtodrive
up to 10 Amps of output current. Care should be
taken to de-rate the output current based on the
thermal conditions in the system such as ambi-
enttemperature,airflowandheatsinking.Maxi-
mum output current could be limited by thermal
limitations of a particular application by taking
advantage of the integrated-over-temperature
protective scheme employed in the SP7655.
The SP7655 incorporates a built-in over-tem-
peratureprotectiontopreventinternaloverheat-
ing.
Vout = 0.80V ( R1 / RSET + 1 ) =>
R1
.
Rset =
[
( Vout / 0.80V ) – 1
]
Where R1 = 68.1K1 and for VOUT = 0.80V
setting, simply remove RSET from the board. Fur-
thermore, one could select the value of the R1
and RSET combination to meet the exact output
voltagesettingbyrestrictingR1resistancerange
such that 50K1 < R1 < 100K1 for overall
system loop stability.
Rev E: 3/2ꢀ/07
SP7655 Wide Input Voltage Range 8A, 300kHz, Buck Regulator
© 2007 Sipex Corporation
6
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