SC1102 Semtech Corporation, SC1102 Datasheet - Page 7
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SC1102
Manufacturer Part Number
SC1102
Description
SYNCHRONOUS DC/DC CONTROLLER FOR DISTRIBUTED POWER SUPPLY APPLICATIONS
Manufacturer
Semtech Corporation
Datasheet
1.SC1102.pdf
(10 pages)
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September 5, 2000
THEORY OF OPERATION
Synchronous Buck Converter
Primary V
voltage-mode pulse width modulated (PWM) controller.
This section has all the features required to build a
high efficiency synchronous buck converter, including
“Power Good” flag, shut-down, and cycle-by-cycle cur-
rent limit.
The output voltage of the synchronous converter is set
and controlled by the output of the error amplifier. The
external resistive divider reference voltage is derived
from an internal trimmed-bandgap voltage reference
(See Fig. 1). 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 200kHz. The triangular output of the os-
cillator sets the reference voltage at the inverting input
of the 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 ampli-
fier 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 SC1102 as-
sures that the high-side MOSFET driver outputs re-
main in the off state whenever the supply voltage drops
below set parameters. Lockout occurs if V
4.1V. Normal operation resumes once V
4.2V.
Over-Voltage Protection
The over-voltage protection pin (OVP) is high only
when the voltage at SENSE is 20% higher than the tar-
get value programmed by the external resistor divider.
The OVP pin is internally connected to a PNP’s
collector.
© 2000 SEMTECH CORP.
CORE
power is provided by a synchronous,
CC
CC
rises above
SYNCHRONOUS DC/DC CONTROLLER FOR
DISTRIBUTED POWER SUPPLY APPLICATIONS
falls below
Power Good
The power good function is to confirm that the regula-
tor outputs are within +/-10% of the programmed
level. PWRGD remains high as long as this condition
is met. PWRGD is connected to an internal open col-
lector NPN transistor.
Soft Start
Initially, SS/SHDN sources 10µA of current to charge
an external capacitor. The outputs of the error ampli-
fiers are clamped to a voltage proportional to the volt-
age on SS/SHDN. This limits the on-time of the high-
side MOSFETs, thus leading to a controlled ramp-up
of the output voltages.
R
The current limit threshold is set by connecting an
external resistor from the V
voltage drop across this resistor is due to the 200µA
internal 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 un-
contributing 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/SHDN starts to sink 2µA.
When SS/SHDN reaches 0.8V, it then starts to
source 10µA and a new cycle begins.
Hiccup Mode
During power up, the SS/SHDN pin is internally
pulled low until VCC reaches the undervoltage lock-
out level of 4.2V. Once V
SS/SHDN 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
amplifier is clamped to this voltage. When the error
signal reaches the level of the internal triangular os-
cillator, which swings from 1V to 2V at a fixed fre-
quency of 200 kHz, switching occurs. As the error
signal crosses over the oscillator signal, the duty cy-
cle of the PWM signal continues to increase until the
output comes into regulation. If an over-current con-
dition has not occurred the soft-start voltage will con-
tinue to rise and level off at about 2.2V.
DS(ON)
652 MITCHELL ROAD NEWBURY PARK CA 91320
Current Limiting
CC
CC
has reached 4.2V, the
supply to OCSET. The
SC1102
7