sc1402 Semtech Corporation, sc1402 Datasheet - Page 8
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sc1402
Manufacturer Part Number
sc1402
Description
Multi-output, Low-noise Power Supply Controller For Notebook Computers
Manufacturer
Semtech Corporation
Datasheet
1.SC1402.pdf
(18 pages)
Available stocks
Company
Part Number
Manufacturer
Quantity
Price
Part Number:
sc1402ISSTR
Manufacturer:
SEMTECH/美国升特
Quantity:
20 000
Detailed Description
The SC1402 is a multiple-output, high efficiency, ver-
satile power supply controller designed to power bat-
tery operated systems. Four high current gate drive
outputs are supplied to control all MOSFETs in two
synchronous rectified buck converters. These buck
converters can be programmed to operate at either
fixed or adjustable output voltages. The power save
feature of the SC1402 achieves high efficiency over a
wide range of load current. The control and fault moni-
toring circuitry associated with each PWM controller
includes digital softstart, turn-on sequencing, voltage
error amplifier with slope compensation, pulse width
modulator, power save, over-current and over-voltage
and under voltage fault protection. Two linear regula-
tors and a precision reference voltage are also pro-
vided by the SC1402.
PWM Control Block
The two PWM control blocks for the 3V and 5V power
supply outputs are identical. The SC1402 employs
peak current mode control with slope compensation to
provide fast output response to load and line tran-
sients. The PWM control block consists of an analog
PWM modulator followed by PWM logic control. The
analog modulator combines the output current, slope
compensation signal and error voltage to generate a
PWM pulse train. The PWM logic uses the pulse train
from the modulator and other control signals to gener-
ate the output states for the high and low side gate
driver outputs. A block diagram of the PWM control
block is shown in Fig. 1.
An error amplifier generates the difference signal be-
tween the reference voltage and the feedback voltage
to generate the error voltage for the peak current mode
comparator. A nominal gain of 8 is used in the error
amplifier to increase the system loop gain and to re-
duce the load regulation error typically seen in low
loop-gain, current-mode controllers. The increased
gain in the voltage loop is compensated by pole-zero-
pole response of the voltage error amplifier. The cur-
rent feedback signal is summed with the slope com-
pensation signal and compared to the error voltage at
the PWM comparator.
When the power supply is operating in continuous con-
duction mode, the high-side MOSFET is turned on at
the beginning of each switching cycle. The high-side
MOSFET is turned off when the desired duty cycle is
reached. Active shoot-thruough protection delays the
turn-on of the low-side MOSFET until the phase node
drops below 2.5V. The low-side MOSFET remains on
© 2000 SEMTECH CORP.
December 15, 2000
Multi-Output, Low-Noise Power Supply
Controller for Notebook Computers
652 MITCHELL ROAD
until the beginning of the next switching cycle. Again,
active shoot-through protection ensures that the gate
to the low-side MOSFET has dropped low before the
high side MOSFET is turned on.
When PSAVE is enabled (low) and the output current
drops below 25% of its peak level, the PWM logic will
automatically enter PSAVE mode to improve efficiency.
When the controller enters power save, it increases the
regulation point by 0.8%, typically issuing one more
high side pulse as the converter enters PSAVE. The
PWM control then disables switching cycles until the
FB falls below the reference. At light loads the effec-
tive switching frequency will drop dramatically and effi-
ciency will increase because of the reduced gate
charge current required to switch the power stage.
Boosting the regulation point when entering PSAVE
gives the output improved dynamic regulation because
the output voltage is not allowed to droop below the
nominal regulation point. Load current steps, that
cause the converter to come out of PSAVE, will not
cause as large a negative dip in the output voltage.
Gate Drive/Control
The gate driver on the SC1402 are designed to switch
large MOSFETs up to 350kHz. The high side gate
driver is required to drive the gates of the high side
MOSFET above the V+ input. The supply for the gate
driver is generated by charging a bootstrap capacitor
from the VL supply when the low-side driver is on.
Monitoring circuit ensures that the bootstrap capacitor
is charged when coming out of shutdown or fault con-
ditions where the bootstrap capacitor may be depleted.
In continuos conduction mode, the low-side driver out-
put that controls the synchronous rectifier in the power
stage is on when the high-side driver is off. Under light
load conditions the inductor ripple current will approach
the point where it reverses polarity. This is detected by
the low side driver control and the synchronous rectifier
is turned off before the current reverses, preventing
energy drain from the output. The low-side driver oper-
ation is also affected by various fault conditions as de-
scribed in the Fault Protection section.
Internal Bias Supply
The VL linear regulator provides a 5V output that is
used to power the gate drivers, 2.5V reference, and
internal control sections of the SC1402. The regulator
is capable of supplying up to 50mA (including MOS-
NEWBURY PARK CA 91320
SC1402
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