SC1403(2002) Просмотр технического описания (PDF) - Semtech Corporation
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SC1403 Datasheet PDF : 30 Pages
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SC1403
POWER MANAGEMENT
Functional Information
Detailed Description
The SC1403 is a versatile multiple-output power supply controller
designed to power battery operated systems. Out of phase
switching design is adopted to improve signal quality and reduce
input RMS current, therefore reducing size of input filter inductor
and capacitors. The SC1403 provides synchronous rectified buck
control in fixed frequency forced-continuous mode and hysteretic
PSAVE mode for two switching power supplies over a wide load
range. The two switchers have on-chip preset output voltage of
5.0V and 3.3V. An external resistor divider can be used to set the
switcher outputs from 2.5V to 5.5V. The control and fault
monitoring circuitry associated with each PWM controller includes
digital softstart, turn-on sequencing, voltage error amplifier with
built-in slope compensation, pulse width modulator, power save,
over-current, over-voltage and under-voltage fault protection. One
linear regulator and a precision reference voltage are also provided
by the SC1403. The 5V/50mA linear regulator takes input from
the battery to power the gate drivers, however to improve efficiency,
the 5V switcher output is used instead when available. Semtech’s
proprietary Virtual Current SenseTM provides greater advantages
in the aspect of stability and signal to noise ratio than the
conventional current sense method.
PWM control
There are two separate PWM control blocks for the 3V and 5V
switchers. They are out-of-phase with each other. The interleaved
topology offers greater advantage over in-phase solutions. It
reduces steady state input filter requirements by reducing current
drawn from the filter capacitors. To avoid both switchers switching
at the same instance, there is a built-in delay between the on-time
of the 3.3V switcher and 5V switcher, the amount of which depends
on the input voltage (see Out-of-Phase Switching). The PWM
provides two modes of control over the entire load range. The
SC1403 operates in forced continuous conduction mode as a
fixed frequency peak current mode controller with falling edge
modulation. Current sense is done differently than that in the
conventional peak current mode control. Semtech’s proprietary
Virtual Current SenseTM emulates the necessary inductor current
information for proper functioning of the IC. In order to
accommodate a wide range of output filters, a COMP pin is also
available for compensating the error amplifier externally. A nominal
gain of 18 is used in the error amplifier to further improve the
system loop gain response and the output transient behavior. When
the switcher is operating in continuous conduction mode, the high-
side MOSFET is turned on at the beginning of each switching cycle.
It is turned off when the desired duty cycle is reached. Active shoot-
through protection delays the turn-on of the lower MOSFET until
the phase node drops below 2.5V. The low-side MOSFET remains
on until the beginning of the next switching cycle. Again, active
shoot-through protection ensures
PRELIMINARY
that the gate to the low-side MOSFET has dropped low before the
high-side MOSFET is turned on. Under light load conditions when
the PSAVE pin is low, the SC1403 operates as a hysteretic controller
in the discontinuous conduction mode to reduce its switching
frequency and switching bias current. The switching of the output
MOSFET does not depend on a given oscillator frequency, but on
the hysteretic FB trip voltage set around the reference. When
entering PSAVE mode, if the minimum (valley) inductor current
measured across the CSH and CSL pins is below the PSAVE
threshold for four switching cycles, the virtual current sensing
circuitry is shutdown and PWM switches from forced continuous
to hysteretic mode. If the minimum (valley) inductor current is
above the threshold for four switching cycles, PWM control changes
from hysteretic to forced continuous mode. The SC1403 provides
built-in hysteresis to prevent chattering between the two modes
of operation.
Gate Drive / Control
The gate drivers on the SC1403 are designed to switch large
MOSFETs up to 350KHz. The high-side gate driver is required to
drive the gates of high-side MOSFET above the V+ input. The supply
for the gate drivers is generated by charging a boostrap capacitor
from the VL supply when the low-side driver is on. Monitoring
circuitry ensures that the bootstrap capacitor is charged when
coming out of shutdown or fault conditions where the bootstrap
capacitor may be depleted. In continuous conduction mode, the
low-side driver output that controls the synchronous rectifier in
the power stage is on when the high-side driver is off. Under light
load conditions when PSAVE pin is low, 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 operation is also affected by various
fault conditions as described 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 section of the
SC1403. The regulator is capable of supplying up to 50mA (in-
cluding MOSFET gate charge current). The VL pin should be by-
passed to GND with 4.7uF to supply the peak current require-
ments of the gate driver outputs. The regulator receives its input
power from the V+ battery input. Efficiency is improved by provid-
ing a boot-strapping mode for the VL bias. When the 5V SMPS
output voltage reaches 5V, internal circuitry detects this condition
and turns on a PMOS pass device between CSL5 and VL. The
internal VL regulator is then disabled and the VL bias is provided
by the high efficiency switcher.
The REF output is accurate to +/- 2% over temperature. It is capable
of delivering 5mA max and should be bypassed with 1uF minimum
capacitor. Loading the REF pin will reduce the REF voltage slightly.
2002 Semtech Corp.
9
www.semtech.com
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