SC1404 datasheet(9/25 Pages) SEMTECH

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 2004 Semtech Corp.

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SC1404

POWER MANAGEMENT

Detailed Description

The SC1404 is a versatile multiple-output power supply controller

for battery operated systems. The SC1404 provides synchronous

rectified buck control in fixed frequency forced-continuous (PWM)

mode and hysteretic PSAVE mode, for two switching power supplies

over a wide load range. Out of phase switching reduces input

noise and RMS current, which reduces the input filter inductors

and capacitors. The two switchers have on-chip preset output

voltages of 5.0V and 3.3V.

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. Two linear regulators and a precision

reference voltage are also provided. The 5V/30mA linear regulator

(VL) uses battery power to feed the gate drivers. For improved

efficiency, VL automatically switches over the +5V converter output

if available. The 12V linear regulator supplies up to 200mA.

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. This interleaved

topology reduces 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).

The PWM provides two modes of control over the entire load range.

The SC1404 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 error

amplifier gain of 18 improves the system loop gain and the output

transient behavior.

When 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 will delay the lower mosfet turn-on 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 low-side mosfet gate voltage drops low before

the high-side mosfet turns on.

Under light load conditions when the PSAVE# pin is low, the

SC1404 operates hysteretically 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 voltage set around the

nominal output voltage. When entering PSAVE# mode (from heavy

to light load), if the minimum (valley) inductor current measured

at CSHx-CSLx is below the Zero Cross threshold (typically 5 mV) for

four switching cycles, the virtual current sense circuitry will shut off

and the mode changes to hysteretic mode. As the load current

increases, if the minimum (valley) inductor current is above the

threshold for four switching cycles, the converter stops psave mode

and enters PWM operation. The change in frequency between

hysteretic psave and PWM mode provides hysteresis to inhibit

chattering between the two modes of operation.

Gate Drive / Control

The gate drivers on the SC1404 are designed to switch large

mosfets. The high-side gate driver must drive the gate 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. 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 is a 5V output that powers the gate drivers,

2.5V reference and internal controls of the SC1404. The regulator

is capable of supplying up to 30mA (including mosfet gate charge

current). The VL pin should be bypassed to GND with 4.7uF to

supply the large gate drive current pulses.

The regulator receives input power from the V+ battery input. Effi-

ciency is improved by providing a bootstrap for the VL output.

When the 5V SMPS output voltage reaches 5V, internal circuitry

turns on a PMOS 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 seen in the following table.

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