lm27953tlx National Semiconductor Corporation, lm27953tlx Datasheet - Page 8

• Manufacturer Part Number: lm27953tlx
• Description: White Led Driver With Four Led Current Sinks And 3/2x Switched Capacitor Boost
• Manufacturer: National Semiconductor Corporation

www.national.com
Block Diagram
Circuit Description
OVERVIEW
The LM27953 is primarily intended for Lithium-Ion battery
driven white-LED drive applications, and is well suited to
drive white LEDs that are used for backlighting small-format
displays. The part has four matched constant-current out-
puts, each capable of driving up to 20mA (or more) through
white LEDs. The well-matched current sources ensure the
current through all the LEDs is virtually identical. This keeps
brightness of all LEDs matched to near perfection so that
they can provide a consistent backlight over the entire dis-
play.
CHARGE PUMP
The core of the LM27953 is a 1.5x/1x dual-mode charge
pump. The input of the charge pump is connected to the V
pin. The recommended input voltage range of the LM27953
is 2.7V to 5.5V. The output of the charge pump is the P
pin (“Pump OUTput”). The output voltage of the charge
pump is unregulated and varies with input voltage and load
current.
The charge pump operates in the 1.5x mode when the input
voltage is below 4.75V (typ.). In this mode, the input-to-
output voltage gain of the charge pump is 1.5, and the
voltage at the output of the charge pump will be approxi-
mately 1.5x the input voltage (V(P
the 1.5x mode, the charge pump provides the voltage boost
that is required to drive white LEDs from a Li-Ion battery.
OUT
) ≈ 1.5 * V
IN
). When in
OUT
IN
8
(White LEDs typically have a forward voltage in the range of
3.3V to 4.0V. A Li-Ion battery typically is not considered to be
fully discharged until the battery voltage falls to 3.0V (ap-
prox.) )
The charge pump operates in the 1x mode when the input
voltage is above 4.75V (typ.). In these conditions, voltage
boost is not required to drive the LEDs, so the charge pump
merely passes the input voltage to P
This reduces the input current and the power dissipation of
the LM27953 when the input voltage is high.
REGULATED CURRENT OUTPUTS
The matched current outputs are generated with a precision
current mirror that is biased off the charge pump output.
Matched currents are ensured with the use of tightly
matched internal devices and internal mismatch cancellation
circuitry.
There are four regulated common anode current outputs.
There is an ON/OFF control pin for the group (EN).
The DC current through the LEDs is programmed with an
external resistor. Changing currents on-the-fly can be
achieved with the use of digital pulse (PWM) signals.
ENABLE PIN: EN
The LM27953 has an enable pin that has active-high logic
(HIGH = ON). There is an internal pull-down resistor (300kΩ
typ.) that is connected internally between the enable pin and
GND.
OUT
(V(P
OUT
20128003
) ≈ V
IN
).