SC2602L Semtech Corporation, SC2602L Datasheet - Page 6

• Manufacturer Part Number: SC2602L
• Description: Synchronous Voltage Mode Controller
• Manufacturer: Semtech Corporation

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Soft Start
Initially, SS/SHDN sources 10µA of current to charge an
external capacitor. The outputs of the error amplifiers
are clamped to a voltage proportional to the voltage on
SS/SHDN. This limits the on-time of the high-side
MOSFETs, thus leading to a controlled ramp-up of the
output voltages.
R
The current limit threshold is set by connecting an ex-
ternal resistor from the V
age drop across this resistor is due to the 200µA inter-
nal sink sets the voltage at the pin. This voltage is com-
pared to the voltage at the PHASE node. This compari-
son is made only when the high-side drive is high to
avoid false current limit triggering due to uncontributing
measurements from the MOSFETs off-voltage. When
the voltage at PHASE is less than the voltage at OCSET,
an overcurrent condition occurs and the soft start cycle
is initiated. The synchronous switch turns off and SS/
SHDN starts to sink 1.5µA. When SS/SHDN reaches
0.8V, it then starts to source 10µA and a new cycle be-
gins.
Hiccup Mode
During power up, the SS/SHDN pin is internally pulled
low until VCC reaches the undervoltage lockout level of
4.2V. Once V
released and begins to source 10µA of current to the
external soft-start capacitor. As the soft-start voltage
rises, the output of the internal error amplifier is clamped
to this voltage. When the error signal reaches the level
of the internal triangular oscillator, which swings from
1V to 2V at a fixed frequency of 200kHz, switching oc-
curs. As the error signal crosses over the oscillator sig-
nal, the duty cycle of the PWM signal continues to in-
crease until the output comes into regulation. If an over-
current condition has not occurred the soft-start voltage
will continue to rise and level off at about 2.2V.
Applications Information (Cont.)
 2005 Semtech Corp.
POWER MANAGEMENT
DS(ON)
Current Limiting
CC
has reached 4.2V, the SS/SHDN pin is
CC
supply to OCSET. The volt-
6
An over-current condition occurs when the high-side drive
is turned on, but the PHASE node does not reach the
voltage level set at the OCSET pin. The PHASE node is
sampled only once per cycle during the valley of the tri-
angular oscillator. Once an over-current occurs, the high-
side drive is turned off and the low-side drive turns on
and the SS/SHDN pin begins to sink 1.5µA. The soft-
start voltage will begin to decrease as the 1.5µA of cur-
rent discharges the external capacitor. When the soft-
start voltage reaches 0.8V, the SS/SHDN pin will begin
to source 10µA and begin to charge the external capaci-
tor causing the soft-start voltage to rise again. Again,
when the soft-start voltage reaches the level of the in-
ternal oscillator, switching will occur.
If the over-current condition is no longer present, nor-
mal operation will continue. If the over-current condition
is still present, the SS/SHDN pin will again begin to sink
1.5µA. This cycle will continue indefinitely until the over-
current condition is removed.
In conclusion, below is shown a typical “12V Application
Circuit” which has a BSTH voltage derived by
bootstrapping input voltage to the PHASE node through
diode D1. This circuit is very useful in cases where only
input power of 12V is available.
In order to prevent substrate glitching, a small-signal di-
ode should be placed in close proximity to the chip with
cathode connected to PHASE and anode connected to
PGND.
SC2602L
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