SC4806MLTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
Manufacturer
SC4806MLTRT Datasheet PDF : 22 Pages
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SC4806
POWER MANAGEMENT
Application Information
SC4806 is a versatile double ended, high speed, low
power, pulse width modulator optimized for applications
requiring minimum space.
The device contains all of 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.
SC4806 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 auto-restart
over-current circuitry initiates the soft-start cycle.
SC4806 dual output drive stages are arranged for double
ended configurations. 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%.
SC4806 also provides flexibility with programmable
LUVLO thresholds, with built-in hysteresis.
POWER SUPPLY
A single supply, VCC is used to provide the bias for the
internal reference, oscillator, drivers, and logic circuitry
of SC4806.
PWM CONTROLLER
To generate PWM control signal, 1/3 of external feed-
back signal to FB pin by an internal 3 to 1 resistor divider
compares to the combined current signal if an external
resistor is connected from ILIM to RAMP. The value of
the resistor will determine the level of slope compensa-
tion. The slope signal to RAMP is generated from either
input voltage or VREF with external RC. Voltage mode of
operation can be achieved if the slope signal is only used.
Two levels of undervoltage lockout are also available.
The LUVLO (line under voltage lockout) pin via an exter-
nal resistive divider programs input voltage turn-on level.
During the LUVLO, the driver outputs are disabled and
the soft-start is reset.
The VCC UVLO (under voltage lockout) determines VCC
voltage turn-on level. Once VCC exceeds the UVLO limit,
the internal reference, oscillator, drivers and logic are
powered up.
SYNC is a positive edge triggered input with a threshold
set to 1.75V. 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 controller operation, SYNC should be
grounded or connected to an external synchronization
clock within the SYNC frequency range. In the Bi-phase
operation mode, a very unique oscillator is utilized to al-
low two SC4806s to be synchronized together and work
out of phase. This feature is set up by simple connection
of the SYNC input of one part to the RC pin of the other.
The master oscillator forces the two PWMs to operate
out of phase. This feature minimizes the input and out-
put ripples, and may reduce input and output capaci-
tors.
SC4806 is a double ended PWM controller that can be VCC UNDER VOLTAGE LOCK OUT
used in voltage or current mode applications. The oscil-
lator frequency is programmed by a resistor and a ca- Depending on the application and the voltages available,
pacitor network connected to an external reference pro- the SC4806 (UVLO = 8V) can be used to provide the VCC
vided by the SC4806. The two outputs, OUTA and OUTB, undervoltage lock out function to ensure the converters
are 180 degrees out-of-phase and run at half of the controlled start up.
oscillator frequency.
Before the VCC UVLO has been reached, the internal ref-
An external error amplifier will provide the error signal to erence, oscillator, OUTA/OUTB drivers, and logic are dis-
the FB pin of the SC4806. The current limit input and abled.
external slope compensation are provided separately via
the ILIM and RAMP pins. The current limit signal from a LINE UNDER VOLTAGE LOCK OUT
sense resistor or a current sense transformer is used
for the peak current and auto-restart overcurrent The SC4806 also provides a line undervoltage (LUVLO =
comparators.
Vref) function. The LUVLO pin is programmed via an ex-
2006 Semtech Corp.
8
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