SC4905A/B
POWER MANAGEMENT
Application Information
THEORY OF OPERATION
PRELIMINARY
The SC4905 is a versatile 10 pin BICMOS primary side This feed forward action provides an immediate duty cycle
adjustment while maintaining a constant oscillator fre-
quency.
voltage mode controller optimized for applications requir-
ing minimum space such as isolated DC-DC and off-line
switching power supplies.
A maximum duty cycle can be programmed by connect-
ing a resistor divider from the VFF to the DMAX pin. The
scaling of the VFF signal will set the maximum duty cycle
percentage.
The device contains all of the control and drive circuity
required for isolated or non-isolated power supplies,
where an external error amplifier is used. Fixed oscillator
frequency up to 1MHz can be programmed by an exter-
nal RC network.
An external error amplifier will provide the error signal to
the FB pin of the SC4905.
The SC4905 is a voltage mode controller, utilizing a feed A current sense input is provided via the ILIM pin. The
forward scheme to accommodate for any variations in
the input supply voltage resulting in a duty cycle
adjustment. This feed forward action results in an
improved dynamic performance of the converter.
current sense input from a sense resistor is used for the
peak current limit comparator.
Once VDD has exceeded the UVLO (VDD under voltage
lock out) limit, the internal reference, oscillator, drivers and
logic are powered up.
The SC4905 also provides a programmable maximum duty
cycle to prevent core saturation when a transformer is used.
As an added level of protection, SC4905 provides a cycle
SYNC is a positive edge triggered input with a threshold
set to 0.5*VFF.
By connecting a faster external control signal to the SYNC
pin, the internal oscillator frequency will be synchronized
to the positive edge of the external control signal. In a
single controller operation, SYNC could be grounded or
connected to an external synchronization clock within the
SYNC frequency range (see page 3).
by cycle peak current limit during an over current condition.
SUPPLY
A single supply, VDD is used to provide the bias for the
internal reference, oscillator, drivers, and logic circuitry
of the SC4905.
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.
PWM CONTROLLER
The SC4905 is a BICMOS primary side voltage mode
controller for use in isolated DC-DC and off-line switch-
ing power supplies. It is a highly integrated solution, re-
quiring 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 volt-
age monitoring, supply UVLO, low start-up current, low
voltage current limit threshold and user accessible refer-
ence.
Device
Typ. Vcc UVLO
4.5V
Reference Voltage
4V
5V
SC4905A
SC4905B
12.5V
Two voltage options are available for the SC4905. The
SC4905A version has a typical VDD under voltage of 4V,
and a 4V reference, while the SC4905B version provides a
12V VDD UVLO, and a 5V reference.
The Oscillator frequency is programmed by a resistor and
a capacitor network connected to the line supply voltage .
Any variations in the input supply voltage result in a duty
cycle adjustment, provided by the change of the oscillator
peak voltage via the VFF pin.
2002 Semtech Corp.
7
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SEMTECH [ SEMTECH CORPORATION ]
