lm2758tlx National Semiconductor Corporation, lm2758tlx Datasheet - Page 8

• Manufacturer Part Number: lm2758tlx
• Description: Switched Capacitor Flash Led Driver In Micro Smd
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
CIRCUIT DESCRIPTION
The LM2758 is an adaptive 1x and 1.5x CMOS charge pump,
optimized for driving Flash LEDs in camera phone and other
portable applications. It provides a constant current of up to
500 mA (typ.) for Flash mode and 107 mA (typ.) for Torch
mode.
There are four modes of operation for LM2758: the Flash
Mode, Torch Mode, Indicator Mode and Shutdown Mode (see
EN1 and EN2 truth table). Torch and Flash modes sink a
constant DC current while Indicator mode operates in pulsat-
ing DC at 1/32 positive duty cycle with same current magni-
tude as Torch mode. The LED is driven from CP
connected to the current sink. LED drive current mode is pro-
grammed by connecting a resistor, R
pin, I
or 1.5x) and adjustable impedance (R
put voltage that would account for LED forward voltage drop
and headroom for the current sink to drive desired current
through LED.
CHARGE PUMP AND GAIN TRANSITIONS
The input to the 1x/1.5x charge pump is connected to the VIN
pin, and the loosely regulated output of the charge pump is
connected to the CP
voltage is less than 4.7V, the output voltage is approximately
equal to the input voltage. When input voltage is over 4.7V
the output voltage gets regulated to 4.7V. In 1.5x mode, the
output voltage is always less than or equal to 4.7V over entire
input voltage range.
The charge pump’s gain is selected depending on the head-
room voltage across the current sink of LM2758. When head-
room voltage V
the charge pump gain transition happens from 1x to 1.5x to
maintain current regulation across the LED. Once the charge
pump transition to a higher gain, it will remain at that gain for
as long as the device remains enabled. Shutting down and
then re-enabling the device will resets the gain mode to the
minimum gain required to maintain the load.
SOFT START
The LM2758 contains internal soft-start circuitry to limit inrush
currents when the part is enabled. Soft start is implemented
internally with a controlled turn-on of the internal voltage ref-
erence.
CURRENT LIMIT PROTECTION
The LM2758 charge pump contains current limit protection
circuitry that protects the device during V
SET
. LM2758 also controls CP
GDX
(at LED pin) drops below 300 mV (typ.)
OUT
pin. In 1x mode, as long as the input
OUT
OUT
SET
with variable gain (1x
OUT
, to the current set
) to provide an out-
fault conditions
OUT
and
8
where excessive current is drawn. Output current is limited to
1.2A typically.
LOGIC CONTROL PINS
LM2758 has two logic pins, EN1 and EN2. The operating
modes of the part function according to the tables below:
FLASH TIME-OUT FEATURE
Flash Time-out Protection Circuitry disables the current sinks
when the signal on EN1 and EN2 is held high for more than
814 msec (typ.). This prevents the device from self-heating
due to the high power dissipation during Flash conditions.
During the time-out condition, voltage will still be present on
CP
current through the Flash LED. When the device goes into a
time-out condition, placing a logic Low signal on EN1 and EN2
will reset the time-out; a subsequent logic High signal on EN1
or EN2 will return the device to normal operation.
SETTING LED CURRENTS
The current through the LED can be set by connecting an
appropriately sized resistor R
LM2758 and GND.
The LED current in Torch mode is approximately 1639 times
greater than the current of I
mode is approximately 7650 times of the same I
The feedback loop of an internal amplifier sets the voltage of
the I
in the equations below:
The maximum recommended current through LED is 500 mA
in Torch mode and 700 mA in Flash mode. Using the part in
conditions where the junction temperature might rise above
the rated maximum requires that the operating ranges and/or
conditions be de-rated. The printed circuit board also must be
carefully laid out to account for high thermal dissipation in the
part.
OUT
SET
EN1
0
1
0
1
but the current sinks will be shut off, resulting in no
pin to 1.3V (typ.). The statements above are simplified
I
LED
= GAIN
EN2
0
0
1
1
FLASH/TORCH
SET
SET
, while the LED current in Flash
between the I
x (1.3/R
Shutdown
Indicator
Mode
Torch
Flash
SET
SET
)
SET
pin of the
current.