sc202amltrt Semtech Corporation, sc202amltrt Datasheet - Page 14

• Manufacturer Part Number: sc202amltrt
• Description: 3.5mhz, 500ma Step-down Regulator With Integrated Inductor And Digital Programmable Output
• Manufacturer: Semtech Corporation

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
+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
until the upper threshold again is reached. Note that the
output voltage is regulated hysteretically while in PSAVE
mode between V
cycle while in PSAVE mode are solely determined by V
and V
switching frequency being much lower than the PWM
mode frequency.
If the output load current increases enough to cause V
to decrease below the PSAVE exit threshold (V
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
undershoot when the SC202A exits PSAVE. Figure 2 illus-
trates the transitions from PWM mode to PSAVE mode and
back to PWM mode.
Figure 2 — Transitions Between PWM and PSAVE Modes
V
Demand
OUT
V
Load
(I
OUT
OUT
+2.5%
OUT
V
V
PWM Mode at
OUT
-2%
Medium/High
LX
)
until PWM mode resumes. This can result in the
Load
OUT
and V
BURST
OFF
PSAVE Mode at
OUT
Light Load
Time
+ 2.5%. The period and duty
OUT
PWM Mode at
Medium/High
OUT
and continues
Load
PSAVE
to minimize
OUT
EXIT
-2%), the
OUT
OUT
IN
Protection Features
The SC202A provides the following protection features:
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
steps, the device continues in PSAVE mode at the end of
soft-start; otherwise, it goes into PWM mode. Note the
V
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
exceeds the regulation voltage by 15%, the PWM drive is
disabled. Switching does not resume until V
below the regulation voltage by 2%.
Current Limit
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
resistance. The load must fall below I
exit fold-back current limit mode. This function makes the
device capable of sustaining an indefinite short circuit on
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
are both disabled, tri-stating the LX output. When the
junction temperature drops by the hysteresis value (20°C),
OUT
•
•
•
•
•
ripple in PSAVE mode can be larger than the ripple in
Soft-Start Operation
Over-Voltage Protection
Current Limit
Thermal Shutdown
Under-Voltage Lockout
OUT
reaches 90% of the target within the first 2
FB-LIM
FB-LIM
for the device to
OUT
and the load
has fallen
OUT
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