SC4614 데이터 시트보기 (PDF) - Semtech Corporation
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SC4614 Datasheet PDF : 14 Pages
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SC4614
POWER MANAGEMENT
Applications Information
THEORY OF OPERATION
The SC4614 is a high-speed, voltage mode PWM con-
troller that provides the control and protection features
necessary for a synchronous buck converter.
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 Rset from the OCS pin to
the DRV pin, as shown in Fig. 1.
As shown in the block diagram of the SC4614, the volt-
age-mode PWM controller consists of an error amplifier,
a 500kHz 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 PWM controller works with soft-
start and fault monitoring circuitry to meet application
requirements.
12V
7
6
V CC
DRV
Rs et
3.3k
2
OCS
S C461 4
GND
5
UVLO, Start Up and Shut Down
To initiate the SC4614, a supply voltage is applied to the
Vcc pin. The top gate (DH) and bottom gate (DL) are held
low until Vcc voltage exceeds 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 top and bottom gates of PWM control-
ler begin to switch. The switching regulator output is slowly
ramping up for a soft turn-on.
Fig. 1. Programming load trip point
350
325
300
275
250
225
If the supply voltages at the Vcc pin falls below UVLO
200
threshold during a normal operation, the SS capacitor
175
begins to discharge. When the SS voltage reaches 0.4V,
150
the PWM controller controls the switching regulator out-
0
100
200
300
400
500
600
put to ramp down slowly for a soft turn-off.
Rset (k -ohm)
Hiccup Mode Short Circuit Protection
The SC4614 uses low-side MOSFET Rdson sensing for
over current protection. In every switching cycle, after
the bottom MOSFET is on for 150ns, the SC4614 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
SC4614 will discharge the internal SS capacitor and shut-
down both outputs. After waiting for around 10 millisec-
onds, the SC4614 begins to charge the 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.
Fig. 2. Pull up resistor (Rset) vs. trip voltage Vpn
The resistor Rset can be found in Fig. 2 for a given phase
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 Rdson of the low-side MOSFET:
Vpn = I peak ´ Rds _ on
The soft start time of the SC4614 is fixed at around
1.5ms. Therefore, the maximum soft start current is de-
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