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ADM8845ACP-REEL Datasheet(PDF) 11 Page - Analog Devices

Part # ADM8845ACP-REEL
Description  Charge Pump Driver for LCD White LED Backlights
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

ADM8845ACP-REEL Datasheet(HTML) 11 Page - Analog Devices

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ADM8845
Rev. 0 | Page 11 of 20
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 k
0.17 V
20 mA
7.08 k
0.20 V
30 mA
4.72 k
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
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.
FB1
FB2
FB3
FB4
FB5
FB6
VOUT
C1
1
µF
CTRL1
CTRL2
C2
1
µF
RSET
ISET
C3
2.2
µF
ADM8845
PWM INPUT
OR HIGH/LOW
PWM INPUT
OR HIGH/LOW
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.


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