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SC4905A Datasheet(PDF) 5 Page - Semtech Corporation

Part No. SC4905A
Description  HIGH PERFORMANCE VOLTAGE MODE PWM CONTROLLER
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Maker  SEMTECH [Semtech Corporation]
Homepage  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC4905A Datasheet(HTML) 5 Page - Semtech Corporation

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SC4905A/B
POWER MANAGEMENT
PRELIMINARY
 2002 Semtech Corp.
www.semtech.com
5
Pin Descriptions
VDD: The supply input for the device. Once VDD has ex-
ceeded the UVLO limit, the internal reference, oscillator,
drivers and logic are powered up. This pin should be by-
passed with a low ESR capacitance right at the IC pin to
minimize noise problems, and to ensure proper opera-
tion.
FB: Input to the PWM comparator with an offset voltage
of 700mV. The feedback analog signal from the output
of an error amplifier or an Optoisolator will be connected
to this pin to provide regulation.
VFF: The VFF pin provides the controller with a voltage
proportional to the power supply input voltage to achieve
feed-forward function. RM plus RB in conjunction with RT
will set the Vff level (see page 1 circuit).
()
()
M
B
T
M
B
R
R
R
R
R
VFF
+
+
+
=
DMAX: Programmable duty cycle is achieved via resis-
tive divider from the VFF. The duty cycle percentage is
set by the ratio of the divider RM, and RB (see page 1
circuit) from the VFF signal. When RM is shorted, maxi-
mum duty cycle of 100% is achieved. RM plus RB in con-
junction with RT will also be used as the divider to set the
Vff level.
VFF
VDMAX
%
DutyCycle =
RC: The oscillator programming pin. The oscillator should
be referenced to Vin to achieve the line feed forward func-
tion. Only two components are required to program the
oscillator, a resistor R
OSC (tied to the Vin and RC), and a
capacitor C
OSC (tied to the RC and GND). Since the peak
oscillator voltage is VFF, constant frequency operation is
maintained over the full power supply. When the DMAX
pin is shorted to the VFF pin, the oscillator can run at the
largest duty cycle possible.
Following formula can be used for a close approximation
of the Oscillator Frequency.
()
05
.
1
VFF
C
R
2
VFF
Vin
F
OSC
OSC
OSC
Where VFF is the voltage at the VFF pin at a given Vin,
frequency is in Hertz, resistance in ohms, and capaci-
tance in farads.
The recommended range if timing resistors is between
10 kohm and 500kohm and range of timing capacitors
is between 100pF and 1000pF. Timing resistors less
than 10 kohm should be avoided.
Refer to layout guide lines on page 16 to achieve best
results.
SYNC: SYNC is a positive edge triggered input with a
threshold precisely set to
0.5*VFF
In the Bi-Phase operation mode SYNC pins should be con-
nected to the Cosc (Timing Capacitors) of the other con-
troller. This will force a 180° out of phase operation.
(see page9).
In a single controller operation, SYNC could be grounded
or connected to an external synchronization clock with Fre-
quency higher than the on board oscillator Frequency (see
page 2).
ILIM: Current sense input is provided via the ILIM pin.
The current sense input from a sense resistor provides a
pulse by pulse current limit by terminating the PWM pulse
when the input is above 200mV.
GND: Device power and analog ground. Careful atten-
tion should be paid to the layout of the ground planes
(see page 12).
OUT: The output is intended to drive an external FET driver
or other high impedance circuit. The output voltage swings
from GND to Vref with a typical output impedance of 500Ω.
REF: The REF pin provides a 4 or 5V user accessible
voltage reference. This pin should be decoupled with a
1
µF capacitor.


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