LM27961TLEV National Semiconductor, LM27961TLEV Datasheet - Page 9

• Manufacturer Part Number: LM27961TLEV
• Description: BOARD EVALUATION LM27961TL
• Manufacturer: National Semiconductor

Specifications of LM27961TLEV

• Current - Output / Channel: 20mA
• Outputs And Type: 7, Non-Isolated
• Voltage - Output: 4 V
• Features: Charge Pump
• Voltage - Input: 2.7 ~ 5.5V
• Utilized Ic / Part: LM27961
• Lead Free Status / RoHS Status: Not applicable / Not applicable

Block Diagram
Circuit Description
OVERVIEW
The LM27961 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 seven 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 LM27961 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 LM27961
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-
OUT
IN
9
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.
(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 LM27961 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 seven regulated current outputs. These seven
outputs are split into two groups, a group of 4 common
OUT
) ≈ 1.5 * V
OUT
(V(P
IN
OUT
20127903
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). When in
) ≈ V
IN
).