SC198AMLTRT Semtech, SC198AMLTRT Datasheet - Page 11

• Manufacturer Part Number: SC198AMLTRT
• Description: IC DUAL DC-DC BUCK CONV 20-MLPQ
• Manufacturer: Semtech
• Type: Step-Down (Buck)r

Specifications of SC198AMLTRT

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: Programmable
• Current - Output: 800mA
• Frequency - Switching: 1MHz
• Voltage - Input: 2.7 ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 20-MLPQ
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: SC198AMLTR

Application Information (continued)
In PSAVE mode (see Figure 1), VOUT regulation is con-
trolled by bursts in switching. While the output voltage is
between the low and high voltage thresholds, switching
is stopped to reduce loss. When the voltage drops to the
lower threshold, the switch bursts at a fi xed on time until
the upper threshold is reached. At this point the output
voltage is allowed to ramp down to the lower threshold
without switching to conserve power. In the PSAVE mode,
less switching is used. This results in less power loss which
can be seen in the diff erence of effi ciency between PWM
mode and PSAVE mode at light loads.
The PSAVE switching burst frequency is controlled such
that the inductor current ripple during the burst is similar
to that in PWM mode.
The SC198A automatically detects when to exit PSAVE
mode. The device sets a maximum peak current in PSAVE
mode, and then detects when VOUT falls due to the load
increasing above the level that the peak current will
support.
For the SC198A to exit PSAVE mode, the load must be
increased, which causes VOUT to decrease until the PSAVE
exit threshold is reached. PSAVE levels are set high to
minimize the undershoot when the SC198A exits PSAVE.
Figure 1 shows PSAVE mode operation and exiting into
PWM mode at increased load.
Thershold
Threshold
Thershold
Inductor
Current
PSAVE
Upper
Lower
Limit
Limit
V
Exit
OUT
0A
BURST
PSAVE Mode at
Moderate Load
Figure 1 — PSAVE Operation
OFF
BURST
Higher Load
OFF
Applied
Time
PSAVE Mode at
High Load
BURST
PWM Mode at
High Load
PWM Mode
Protection Features
The SC198A provides the following protection features:
Thermal Shutdown
The SC198A has a thermal shutdown feature to protect
against damage if the junction temperature exceeds
145°C. In thermal shutdown, the on-chip power devices
are disabled, tri-stating the LX output. Switching resumes
when the temperature drops by 10°C. During this time, if
the output voltage decreases by more than 60% of its
programmed value, a soft-start will be invoked.
Current Limit
The internal power devices are protected by current limit
functions. In the event of a short to ground on the output,
the part enters frequency foldback mode. This causes the
switching frequency to decrease by a factor determined
by the output voltage. This prevents the inductor current
from staircasing.
Over-Voltage Protection
In the event of an over-voltage on the output in switcher
mode, the PWM drive is disabled, eff ectively tri-stating
the LX output. The part will not resume switching until
the output voltage has fallen below 2% of the regulation
voltage.
Soft-Start
The soft-start mode operates by limiting the current
through the inductor and controlling the switching fre-
quency. The current limit is increased over several
milliseconds in discrete steps. This has the desired eff ect
of limiting in-rush current from the input supply. The soft-
start mode is cancelled once the output reaches
regulation. Soft-start is only re-enabled by power cycling,
toggling enable, a UVLO event, or shutdown.
•
•
•
•
Thermal Shutdown
Current Limit
Over-Voltage Protection
Soft-Start
SC198A
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