SC111 Semtech Corporation, SC111 Datasheet - Page 5

• Manufacturer Part Number: SC111
• Description: 0.8AMP POSITIVE VOLTAGE REGULATOR
• Manufacturer: Semtech Corporation

PRELIMINARY - October 9, 2000
THEORY OF OPERATION
Synchronous Buck Converter
V
voltage-mode pulse width modulated (PWM) controller.
This section has all the features required to build a
high efficiency synchronous buck converter for termi-
nation of power application.
The output voltage of the synchronous converter is set
and controlled by the output of the error amplifier. The
resistive divider generates reference voltage for the
error amplifier from an external 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 os-
cillator sets the reference voltage at the inverting input
of the PWM comparator. The non-inverting input of the
comparator receives it’s input voltage from the error
amplifier. When the oscillator output voltage drops be-
low 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 voltage, the comparator output goes
low. This pulls DH low, turning off the high-side FET,
and DL is pulled high, turning 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 as-
sures 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
below 4.0V. Normal operation resumes once V
above 4.2V.
Soft Start
Initially, SS/ENABLE sources 10µA of current to
charge an external capacitor. The outputs of the error
amplifiers are clamped to a voltage proportional to the
voltage on SS/ENABLE. This limits the on-time of the
high-side MOSFETs, thus leading to a controlled ramp-
up of the output voltages.
© 2000 SEMTECH CORP.
TERMINATION
power is provided by a synchronous,
SINK AND SOURCE DC/DC CONTROLLER FOR
TERMINATION POWER SUPPLY APPLICATIONS
CC
CC
rises
falls
R
The current limit threshold is set by connecting an ex-
ternal resistor from the V
age drop across this resistor is due to the 200µA inter-
nal sink sets the voltage at the pin. This voltage is
compared to the voltage at the PHASE node. This
comparison is made only when the high-side drive is
high to avoid false current limit triggering due to uncon-
tributing measurements from the MOSFET’s off-
voltage. When the voltage at PHASE is less than the
voltage at OCSET, an overcurrent condition occurs
and the soft start cycle is initiated. The synchronous
switcher turns off and SS/ENABLE starts to sink 2µA.
When SS/ENABLE reaches 0.8V, it then starts to
source 10µA and a new cycle begins.
Hiccup Mode
During power up, the SS/ENABLE pin is internally
pulled low until VCC reaches the undervoltage lockout
level of 4.2V. Once V
ABLE pin is released and begins to source 10µA of
current to the external soft-start capacitor. As the soft-
start voltage rises, the output of the internal error am-
plifier is clamped to this voltage. When the error signal
reaches the level of the internal triangular oscillator,
which swings from 1V to 2V at a fixed frequency of 250
kHz, switching occurs. As the error signal crosses
over the oscillator signal, the duty cycle of the PWM
signal continues to increase until the output comes into
regulation. If an over-current condition has not oc-
curred the soft-start voltage will continue to rise and
level off at about 2.2V.
An over-current condition occurs when the high-side
drive is turned on, but the PHASE node does not
reach the voltage level set at the OCSET pin. The
PHASE node is sampled only once per cycle during the
valley of the triangular oscillator. Once an over-current
occurs, both the high-side and low-side drivers turn off
and the SS/ENABLE pin begins to sink 2uA. The soft-
start voltage will begin to decrease as the 2uA of cur-
rent discharges the external capacitor. When the soft-
start voltage reaches 0.8V, the SS/ENABLE pin will be-
gin to source 10uA and begin to charge the external
capacitor causing the soft-start voltage to rise again.
Again, when the soft-start voltage reaches the level of
the internal oscillator, switching will occur.
If the over-current condition is no longer present, nor-
mal operation will continue. If the over-current condi-
tion is still present, the SS/ENABLE pin will again begin
to sink 2uA. This cycle will continue indefinitely until
the over-current condition is removed.
DS(ON)
652 MITCHELL ROAD NEWBURY PARK CA 91320
Current Limiting
CC
has reached 4.2V, the SS/EN-
CC
supply to OCSET. The volt-
SC1110
5