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參數(shù)資料
| 型號(hào)： | ADM8845 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Charge Pump Driver for LCD White LED Backlights |
| 中文描述： | 電荷泵驅(qū)動(dòng)器的液晶顯示白色LED背光源 |
| 文件頁數(shù)： | 11/18頁 |
| 文件大小： | 341K |
| 代理商： | ADM8845 |
Preliminary Technical Data
ADM8845
OUTPUT CURRENT CAPABILITY
The ADM8845 is capable of driving up to 30mA of current to each of the six LEDs given an input voltage of 2.6V to 5.5V. The LED
currents have a max current matching of 1% between any two LED currents. An external resistor R
SET
sets the output current,
approximated by the following equation:
Rev. PrJ 06/04| Page 11 of 18
R
SET
= 120 x (1.18V / I
LED
).
In order for the LED currents to be regulated 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 in order to ensure the desired current and
matching performance is realizable. To ensure the desired current is obtained, apply the following equations to find the minimum input
voltage required:
V
OUT
– V
F
≥
Compliance
V
F
is the LED forward voltage. For 20mA/LED the compliance is 0.2V typ and 0.3V max, see Table 5.
I
LED
15mA
20mA
30mA
R
SET
Typ. Compliance
0.17V
0.20V
0.34V
9.44k Ohms
7.08k Ohms
4.72k Ohms
Table 5. I
LED
, R
SET
and Compliance Table
When the ADM8845 charge pump is loaded with 180mA (six LEDs at 30mA/LED) the ambient operating temperature is reduced, see
Figure 22.
AUTOMATIC GAIN CONTROL
The Automatic Gain Control block controls the operation of the charge pump by selecting the appropriate gain for the charge pump, to
maintain sufficient drive for the LED anodes at the highest power efficiency over a 2.6V to 5.5V input supply range. The charge pump
switching thresholds are:
Gain
1.5x to 2x
2x to 1.5x
1x to 1.5x
1.5x to 1x
Threshold
3.33V
3.36V
4.77V
4.81V
Table 6. Charge Pump Switching Thresholds
CURRENT MATCHING
The 1% max current matching performance is defined by the following two equations:
I
AVG
= (I
MAX
+ I
MIN
)/2
Max Matching Error = [(I
MAX
– I
AVG
)/I
AVG
] * 100
or
Min Matching Error = [(I
MIN
– I
AVG
)/I
AVG
] * 100
Where I
MAX
is the largest I
LED
current and I
MIN
is the smallest I
LED
current.
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