SC2621A Semtech Corporation, SC2621A Datasheet - Page 6

• Manufacturer Part Number: SC2621A
• Description: Voltage Mode PWM Controller
• Manufacturer: Semtech Corporation

THEOR
THEOR
UVLO, Start-up and Shutdown
UVLO, Start-up and Shutdown
Hiccup Mode Short Circuit Protection
Hiccup Mode Short Circuit Protection
THEOR
THEORY OF OPERA
THEOR
The SC2621A integrates a high-speed, voltage mode
PWM controller with a linear controller into a single pack-
age. It is designed to control two independent output
voltages for high performance graphic card applications.
As shown in the block diagram of the SC2621A, the volt-
age-mode PWM controller consists of an error amplifier,
a 450kHz ramp generator, a PWM comparator, a RS latch
circuit, and two MOSFET drivers. The buck converter out-
put voltage is fed back to the error amplifier negative
input and is regulated to a reference voltage level. The
error amplifier output is compared with the ramp to gen-
erate a PWM wave, which is amplified and used to drive
the MOSFETs in the buck converter. The PWM wave at
the phase node with the amplitude of Vin is filtered out
to get a DC output. The linear controller is an error ampli-
fier. It provides the gate drive and output voltage control
for a linear regulator. Both PWM controller and linear con-
troller work with soft-start and fault monitoring circuitry
to meet application requirement.
UVLO, Start-up and Shutdown
UVLO, Start-up and Shutdown
UVLO, Start-up and Shutdown
To initiate the SC2621A, a supply voltage is applied to
Vcc pin. The top gate (DH) and bottom gate (DL) are held
low until Vcc voltage exceed UVLO (Under Voltage Lock
Out) threshold, typically 4.0V. Then the internal Soft-Start
(SS) capacitor begins to charge, the top gate remains
low, and the bottom gate is pulled high to turn on the
bottom MOSFET. When the SS voltage at the capacitor
reaches 0.4V, the linear controller is enabled and the
LDO output is turned on. Meanwhile, the top and bottom
gates of PWM controller begin to switch. The switching
regulator output is slowly ramping up for a soft turn-on.
If the supply voltages at Vcc pin falls below UVLO thresh-
old during a normal operation, the SS capacitor begins
to discharge. When the SS voltage reaches 0.4V, the
PWM controller controls the switching regulator output
to ramp down slowly for a soft turn-off. Meanwhile, the
linear controller is disabled and LDO output is turned off.
Hiccup Mode Short Circuit Protection
Hiccup Mode Short Circuit Protection
Hiccup Mode Short Circuit Protection
The SC2621A uses low-side MOSFET R
over current protection. In every switching cycle, after
the bottom MOSFET is on for 150ns, the SC2621A de-
tects the phase node voltage and compares it with an
internal setting voltage. If the phase node is lower than
the setting voltage, an overcurrent condition occurs. The
SC2621A will discharge the internal SS capacitor and
POWER MANAGEMENT
Applications Information
2007 Semtech Corp.
www.DataSheet4U.com
Y OF OPERA
Y OF OPERA
Y OF OPERATION
Y OF OPERA
TION
TION
TION
TION
dson
sensing for
6
shutdown both outputs. After waiting for around 5 milli-
seconds, the SC2621A begins to charge SS capacitor
again and initiates a fresh startup. The startup and shut-
down cycle will repeat until the short circuit is removed.
This is called a hiccup mode short circuit protection.
To program a load trip point for short circuit protection, it
is recommended to connect a 3.3k resistor from the OCS
pin to the ground, and a resistor R
the DRV pin, as shown in Fig. 1.
Fig. 1. Programming load trip point
Fig. 2. Pull up resistor (R
The resistor R
node voltage Vpn at the load trip point. This voltage is
the product of the inductor peak current at the load trip
point and the R
The soft start time of the SC2621A is fixed at around
V
pn
350
350
325
325
300
300
275
275
250
250
225
225
200
200
175
175
150
150
=
0
0
I
peak
´
R
Rset
Rset
100
100
set
3.3k
3.3k
ds
dson
_
can be found in Fig. 2 for a given phase
on
10
10
2
2
of the low-side MOSFET:
200
200
OCS
OCS
DRV
DRV
12V
12V
SC2621A
SC2621A
set
Rset (k -ohm)
Rset (k -ohm)
V CC
V CC
GND
GND
) vs. trip voltage Vpn
8
8
12
12
300
300
set
from the OCS pin to
400
400
SC2621A
www.semtech.com
500
500
600
600