6 2004 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC1110
Block Diagram
Applications Information
THEORY OF OPERATION
Synchronous Buck Converter
V
TERMINATION
power is provided by a synchronous, voltage-
mode pulse width modulated (PWM) controller. This sec-
tion has all the features required to build a high efficiency
synchronous buck converter for termination of power
application.
The output voltage of the synchronous converter is set
and controlled by the output of the error amplifier. The
external resistive divider generates reference voltage for
the error amplifier from the chipset voltage which is
usually 2.5V. The inverting input of the error amplifier
receives its voltage from the SENSE pin.
The internal oscillator uses an on-chip capacitor and
trimmed precision current sources to set the oscillation
frequency to 250kHz. The triangular output of the oscil-
lator sets the reference voltage at the inverting input of
the PWM comparator. The non-inverting input of the com-
parator receives it’s input voltage from the error ampli-
fier. When the oscillator output voltage drops below the
error amplifier output voltage, the comparator output
goes high. This pulls DL low, turning off the low-side FET,
and DH is pulled high, turning on the high-side FET (once
the cross-current control allows it). When the oscillator
voltage rises back above the error amplifier output volt-
age, the comparator output goes low. This pulls DH low,
turning off the high-side FET, and DL is pulled high, turn-
ing on the low-side FET (once the cross-current control
allows it).
As SENSE increases, the output voltage of the error
amplifier decreases. This causes a reduction in the on-
time of the high-side MOSFET connected to DH, hence
lowering the output voltage.
Under Voltage Lockout
The under voltage lockout circuit of the SC1110 assures
that both the high-side MOSFET driver outputs remain in
the off state whenever the supply voltage drops below
set parameters. Lockout occurs if V
CC
falls below 4.1V.
Normal operation resumes once V
CC
rises above 4.2V.
