3.3 µF

1 µF

1 µF

LM27951

D1

3.3 µF

EN

Voltage

Reference

1x/1.5x

Charge

Pump

Current

Control

Gain

Select

OSC

D4

D2

D3

PWM

Regulated

Current Sources

GND

7

LM27951

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SNVS416C – NOVEMBER 2005 – REVISED FEBRUARY 2016

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7 Detailed Description

7.1 Overview

The LM27951 is an adaptive 1.5×/1× CMOS charge pump, optimized for driving white LEDs used in small-format

display backlighting. It provides four constant current outputs capable of sourcing up to 30 mA through each

LED. The well-matched current sources ensure the current through all the LEDs are virtually identical, providing

a uniform brightness across the entire display.

modulated (PWM) signal to the dedicated PWM input pin.

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Charge Pump

loosely regulated charge pump of the device has both open loop and closed loop modes of operation. When the

actively selected to maintain regulation based on LED forward voltage and load requirements. This allows the

charge pump to stay in the most efficient gain (1×) over as much of the input voltage range as possible, reducing

the power consumed from the battery.

7.3.2 Soft Start

The LM27951 contains internal soft-start circuitry to limit input inrush currents when the part is enabled. Soft start

is implemented internally with a controlled turnon of the internal voltage reference. Due to the soft-start circuitry,

startup time of the LM27951 is approximately 330 µs (typical).

7.3.3 Thermal Protection

Internal thermal protection circuitry disables the LM27951 when the junction temperature exceeds 150°C

(typical). This feature protects the device from being damaged by high die temperatures that might otherwise

result from excessive power dissipation. The device recovers and operate normally when the junction

temperature falls below 140°C (typical). It is important that the board layout provide good thermal conduction to

keep the junction temperature within the specified operating ratings.