Datasheet

LM2757
Supercapacitor
Flash Charger
Flash
LED(s)
V
BAT
Charge (M1)
M0
V
OUT
= 5V
Flash
2.7 k:
2.7 k:
0.6F, 5V, Dual
Supercapacitor
R R
USB OTG
Transceiver
LM2757 (Host
Mode V
BUS
Power)
Dual Role
Application
Processor
EN (V
M1
)
V
BAT
(System Voltage)
V
OUT
/ V
BUS
(5V)
ID
D+
D-
GND
V
BUS
V
M0
USB Connector
LM2757
SNVS536E –OCTOBER 2007–REVISED MAY 2013 www.ti.com
Application Circuits
USB OTG / Mobile HDMI Power Supply
The 5V output mode will normally be used for the USB OTG / Mobile HDMI 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/HDMI mode of the application, the LM2757 can be enabled to drive the power bus line (Host), or
disabled to put its output in high impedance allowing an external supply to drive the bus line (Slave). In addition
to the high impedance-backdrive protection, the output current limit protection is 250mA (typ.), well within the
USB OTG and HDMI requirements.
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 Supercapicators for LED flash applications where the voltage
on the capacitor is charged to a fixed value. This is due to the possible power management issues that could
arise as a result of the high flash current and wide tolerance ranges (V–I characteristics) 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 voltage 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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