SC1403 Просмотр технического описания (PDF) - Semtech Corporation
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SC1403 Datasheet PDF : 31 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. 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. Out of phase switching
improves signal quality and reduces input RMS current, therefore
reducing size of input filter inductors and capacitors. Lossless
current sensing eliminates the need for discrete current sense
resistors. The two switchers have on-chip preset output voltages
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 circuitry for
each PWM controller includes digital softstart, voltage error
amplifier with built-in slope compensation, pulse width modulator,
power save, overcurrent, overvoltage and undervoltage fault
protection. One linear regulator and a precision reference voltage
are also provided. The 5V/30mA linear regulator uses battery power
to feed the gate drivers; for improved 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 each switcher.
They are switched out-of-phase with each other. The interleaved
topology 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 turn-on of the 3.3V switcher and 5V switcher, the amount of
which depends on the input voltage (see Out-of-Phase Switching).
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 feedback
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
inhibit 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.
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 in 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 start 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 1V. The low-side mosfet remains
on 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 turns on.
Internal Bias Supply
The VL linear regulator provides a 5V output used to power the
gate drivers, 2.5V reference and internal control section of the
SC1403. The regulator is capable of supplying up to 30mA (in-
cluding mosfet gate charge current). The VL pin should be bypassed
to GND with 4.7uF to supply the peak current requirements of the
gate driver outputs.
The regulator receives its input power from the V+ battery input.
Efficiency is improved by providing a boot-strapping mode for the
VL bias. When the 5V SMPS output voltage reaches 5V, internal
circuitry turns on a pmos pass device between CSL5 and VL. The
internal VL regulator is then disabled and VL bias is provided by
the high efficiency 5V 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
as shown in the following table.
2004 Semtech Corp.
9
United States Patent No. 6,377,032
www.semtech.com
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