6 2006 Semtech Corp.
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POWER MANAGEMENT
SC1102/SC1102A
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
DS(ON)
Current Limiting
The current limit threshold is set by connecting an exter-
nal resistor from the V
CC
supply to OCSET. The voltage
drop across this resistor is due to the 200µA internal
sink sets the voltage at the pin. This voltage is compared
to the voltage at the PHASE node. This comparison is
made only when the high-side drive is high to avoid false
current limit triggering due to uncontributing measure-
ments 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 2µA. When SS/SHDN reaches 0.8V, it then starts
to source 10µA and a new cycle begins.
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
CC
has reached 4.2V, the SS/SHDN pin is
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/500kHz, switching
occurs. As the error signal crosses over the oscillator
signal, 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.)
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 2uA. The soft-start
voltage will begin to decrease as the 2uA of current dis-
charges the external capacitor. When the soft-start volt-
age reaches 0.8V, the SS/SHDN pin will begin to source
10uA and begin to charge the external capacitor causing
the soft-start voltage to rise again. Again, when the soft-
start voltage reaches the level of the internal oscillator,
switching will occur.
If the over-current condition is no longer present, normal
operation will continue. If the over-current condition is
still present, the SS/SHDN pin will again begin to sink
2uA. 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.
