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Электронный компонент: SC4905A

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1
www.semtech.com
SC4905A/B
High Performance Voltage Mode
PWM Controller
POWER MANAGEMENT
PRELIMINARY
Revision 4, April 2002
Description
Features
Applications
Typical Application Circuit
K
Operation to 1MHz
K
Accurate programmable maximum duty cycle
K
Integrated oscillator/voltage feed forward
compensation
K
Line voltage monitoring
K
External frequency synchronization
K
Bi-phase mode of operation for ripple reduction
K
Under 100A start-up current
K
Accessible reference voltage
K
VDD undervoltage lockout
K
-40C to 105C operating temperature
K
10 lead MSOP package
K
Telecom equipment and power supplies
K
Networking power supplies
K
Power over LAN applications
K
Industrial power supplies
K
Isolated power supplies
The SC4905A/B is a 10 pin BICMOS primary side voltage
mode controller for use in Isolated DC-DC and off-line
switching power supplies. It is a highly integrated solution,
requiring few external components. The device features a
high speed oscillator with integrated feed forward
compensation, accurately programmable maximum duty
cycle, voltage mode of operation, line voltage monitoring,
supply UVLO, low start up current, low voltage current limit
threshold and user accessible reference.
The SC4905A/B device operates at a fixed frequency,
highly desirable for Telecom applications. Features a
separate SYNC pin which simplifies synchronization to
an external clock. Feeding the oscillator of one device to
the SYNC of another forces biphase operation (180
degrees apart) which reduces input ripple and filter size.
The SC4905A has a turn-on threshold of 4.5V and the
SC4905B has a turn-on threshold of about 12 volts.
These devices are available in the 10 lead MSOP package.
background image
SC4905A/B
POWER MANAGEMENT
PRELIMINARY
2002 Semtech Corp.
www.semtech.com
2
Absolute Maximum Ratings
Electrical Characteristics
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Exceeding the specifications below may result in permanent damage to the device, or device malfunction. Operation outside of the parameters specified in the
Electrical Characteristics section is not implied.
background image
SC4905A/B
POWER MANAGEMENT
PRELIMINARY
2002 Semtech Corp.
www.semtech.com
3
Electrical Characteristics (Cont.)
Note 1: Guaranteed by design. Not 100% tested in production.
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Unless otherwise specified, V
DD
= 12V, V
IN
= 48V, R
OSC
= 499k, C
OSC
= 220pF, R
T
= 280k, R
M
= 2k, R
B
= 8.25k, C
VDD
= 0.1uF, and no load on the outputs.
background image
SC4905A/B
POWER MANAGEMENT
PRELIMINARY
2002 Semtech Corp.
www.semtech.com
4
Pin Configuration
Ordering Information
Note:
Only available in tape and reel packaging. A reel
contains 2500 devices.
(Top view)
(MSOP-10)
VDD
FB
VFF
DMAX
RC
SYNC
ILIM
GND
OUT
REF
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background image
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 500.
REF: The REF pin provides a 4 or 5V user accessible
voltage reference. This pin should be decoupled with a
1
F
capacitor.