SC1403 datasheet(9/30 Pages) SEMTECH

9 / 30 page

 2002 Semtech Corp.

www.semtech.com

SC1403

POWER MANAGEMENT

PRELIMINARY

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

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.

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