ADM8843(Rev0) Просмотр технического описания (PDF) - Analog Devices

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ADM8843
(Rev.:Rev0)

4 White LED Backlight Driver

Analog Devices 

ADM8843

AUTOMATIC GAIN CONTROL

The automatic gain control block controls the operation of the

charge pump by selecting the appropriate gain for the charge

pump. Doing so 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 5.

Table 5. 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

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 at

either 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

ADM8843, 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

3.4V IIN

PWM INPUT

OR

HIGH/LOW

PWM INPUT

OR

HIGH/LOW

RSET

VCC

ADM8845

CTRL1

VOUT

FB1

CTRL2

FB2

FB3

FB4

ISET

C3

2.2µF

Figure 19. Digital PWM Brightness Control Application Diagram

By applying a digital PWM signal to the digital input control

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

and/or main displays. The ADM8843’s four white LEDs are

organized into two groups, main display (FB1 to FB3) and sub

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

The ADM8843’s main and sub display brightness can be

controlled together or separately. It does this by applying a

digital PWM signal to both the CTRL1 and CTRL2 pins. The

duty cycle of the applied digital PWM signal determines the

brightness of the main and sub displays together. Varying the

duty cycle of the applied PWM signal varies the brightness of

the main and sub displays from 0% to 100%.

By holding CTRL1 low and applying a digital PWM signal to

CTRL2, the sub display is turned off and the main display is

turned on. Then the brightness of the main display is determined

by the duty cycle of the applied digital PWM signal.

By applying a digital PWM signal to CTRL1 and holding CTRL2

low, the sub display is turned on and the main display is turned

off. Then the brightness of the sub display is determined by the

duty cycle of the applied digital PWM signal.

By applying a digital PWM signal to CTRL1 and holding

CTRL2 high, the sub display is turned on and the main display

is turned on. Then the brightness of the sub display is determined

by the duty cycle of the applied digital PWM signal. The bright-

ness of the main display is set to the maximum (maximum is set

by RSET).

By holding CTRL1 high and applying a digital PWM signal to

CTRL2, the sub display is turned on and the main display is

turned on. Then the brightness of the main display is determined

by the duty cycle of the applied digital PWM signal. The bright-

ness of the sub display is set to the maximum (maximum is set

by RSET).

When CTRL1 and CTRL2 are low, the LED current control

sinks shutdown. Shutdown of the charge pump is delayed by

15 ms. This timeout period, tCP, allows the ADM8843 to

determine if a digital PWM signal is present on CTRL1 and

CTRL2, or if the user has selected a full chip shutdown (see

Figure 20).

If digital PWM brightness control of the LEDs is not required, a

constant Logic Level 1 (VCC) or 0 (GND) must be applied.

The four white LED in the ADM8843 are arranged into two

groups, sub and main. It is possible to configure the four LEDs

as in Table 6. Refer also to Figure 20.

Rev. 0 | Page 10 of 16

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