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SC202A
Applications Information (continued)
setting, optimizing efficiency for all possible load currents
in PWM or PSAVE mode. While in PSAVE mode, output
voltage regulation is controlled by a series of switching
bursts. During a burst, the inductor current is limited to a
peak value which controls the on-time of the PMOS switch.
After reaching this peak, the PMOS switch is disabled and
the inductor current decreases to near 0mA. Switching
bursts continue until the output voltage climbs to V OUT
+2.5% or until the PSAVE current limit is reached. Switching
is then stopped to eliminate switching losses, enhancing
overall efficiency. Switching resumes when the output
voltage reaches the lower threshold of V OUT and continues
until the upper threshold again is reached. Note that the
output voltage is regulated hysteretically while in PSAVE
mode between V OUT and V OUT + 2.5%. The period and duty
cycle while in PSAVE mode are solely determined by V IN
and V OUT until PWM mode resumes. This can result in the
switching frequency being much lower than the PWM
mode frequency.
If the output load current increases enough to cause V OUT
to decrease below the PSAVE exit threshold (V OUT -2%), the
device automatically exits PSAVE and operates in continu-
ous PWM mode. Note that the PSAVE high and low
threshold levels are both set at or above V OUT to minimize
undershoot when the SC202A exits PSAVE. Figure 2 illus-
trates the transitions from PWM mode to PSAVE mode and
back to PWM mode.
Protection Features
The SC202A provides the following protection features:
? Soft-Start Operation
? Over-Voltage Protection
? Current Limit
? Thermal Shutdown
? Under-Voltage Lockout
Soft-Start
The soft-start sequence is activated after a transition from
an all zeros CTL code to a non-zero CTL code enables the
device. At start-up, the PMOS current limit is stepped
through four levels: 25%, 40%, 60%, and 100%. Each step
is maintained for 60μs following an internal reference start
up of 20μs, resulting in a total nominal start-up period of
260μs. If V OUT reaches 90% of the target within the first 2
steps, the device continues in PSAVE mode at the end of
soft-start; otherwise, it goes into PWM mode. Note the
V OUT ripple in PSAVE mode can be larger than the ripple in
PWM mode.
Over-Voltage Protection
Over-voltage protection ensures the output voltage does
not rise to a level that could damage its load. When V OUT
exceeds the regulation voltage by 15%, the PWM drive is
disabled. Switching does not resume until V OUT has fallen
below the regulation voltage by 2%.
Current Limit
Load
Demand
( I OUT )
V OUT +2.5%
V OUT
V OUT -2%
OFF
BURST
The SC202A switching stage is protected by a current limit
function. If the output load exceeds the PMOS current
limit for 32 consecutive switching cycles, the device enters
fold-back current limit mode and the output current is
limited to approximately 150mA. Under these conditions,
the output voltage will be the product of I FB-LIM and the load
resistance. The load must fall below I FB-LIM for the device to
exit fold-back current limit mode. This function makes the
device capable of sustaining an indefinite short circuit on
V LX
PWM Mode at
Medium/High
Load
PSAVE Mode at
Light Load
Time
PSAVE
EXIT
PWM Mode at
Medium/High
Load
its output under fault conditions.
Thermal Shutdown
The SC202A has a thermal shutdown feature to protect
the device if the junction temperature exceeds 160°C.
During thermal shutdown, the PMOS and NMOS switches
Figure 2 — Transitions Between PWM and PSAVE Modes
are both disabled, tri-stating the LX output. When the
junction temperature drops by the hysteresis value (20°C),
14
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