Application Information

CIRCUIT DESCRIPTION

The LM27951 is an adaptive 1.5x/1x CMOS charge pump,

optimized for driving white LEDs used in small-format dis-

play backlighting. It provides four constant current outputs

capable of sourcing up to 30mA through each LED. The

well-matched current sources ensure the current through all

the LEDs are virtually identical, providing a uniform bright-

ness across the entire display.

Each current source is internally connected to the charge

pump output, V

connecting a resistor, R

brightness is adjusted by applying a Pulse Width Modulated

(PWM) signal to the dedicated PWM input pin.

CHARGE PUMP

The input to the 1.5x/1x charge pump is connected to the V

IN

pin, and the loosely regulated output of the charge pump is

connected to the V

range of the LM27951 is 3.0V to 5.5V. The device’s loosely-

regulated charge pump has both open loop and closed loop

modes of operation. When the device is in open loop, the

voltage at V

input. When the device is in closed loop, the voltage at V

OUT

is loosely regulated to 4.5V (typ.). The charge pump gain

transitions are 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 (1x) over

as much of the input voltage range as possible, reducing the

power consumed from the battery.

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 turn-on of the inter-

nal voltage reference. Due to the soft-start circuitry, startup

time of the LM27951 is approximately 330µs (typ.).

ENABLE AND PWM PINS

The LM27951 has 2 logic control pins. Both pins are active-

high logic (HIGH = ON). There is an internal pull-down

resistor (150k

Ω typ.) connected between the enable pin (EN)

and GND. There is no pull-up or pull-down connected to the

Pulse Width Modulated (PWM) pin.

The EN pin is the master enable pin for the part. When the

mode. In this mode, all internal circuitry is OFF and the part

the charge pump and regulate the output voltage to its

nominal value.

The PWM pin serves as a dedicated logic input for LED

brightness control. When the voltage on this pin is low

will flow through the LEDs. When the voltage on this pin is

to the current level set by the resistor connected to the I

SET

pin.

SETTING LED CURRENTS

The current through the four LEDs connected to D

set to a desired level simply by connecting an appropriately

sized resistor (R

and GND. The LED currents are proportional to the current

that flows out of the I

greater than the I

nal amplifier sets the voltage of the I

The statements above are simplified in the equations below:

I

R

ADJUSTING LED BRIGHTNESS (PWM control)

Perceived LED brightness can be adjusted using a PWM

control signal on the LM27951 PWM logic input pin, turning

the current sources ON and OFF at a rate faster than per-

ceptible by the human eye. When this is done, the total

brightness perceived is proportional to the duty cycle (D) of

the PWM signal (D = the percentage of time that the LED is

on in every PWM cycle). A simple example: if the LEDs are

driven at 15mA each with a PWM signal that has a 50% duty

cycle, perceived LED brightness will be about half as bright

as compared to when the LEDs are driven continuously with

15mA.

The minimum recommended PWM frequency is 100Hz. Fre-

quencies below this may be visible as flicker or blinking. The

maximum recommended PWM frequency is 1kHz. Frequen-

cies above this may cause interference with internal current

driver circuitry and/or noise in the audible range. Due to the

regulation control loop, the maximum frequency and mini-

mum duty cycle applied to the PWM pin should be chosen

such that the minimum ON time is no less than 30µs in

duration. If a PWM signal is applied to the EN pin instead,

the maximum frequency and minimum duty cycle should be

chosen to accommodate both the LM27951 startup time

(330µs typ.) and the 30µs control loop delay.

The preferred method to adjust brightness is to keep the

master EN voltage ON continuously and apply a PWM signal

to the dedicated PWM input pin. The benefit of this type of

connection can be best understood with a contrary example.

When a PWM signal is connected to the master enable (EN)

pin, the charge pump repeatedly turns on and off. Every time

the charge pump turns on, there is an inrush of current as the

capacitors, both internal and external, are recharged. This

inrush current results in a current spike and a voltage dip at

the input of the part. By applying the PWM signal to PWM

logic input pin, the charge pump remains active, resulting in

much lower input noise.

In cases where a PWM signal must be connected to the EN

pin, measures can be taken to reduce the magnitude of the

charge-pump turn-on transient response. More input capaci-

tance, series resistors and/or ferrite beads may provide ben-

efits. If the current spikes and voltage dips can be tolerated,

connecting the PWM signal to the EN pin does provide a

benefit of lower supply current consumption. When the PWM

signal to the EN pin is low, the LM27951 will be shutdown

and input current will only be a few micro-amps. This results

in a lower time-averaged input current than the prior sugges-

tion, where EN is kept on continuously.

MAXIMUM OUTPUT CURRENT, MAXIMUM LED

VOLTAGE, MINIMUM INPUT VOLTAGE

The LM27951 can drive 4 LEDs at 30mA each from an input

voltage as low as 3.0V, so long as the LEDs have a forward

voltage of 3.6V or less (room temperature).

The statement above is a simple example of the LED drive

capabilities of the LM27951. The statement contains key

application parameters required to validate an LED-drive

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