SC1404(2003) View Datasheet(PDF) - Semtech Corporation
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SC1404 Datasheet PDF : 27 Pages
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SC1404
POWER MANAGEMENT
Functional Information
Detailed Description
The SC1404 is a versatile multiple-output power supply controller
designed to power battery operated systems. Out-of-phase
switching improves signal quality and reduces input RMS current,
thereby reducing the size of input filter capacitors (see Out-of-
Phase Switching). The SC1404 provides synchronous rectified
buck control in fixed-frequency forced-continuous mode and in
hysteretic PSAVE mode, for two switching converters over a wide
load range. The two switchers have on-chip preset output voltages
of 5.0V and 3.3V. The control and fault monitoring circuitry for
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. Two linear regulators and a precision
reference voltage are also provided. The 5V/50mA linear regulator
(VL supply) which powers the SC1404 and gate drivers operates
from the battery (V+ supply). If the 5V converter is running, the
SC1404 disables the linear regulator and connects the VL supply
to the 5V output through an internal switch. The operating current
for the SC1404 and gate drivers then comes from the more
efficient 5V converter. The 12V/200mA linear regulator can supply
200mA. Semtech’s proprietary Virtual Current SenseTM provides
advantages in stability and signal to noise ratio compared to
conventional current sensing.
PWM control
There are two separate PWM control blocks for the 3V and 5V
switchers. They are switched out-of-phase with each other. The
interleaved topology offers advantages over in-phase solutions. It
reduces steady state input filter requirements by reducing current
drawn from the filter capacitors. To avoid both switchers switching
simultaneously, there is a built-in time delay between the two
switchers, 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: 1 - Forced continuous conduction mode
as a fixed-frequency peak current mode controller with falling edge
modulation, and 2 - Hysteretic Power-save mode. 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.
When the switcher operates in continuous conduction mode, the
high-side MOSFET turns on the beginning of each switching cycle.
It is turned off when the desired duty cycle is reached. Active shoot-
through protection will delay the low-side MOSFET turn-on until
the phase node drops below 1.0 V. The low-side MOSFET then
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 is
turned on.
Under light load conditions when the PSAVE pin is low, the SC1404
operates as a hysteretic controller in discontinuous conduction
mode to reduce switching frequency and switching bias current.
The switching frequency then is determined by the hysteretic trip
voltage set around the reference. When entering PSAVE# mode, if
the minimum (valley) inductor current as measured across the
CSH and CSL pins is below the PSAVE# threshold for four switching
cycles, the virtual current sense circuitry is shutdown and PWM
control 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 SC1404 provides built-in hysteresis to
prevent chattering between the two modes of operation.
Gate Drive / Control
The SC1404 gate drivers are designed to switch large MOSFETs
up to 350KHz. The high-side gate driver is required to drive the
gate of high-side MOSFETs above the V+ input. The supply for the
gate drivers is generated by charging a boostrap capacitor from
the VL supply while 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 low-side MOSFET is on when the
high-side driver is off. Under light load conditions when the PSAVE#
input 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 low-side MOSFET is turned off before the current
goes significantly negative and causes energy drain from the output.
The low-side driver operation is also affected by various fault
conditions as described in the Fault Protection section.
External Compensation
The COMP pin allows external compensation of the feedback loop.
This allows greater flexibility when choosing output filters, resulting
in reduced cost and smaller size compared to a fixed compensation
approach. A nominal gain of 18 for the error amplifier improves
the system loop gain and output transient response.
Internal Bias Supply
The VL linear regulator provides a 5V output that powers the gate
drivers, 2.5V reference, and internal controls of the SC1404. The
VL supply can provide up to 50mA, but this must include MOSFET
gate drive current. The VL pin should be bypassed to GND with
4.7uF to supply the peak gate drive curents. The VL regulator
receives input power from the V+ battery input. Efficiency is im-
proved by providing a boot-strap 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 pro-
vided by the high efficiency 5V switcher.
2003 Semtech Corp.
9
www.semtech.com
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