lm2791ldx-l National Semiconductor Corporation, lm2791ldx-l Datasheet - Page 8

• Manufacturer Part Number: lm2791ldx-l
• Description: Current Regulated Switched Capacitor Led Driver With Analog Brightness Control
• Manufacturer: National Semiconductor Corporation

www.national.com
Circuit Description
The LM2791 provides two matched current sources for driv-
ing high forward voltage drop LEDs from Li-Ion battery
sources. The device has on-chip current regulators which
are composed of current mirrors with a 25 to 1 ratio. The
mirrors control the LED current without using current limiting
resistors in the LED current path. The device can drive up to
a total of 36mA through the LEDs.
Application Information
SOFT START
LM2791 includes a soft start function to reduce the inrush
currents and high peak current during power up of the de-
vice. Soft start is implemented internally by ramping the
bandgap more slowly than the applied voltage. This is done
by holding the bandgap in shutdown for a short time. During
soft start, the switch resistances limit the inrush current used
to charge the flying and hold capacitors.
SHUTDOWN MODE
A shutdown pin (SD or /SD) is available to disable the
LM2791 and reduce the quiescent current to 1µA maximum.
The LM2791 is available with both senses of shutdown
polarity.
During normal operation mode of the ’-L’ options, an active
high logic signal to the SD pin or tying the SD pin to V
enable the device. Pulling SD low or connectingSD to
ground will disable the device.
During normal operation mode of the ’-H’ options, an active
low logic signal to the SD pin or tying the SD pin to GND, will
enable the device. Pulling SD high or connecting SD to V
will disable the device.
IN
, will
FIGURE 1.
IN
8
The LED brightness can be controlled by both analog and or
digital methods. The digital technique uses a PWM (Pulse
Width Modulation) signal applied to the shutdown input. The
analog technique applies an analog voltage to the brightness
(BRGT) pin (see Application Information sections). For low-
est cost, the LM2791 can be used for constant brightness by
grounding BRGT and enabling the shutdown pin.
CAPACITOR SELECTION
Low equivalent series resistance (ESR) capacitors such as
X5R or X7R are recommended to be used for C
and C
less than or equal to 0.3 ohms ESR value are recommended
for this application. Table 1 below lists suggested capacitor
suppliers for the typical application circuit.
SCHOTTKY DIODE SELECTION
A schottky diode (D1) must be used between V
for proper operation. During start-up, the low voltage drop
across this diode is used to charge C
oscillator. It is necessary to protect the device from
Manufacturer
Taiyo Yuden
MuRata
HOLD
TDK
TABLE 1. Low ESR Capacitor Manufactures
for best performance. Ceramic capacitors with
(847) 803
(800) 831
(800) 348
Contact
6100
9172
2496
www.component.tdk.com
www.t-yuden.com
www.murata.com
OUT
website
20018304
and start the
IN
IN
and P
, C1, C2,
OUT