SC412A
POWER MANAGEMENT
Applications Information
SC412A Synchronous Buck Controller
The SC412A is a synchronous power supply controller which
simplifies the task of designing a power supply suitable for
powering low voltage circuits.
Battery and +5V Bias Supplies
The SC412A requires an external +5V bias supply in ad-
dition to the battery. If stand-alone capability is required,
the +5V supply can be generated with an external linear
regulator.
Pseudo-Fixed-Frequency Constant On-Time
PWM Controller
The PWM control method is a constant-on-time, pseudo-
fi
xed-frequency PWM controller, see Figure 1. The ripple
voltage seen across the output capacitor’s ESR provides
the PWM ramp signal, eliminating the need for a current
sense resistor. The on-time is determined by an internal
one-shot whose period is proportional to output voltage,
and inversely proportional to input voltage. A separate
one-shot sets the minimum off-time (typically 350ns).
The typical operating frequency is 325kHz. It is possible
to raise or lower the operating frequency with external
components, refer to the section on Switching Frequency
Variations.
TON
VBAT
V
PHASE
C
IN
V
PHASE
L
Q2
ESR
V
OUT
/FB Ripple
FB Threshold
0.75V
On-Time One-Shot (T
ON
)
The internal on-time one-shot comparator has two inputs.
One input looks at the output voltage via the VOUT pin,
while the other input samples the input voltage via the LX
pin and converts it to a proportional current which charges
an internal on-time capacitor.
The TON time is the time required for this capacitor to
charge from zero volts to VOUT, thereby making the on-
time directly proportional to output voltage and inversely
proportional to input voltage. This implementation results
in a fairly constant switching frequency without the need of
a clock generator. The internal frequency is optimized for
325kHz. The general equation for the on-time is:
T
ON
(nsec) = 2560 • (V
OUT
/V
BAT
) + 35
VOUT Voltage Selection
Output voltage is regulated by comparing VOUT as seen
through a resistor divider to the internal 0.75V reference,
see Figure 2. The output voltage is set by the equation:
V
OUT
= 0.75 • (1 + R1/R2)
VOUT
R1
R2
To FB pin
Q1
V
OUT
+
C
OUT
Figure 2.
Figure 1.
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