SC1409

© 2000 SEMTECH CORP.

652 MITCHELL ROAD NEWBURY PARK CA 91320

1 A Synchronous Switching Regulator

June 28, 2000

6

THEORY OF OPERATION

General

The SC1409 is a synchronous buck controller with

on-board FETs. It uses top FET sensing for current limit

together with a soft-start circuit and hiccup overcurrent

protection. The EN/SS pin can be pulled low to put the

SC1409 in a low quiescent shutdown state. It uses a

wide bandwidth error amplifier for optimum transient

response and minimum external compensation require-

ments. Overcurrent limit is preset at 1.5A, but can be

set at a lower value by use of an external ISET resistor.

EN/SS

When power is first applied to the device, the EN/SS

pin sources approximately 1uA to the external soft-start

capacitor, most of the rest of the internal circuitry is

disabled and the device is in low-power mode. If the

soft-start pin is held low, the SC1409 will remain in low-

power mode, drawing approximately 5uA from the sup-

ply. Once the soft start pin is released, the external soft-

start capacitor will charge up slowly to the EN/SS

threshold, where the source current will increase to

approximately 30uA, charging the soft-start capacitor

more quickly. The voltage at the soft-start pin modifies

the internal reference voltage and hence controls the

output voltage (see Soft-Start control of Vout). Once

the soft start capacitor has charged to approximately

3V, the reference voltage at the input to the error amp

will have reached its final value and the output voltage

will no longer be under the control of the soft-start.

Overcurrent Limit

The SC1409 internally mirrors and scales the current

through the top FET. This scaled, mirrored current is

used to detect an overcurrent condition. In the event an

overcurrent is detected, the output is immediately dis-

abled and the soft-start capacitor is discharged at

approximately 30uA until SS/EN reaches 1.5V. At this

point the output is enabled and a normal soft-start cycle

is initiated. If the fault persists, the output will be dis-

abled, the soft-start capacitor will continue charging at

30uA until SS/EN reaches 4.5V before the discharge

cycle is initiated.

Some combinations of high output voltage, low induc-

tance and high output capacitance will cause this

“hiccup” mode of operation to continue even if the fault

is cleared. In this case, input power will need to be

cycled, or EN/SS cycled through shutdown to restore

normal operation.

Operating Frequency

The frequency of operation can be set to a nominal

300kHz by grounding the FSET pin, or to a nominal

500kHz by returning the FSET pin to VIN. The SC1409

can also be synchronized to an external clock using the

FSET pin.

LAYOUT GUIDELINES

The SC1409 should preferably be laid out over a

ground plane. Since the power section and the control

section of the IC share a common VIN pin, the VIN pin

must be decoupled to ground by a 0.1uF ceramic ca-

pacitor as close to the IC as possible. The traces be-

tween VIN and the decoupling capacitor and the de-

coupling capacitor and the PGND pin should be 25

mils wide and no more than 100 mils long. The main

input capacitor should be placed within about 1” of the

IC using solid copper areas to connect to the IC.

Ideally AGND should be returned to the output capaci-

tor and connected to the ground plane at that point.

Feedback resistors and all traces associated with them

should be kept away from the phase node. The induc-

tor should be placed as close to the PHASE pin as

possible to minimize EMI.

These principles are illustrated in the Evaluation Board

shown below.

Evaluation Board Top Copper and Assembly Diagrams