SC1409 Semtech Corporation, SC1409 Datasheet - Page 6

• Manufacturer Part Number: SC1409
• Description: 1 A Synchronous Switching Regulator
• Manufacturer: Semtech Corporation

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.
© 2000 SEMTECH CORP.
June 28, 2000
1 A Synchronous Switching Regulator
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
652 MITCHELL ROAD NEWBURY PARK CA 91320
SC1409
6