SC120 SEMTECH [Semtech Corporation], SC120 Datasheet - Page 14

• Manufacturer Part Number: SC120
• Description: Low Voltage Synchronous Boost Converter
• Manufacturer: SEMTECH [Semtech Corporation]

Applications Information (continued)
the inductance value, the input voltage, and the on-time
(D×T). Therefore, the instantaneous inductor current will
be alternately larger and smaller than the average. If the
average output current is suffi ciently small, the minimum
inductor current can reach zero during the off -state. If
the energy stored in the inductor is depleted (if the induc-
tor current decreases to zero) during the off -state, both
FETs turn off for the remainder of the off -state. If this dis-
continuous mode (DM) operation persists, the SC120
transitions to PSAVE operation.
PSAVE Operation
At light loads, the SC120 will operate in PSAVE mode.
PSAVE mode ensures proper regulation when V
close to V
duty cycle above its minimum value. At very low output
load, PSAVE mode will operate more effi ciently than PWM
mode. PSAVE operation is triggered by 256 consecutive
cycles of DM operation in PWM mode, when the output
of the P
low load current.
LIM
OUT
amplifi er falls to 0V during the off -state due to
while the output load is too small to keep the
Figure 2 — PSAVE Operation With Exit to PWM Due To Output Voltage Decay
Inductor
Current
V
OUT
Voltage
240mA
+1.8%
+0.8%
Prog’d
-2%
0A
BURST
PSAVE Mode at
Moderate Load
OFF
BURST
IN
PRELIMINARY
is too
Higher Load
OFF
Applied
Time
PSAVE mode requires fewer circuit resources than PWM
mode. All unused circuitry is disabled to reduce quies-
cent power dissipation. In PSAVE mode, the OUT pin
voltage monitoring circuit remains active and the output
voltage error amplifi er operates as a comparator. PSAVE
regulation is shown in Figure 2. When V
where V
fi xed-period switching occurs to boost the output voltage.
The n-channel FET turns on (on-state) until the inductor
current rises to approximately 240mA. Then the n-channel
FET turns off and the p-channel FET turns on to transfer
the inductor energy to the output capacitor and load for
the duration of the off-state. This cycle repeats until
V
The output capacitor then discharges into the load until
V
When the output current increases above a predeter-
mined level, either of two PSAVE exit conditions will force
the resumption of PWM operation. The fi rst PSAVE exit
criterion is shown in Figure 2. If the PSAVE burst cycle
cannot provide sufficient current to the output, the
output voltage will decrease during the burst. If
OUT
OUT
> 1.018×V
< 1.008×V
BURST
REG
to output decay
PSAVE exit due
is the programmed output voltage, a burst of
REG
REG
, and the burst cycle repeats.
, at which point bot FETs are turned off .
PWM Mode at
High Load
PWM Mode
OUT
< 1.008xV
SC120
REG
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