5
2007 Semtech Corp.
www.semtech.com
SC2603A
POWER MANAGEMENT
Applications Information
Theory of Operation
The SC2603A is a versatile, low-cost, voltage-mode PWM
controller designed for DC/DC power supply applications.
In normal condition, the SC2603A operates at a xed
200kHz. One exception is that when the FB pin voltage
drops to one half, the frequency will be shifted to one
fourth of 200kHz.
SC2603A features include a temperature compensated
voltage reference, triangle wave oscillator, current limit
comparator, frequency shift over-current protection, and
an internally compensated error amplier. Pulse by pulse
current limiting is implemented by sensing the differential
voltage across an external resistor, or an appropriately
sized PC board trace.
Setting the Output Voltage
As shown in Fig.1, an external resistive divider R2 and R4
with its center tap tied to the FB pin sets the output volt-
age.
Soft-Start
Soft-start prevents a DC-DC converter from drawing exces-
sive current from the power source during start up. If the
soft-start time is made sufciently long, then the output will
enter regulation without overshoot. An external capacitor
is connected from the EN/SS pin to the ground. After the
part’s input voltage surpasses the UVLO, the EN/SS pin is
allowed to charge its capacitor with 60µA. When 100mV
is reached at the EN/SS pin, the internal comp node is at
0.4V. The SC2603A will start switching, and the converter
output comes into regulation. Because the FB pin voltage
follows EN/SS pin voltage, nally, the EN/SS pin voltage will
be clamped at 1.25V same as the reference voltage.
Under Voltage Lockout
The under voltage lockout circuit of the SC2603A assures
that the Gate output remains in the off state whenever the
supply voltage drops below set parameters. Lockout occurs
if Vin falls below 3.8V. Normal operation resumes once Vin
rises above 4.2V.
Maximum Duty Cycle
The maximum duty cycle, D
max
denes the upper limit of
power conversion ratio
Clamping Diode D2 at Vin Pin
Since the Vin pin of the SC2603A is rated at 20V, when
the supply voltage is higher than 16V, a Zener diode D2
is required to limit the SC2603A supply voltage, as shown
in Fig.1. The clamping voltage of D2 has to be not higher
than 15V.
Over Current Protection (OCP)
The over-current protection is implemented by a sensing
resistor and an internal current-limit comparator with a
threshold of 70mV. In each switching cycle, if the switching
current through the sensing resistor causing the voltage at
the CS pin reaches 70mV, the SC2603A will immediately
turn the gate pulse off to limit the current through the
power switch.
The gure below shows the current waveforms run into
over-current protection. At t1, the input current Iin begins
to increase as the load Iout builds up until it hits the OCP
threshold at t2. At t2, Vout begins to drop as Iout starts
increasing toward t3 while Vin and Iin remain the same.
At t3, there two events are happening. 1) Vout is at half of
its regulating voltage. At this point, the internal oscillator
runs into frequency shift mode and shifts the frequency to
one fourth of the operating frequency. 2) The diode starts
conducting current without limit causing Vin to fall due to
supply current limit. Finally, Vin and Vout both drops to zero
and Iin becomes the input supply current limit.
A RC lter at the CS pin is necessary to lter out noise to
ensure accurate sensing.
The value of the sensing resistor can be calculated by the
following equation:
Rs = 70mV/Ipeak
−= 1
V25.1
V
RR
OUT
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