SC417/SC427
18
the output of the switching regulator soft-discharges the
output into a 15& internal resistor via the V
OUT
pin. When
EN/PSV is allowed to float, the pin voltage will float to 33%
of the voltage at V5V. The switching regulator turns on
with power-save disabled and all switching is in forced
continuous mode.
When EN/PSV is high (above 44% of the voltage at V5V)
for SC417, the switching regulator turns on with ultra-
sonic power-save enabled. The SC417 ultra-sonic power-
save operation maintains a minimum switching frequency
of 25kHz, for applications with stringent audio
requirements.
When EN/PSV is high (above 44% of the voltage at V5V)
for SC427, the switching regulator turns on with power-
save enabled. The SC427 power-save operation is designed
to maximize efficiency at light loads with no minimum
frequency limits. This makes the SC427 an excellent choice
for portable and battery-operated systems.
Forced Continuous Mode Operation
The SC417/SC427 operates the switcher in Forced
Continuous Mode (FCM) by floating the EN/PSV pin (see
Figure 4). In this mode one of the power MOSFETs is
always on, with no intentional dead time other than to
avoid cross-conduction. This feature results in uniform
frequency across the full load range with the trade-off
being poor efficiency at light loads due to the high-fre-
quency 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
Ultra-sonic Power-save Operation (SC417)
The SC417 provides ultra-sonic power-save operation at
light loads, with the minimum operating frequency fixed
at 25kHz. This is accomplished using an internal timer that
monitors the time between consecutive high-side gate
pulses. If the time exceeds 40祍, 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 threshold, the next DH on-time is trig-
gered. After the on-time is completed the high-side
MOSFET is turned off and the low-side MOSFET turns on.
The low-side MOSFET remains on until the inductor
current ramps down to zero, at which point the low-side
MOSFET is turned off.
Because the on-times are forced to occur at intervals no
greater than 40祍, the frequency will not fall below
~25kHz. Figure 5 shows ultra-sonic power-save
operation.
Applications Information (continued)