10 2006 Semtech Corp. www.semtech.com
SC4808B-1
POWER MANAGEMENT
age divider provides a 3X amplification of the CS signal.
This is used for comparison to the external error amplifier
signal. If an external resistor is connected from CS to the
current sense resistor, the internal current source will pro-
vide a programmable slope compensation. The value of
the resistor will determine the level of compensation. At
higher compensation levels, voltage mode of operation can
be achieved. The error amplifier signal at the FB pin will be
used in conjunction with the CS signal to achieve regula-
tion.
Two levels of undervoltage lockout are also available. The
LUVLO (line under voltage lockout) pin via an external re-
sistive divider will program the undervoltage lockout level.
During the LUVLO, the driver outputs are disabled and the
softstart is reset.
Once VCC has exceeded the UVLO (VCC under voltage lock-
out) limit, the internal reference, oscillator, drivers and logic
are powered up.
SYNC is a positive edge triggered input with a threshold
set to 1.0V.
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. In a single con-
troller operation, SYNC should be grounded or connected
to an external synchronization clock within the SYNC fre-
quency range (see page 3).
In the Bi-phase operation mode a very unique oscillator
is utilized to allow two SC4808B-1 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.
THEORY OF OPERATION
The SC4808B-1 is a versatile double ended, high speed,
low power, pulse width modulator that is optimized for ap-
plications requiring minimum space.
The device contains all of the control and drive circuity re-
quired for isolated or non isolated power supplies where
an external error amplifier is used. A fixed oscillator fre-
quency (up to 1MHz) can be programmed by an external
RC network.
The SC4808B-1 is a peak current or voltage mode
controller, depending on the amount of slope
compensation, programmable with only one external
resistor. The cycle by cycle peak current limit prevents core
saturation when a transformer is used for isolation while
the overcurrent circuitry initiates the softstart cycle.
The SC4808B-1 dual output drive stages are arranged in a
push-pull configuration. Both outputs switch at half the
oscillator frequency using a toggle flip flop. The dead time
between the two outputs is programmable depending on
the values of the timing capacitor and resistors, thus limiting
each output stage duty cycle to less than 50%.
The SC4808B-1 also provides flexibility with programmable
LUVLO thresholds, with built-in hysteresis.
SUPPLY
A single supply, VCC is used to provide the bias for the
internal reference, oscillator, drivers, and logic circuitry of
SC4808B-1. To ensure proper operation during start up,
VCC slew rate of less than 10V/mS is recommended.
PWM CONTROLLER
SC4808B-1 is a double ended PWM controller that can be
used in voltage or current mode applications. The
SC4808B-1 provides a 4.4V VCC UVLO, and a 3.125V ref-
erence. The oscillator frequency is programmed by a resis-
tor and a capacitor network connected to an external refer-
ence provided by the SC4808B-1. The two outputs, OUTA
and OUTB, are 180 degrees out of phase and run at half of
the oscillator frequency.
An external error amplifier will provide the error signal to
the FB pin of the SC4808B-1.
The current sense input and internal slope compensation
are both provided via the CS pin. The current sense input
from a sense resistor is used for the peak current and
overcurrent comparators. An internal 1 to 3 feedback volt-
Application Information
