SC4808B-2_09 Datasheet, PDF (12/28 Pages) Semtech Corporation – High Performance Dual Ended PWM Controller

SC4808B-2
POWER MANAGEMENT
Application Information (Cont.)
SYNC/Bi-Phase operation
FEED BACK
In noise sensitive applications where synchronization of
the oscillator frequency to a reference frequency is required,
the SYNC pin can accept the external clock. By connecting
an external control signal to the SYNC pin, the internal os-
cillator frequency will be synchronized to the positive edge
of the external control signal. SYNC is a positive edge trig-
gered input with a threshold set to 1.0V (SC4808B-2).
In a single controller operation, SYNC should be grounded
or connected to an external synchronization clock within
the SYNC frequency range (see page 3).
The error signal from the output of an external error ampli-
fier such as SC431 or SC4431 is applied to the inverting
input of the PWM comparator at the FB pin either directly
or via an opto coupler for the isolated applications. For best
stability, keep the FB trace length as short as possible.
Vref
R37
2.2k
FB _C40
22pF
6
3
5
4
MOCD207
R34
5
C38
C39
0.1u 22n
Vout
Vout
C35
R32
C36 R35
1
4
R36
C37
SC4431
R38
Vref
2
REF
Rosc1
Cosc1
U1
5 _REF
4 _FB
GND ⁶
OUTB ⁷
3 _CS
2
RC
1 _SYNC
OUTA ⁸
9
VCC
LUVLO ¹⁰
SC4808
REF
Rosc2
Cosc2
U2
5 _REF
4 _FB
GND ⁶
OUTB ⁷
3 _CS
OUTA ⁸
2
RC
9
VCC
1 _{SYNC LUVLO} 10
SC4808
In the Bi-phase operation mode a very unique oscillator is
utilized to allow two SC4808B-2’s to be synchronized
together and work out of phase. This feature is set up by a
simple connection of the SYNC input to the RC pin of the
other part. The fastest oscillator automatically becomes
the master, forcing the two PWMs to operate out of phase.
This feature minimizes the input and output ripples, and
reduces stress on the capacitors.
The signal at the FB pin is then compared to the 3X ampli-
fied signal from the current sense/ slope compensation
CS pin. Matched out of phase signals are generated to
control the OUTA and OUTB gate drives of the two phases.
A single ramp signal is used to generate the control sig-
nals for both phases, hence achieving a tightly matched
per phase operation.
Voltages below 1.5V at the FB pin, will produce a 0% duty
cycle at the OUTA/OUTB gate drives. This offset is to pro-
vide enough head room for the opto coupler used in iso-
lated applications.
GATE DRIVERS
OUTA and OUTB are out of phase bipolar gate drive output
stages, that are supplied from VCC and provide a peak
source/sink current of about 100mA. Both stages are ca-
pable of driving the logic input of external MOSFET drivers
or a NPN/PNP transistor buffer. The output stages switch
at half the oscillator frequency. When the voltage on the
RC pin is rising, one of the two outputs is high, but during
fall time, both outputs are off. This “dead time” between
the two outputs, along with a slower output rise and fall
time, insures that the two outputs can not be on at the
same time. The dead time is programmable and depends
upon the timing capacitor.
© 2009 Semtech Corp.
12
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