SC4905A/B
POWER MANAGEMENT
PRELIMINARY
Application Information (Cont.)
FEED FORWARD & MAXIMUM DUTY CYCLE
The feed forward function provided by the SC4905 will
improve the dynamic performance of the converter in
response to the changes in the input supply.
In voltage mode controllers without the voltage feed for-
ward circuitry, any changes in the input supply will cause
an error in the output voltage which is sensed by the error
amplifier and eventually is translated to an adjustment in
the duty cycle by the controller. This delay in the response
will cause the slower dynamic performance of the con-
verter.
If the application does not require an upper limit on the
duty cycle, the VFF pin should be connected to the DMAX
pin. In this mode, the duty cycle will be allowed to increase
to the maximum limit of about 100%.
SYNC/Bi-Phase operation
In noise sensitive applications where synchronization of
the oscillator frequency to a reference frequency is re-
quired, the SYNC pin can accept the external clock.By
connecting an external control signal to the SYNC pin,
the Internal oscillator frequency will be synchronized to
the positive edge of the external control signal. SYNC is
a positive edge triggered input with a threshold set to
0.5*VFF.
This problem is resolved by sensing the input supply line
and making the adjustment in the duty cycle immediately
at the PWM controller.
Vin
In a single controller operation, SYNC chould be grounded
or connected to an external synchronization clock within
the SYNC frequency range (see page 3).
SC4905
U1
1
2
3
4
5
10
9
VDD
FB
REF
OUT
R14
280k
R15
499k
VIN
Shut Down
8
VFF
DMAX
RC
GND
ILIM
7
U1
U2
R16
2k
1
2
3
4
5
10
9
1
2
3
4
5
10
9
6
VDD
FB
REF
OUT
VDD
FB
REF
OUT
SYNC
Rosc1
Rosc2
8
8
VFF
DMAX
RC
GND
ILIM
VFF
DMAX
RC
GND
ILIM
R18
C27
220p, 16V
7
7
8.25k
6
6
SYNC
SYNC
Cosc1
Cosc2
SC4905
SC4905
The SC4905 uses the input supply line as the bias for the
oscillator circuitry, and the VFF pin. Any changes in the
line will cause the ramp peak voltage to be adjusted to
the VFF pin voltage while maintaining the oscillator fre-
quency unchanged.
In the Bi-Phase operation mode a very unique oscillator
is utilized to allow two SC4905 to be synchronized
together and work out of phase. This feature is setup by
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 VFF pin can also be used to shut down the SC4905
if it is pulled down to GND by an open collector circuitry.
This can be useful for overvoltage protection or other
control signals.
The SC4905 also provides a programmable duty cycle,
that can be set by an external voltage divider from the
VFF pin. The ratio of the divider will determine the pro-
grammed duty cycle allowed.
VDMAX
DutyCycle%=
VFF
2002 Semtech Corp.
9
www.semtech.com
SEMTECH [ SEMTECH CORPORATION ]
