lm2757tmx National Semiconductor Corporation, lm2757tmx Datasheet - Page 13

• Manufacturer Part Number: lm2757tmx
• Description: Switched Capacitor Boost Regulator With High Impedance Output In Shutdown
• Manufacturer: National Semiconductor Corporation

Application Circuits
USB OTG POWER SUPPLY
The 5V output mode will normally be used for the USB OTG
application. Therefore the LM2757 can be enabled/disabled
by applying a logic signal on only the M1 pin while grounding
the M0 pin. Depending on the USB mode of the application,
the LM2757 can be enabled to drive the USB power bus line
SUPERCAPACITOR FLASH DRIVER
Using the 5V output voltage mode, the LM2757 can be used
to charge a Supercapacitor for LED Flash applications while
limiting the peak current drawn off the battery during the
charge cycle. The LM2757 can be disabled for the Flash
event, placing its output in high impedance with the input. In
this way, all charge for the Flash LED(s) will come directly off
the Supercapacitor and not load the main battery line. The
LM2757 can be enabled/disabled by applying a logic signal
on only the M1 pin while grounding the M0 pin.
Special consideration must be taken when using Supercapi-
cators for LED flash applications where the voltage on the
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(Host), or disabled to put its output in high impedance allowing
an external supply to drive the bus line (Slave).
capacitor is charged to a fixed value. This is due to the pos-
sible power management issues that could arise as a result
of the high flash current and wide tolerance ranges (V–I char-
acteristics) of typical Flash LEDs. If the voltage across the
Flash LED(s) is not managed, damage could occur where a
relatively low Vf LED is overdriven or places excessive volt-
age across the bottom control FET. To help reduce this issue,
the use of a high power current sink is advised in applications
where the forward voltage specification of the Flash LED has
a wide range.
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