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ADM8845

Charge Pump Driver for LCD White LED Backlights

Analog Devices 

OUTPUT CURRENT CAPABILITY

The ADM8845 can drive up to 30 mA of current to each of the

six LEDs given an input voltage of 2.6 V to 5.5 V. The LED

currents have a maximum current matching of 1% between any

two LED currents. An external resistor, RSET, sets the output

current, approximated by the following equation:

RSET = 120 × (1.18 V/ILED)

To regulate the LED currents properly, sufficient headroom

voltage (compliance) must be present. The compliance refers to

the minimum amount of voltage that must be present across the

internal current sinks to ensure that the desired current and

matching performance is realizable. To ensure that the desired

current is obtained, use the following equation to find the

minimum input voltage required:

VOUT – VF ≥ Compliance

where VF is the LED forward voltage. For 20 mA/LED, the

compliance is 0.20 V typ and 0.30 V max (see Table 5).

Table 5. ILED, RSET, and Compliance Table

ILED

RSET

Typ. Compliance

15 mA

9.44 kΩ

0.17 V

20 mA

7.08 kΩ

0.20 V

30 mA

4.72 kΩ

0.34 V

When the ADM8845 charge pump is loaded with 180 mA (six

LEDs at 30 mA/LED), the ambient operating temperature is

reduced (see Figure 21).

AUTOMATIC GAIN CONTROL

The automatic gain control block controls the operation of the

charge pump by selecting the appropriate gain for the charge

pump. This maintains sufficient drive for the LED anodes at

the highest power efficiency over a 2.6 V to 5.5 V input supply

range. The charge pump switching thresholds are described in

Table 6.

Table 6. Charge Pump Switching Thresholds

Gain

Threshold

1.5× to 2×

3.33 V

2× to 1.5×

3.36 V

1× to 1.5×

4.77 V

1.5× to 1×

4.81 V

ADM8845

CURRENT MATCHING

The 1% maximum current matching performance is defined by

the following equations:

IAVG = (IMAX + IMIN)/2

Max Matching Error = [(IMAX – IAVG)/IAVG] × 100

or

Min Matching Error = [(IMIN – IAVG)/IAVG] × 100

where IMAX is the largest ILED current, and IMIN is the smallest ILED

current.

BRIGHTNESS CONTROL WITH A

DIGITAL PWM SIGNAL

PWM brightness control provides the widest brightness control

method by pulsing the white LEDs on and off using the digital

input control pins, CTRL1 and/or CTRL2. PWM brightness

control also removes any chromaticity shifts associated with

changing the white LED current, because the LEDs operate

either at zero current or full current (set by RSET).

The digital PWM signal applied with a frequency of 100 Hz to

200 kHz turns the current control sinks on and off using CTRL1

and/or CTRL2. The average current through the LEDs changes

with the PWM signal duty cycle. If the PWM frequency is much

less than 100 Hz, flicker could be seen in the LEDs. For the

ADM8845, zero duty cycle turns off the LEDs, and a 50% duty

cycle results in an average LED current ILED being half the pro-

grammed LED current. For example, if RSET is set to program

20 mA/LED, a 50% duty cycle results in an average ILED of

10 mA/LED, ILED being half the programmed LED current.

C1

C2

1µF

1µF

ADM8845

VOUT

PWM INPUT

OR HIGH/LOW

CTRL1

PWM INPUT

OR HIGH/LOW

CTRL2

FB1

FB2

FB3

FB4

ISET

FB5

FB6

RSET

C3

2.2µF

Figure 24. Digital PWM Brightness Control Application Diagram

By applying a digital PWM signal to the digital input control

pins, CTRL1 and/or CTRL2 can adjust the brightness of the sub

and/or main displays. The ADM8845’s six white LEDs are

organized into two groups, main display (FB1 to FB4) and sub

display (FB4 to FB6); refer to the Theory of Operation section.

Rev. 0 | Page 11 of 20

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