SC418
5
maintain the desired on-time; on-time is calculated
according to the voltage at the VOUT pin.
V
OUT
Voltage Selection
The switcher output voltage is regulated by comparing
V
OUT
as seen through a resistor divider at the FB pin to the
internal 500mV reference voltage (see Figure 3).
R
1
To FB pin
R
2
V
OUT
Figure 3 Output Voltage Selection
Note that this control method regulates the valley of the
output ripple voltage, not the DC value. The DC value of
V
OUT
is offset by the output ripple according to the follow-
ing equation.
2
V
R
R
1
5
.
0
V
RIPPLE
2
1
OUT
Forced Continuous Mode Operation
The SC48 operates the switcher in Forced Continuous
Mode (FCM) by connecting the PSV pin to VDDA. (The PSV
pin should never exceed the VDDA supply.) See Figure 4
for FCM waveforms. In this mode one of the power
MOSFETs is always on, with no intentional dead time other
than to avoid cross-conduction. This results in more
uniform frequency across the full load range with the
trade-off being reduced efficiency at light loads due to
the high-frequency switching of the MOSFETs.
FB Ripple
Voltage (V
FB
)
FB threshold
DL
DH
Inductor
Current
DC Load Current
DH on-time is triggered when
V
FB
reaches the FB Threshold.
(500mV)
On-time
(T
ON
)
DL drives high when on-time is completed.
DL remains high until V
FB
falls to the FB threshold.
Figure 4 Forced Continuous Mode Operation
Programmable Ultrasonic Power-Save Operation
The device provides programmable ultrasonic power-save
operation at light loads; the minimum operating fre-
quency is programmed by connecting a resistor from PSV
to AGND. The SC48 uses the PSV resistor to set an inter-
nal timer that monitors the time between consecutive
high-side gate pulses. If the time exceeds the programmed
timer, DL drives high to turn the low-side MOSFET on. This
draws current from V
OUT
through the inductor, forcing both
V
OUT
and V
FB
to fall. When V
FB
drops to the 500mV thresh-
old, the next DH on-time is triggered. After the on-time is
completed the high-side MOSFET is turned off and the
low-side MOSFET turns on, and the internal timer is
restarted. The low-side MOSFET remains on until the
inductor current ramps down to zero, at which point the
low-side MOSFET is turned off. This ends the cycle until V
FB
falls below the 500mV threshold, or the internal timer
forces another DL turn-on.
Because the period between on-times is limited to a
maximum value, a minimum operating frequency is main-
tained. Figure 5 shows ultrasonic power-save operation.
Applications Information (continued)
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