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| 型號(hào)： | AAT1236IRN-T1 |
| 廠商： | Advanced Analog Technology,lnc. |
| 元件分類： | LED驅(qū)動(dòng)器 |
| 英文描述： | High Efficiency White LED Drivers for Backlight and Keypad |
| 中文描述： | 高效率白光LED的背光和鍵盤(pán)驅(qū)動(dòng)程序 |
| 文件頁(yè)數(shù)： | 17/24頁(yè) |
| 文件大小： | 1009K |
| 代理商： | AAT1236IRN-T1 |
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AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
1236.2007.02.1.1
17
Over-Voltage Protection
The over-voltage protection circuit consists of a
resistor network connected from the output voltage
to the OV pin (see Figure 9). This over voltage pro-
tection circuit prevents damage to the device when
one of the five channels has an open LED circuit.
The AAT1236 continues to operate; however, the
LED current in the remaining channels is no longer
regulated and the actual LED current will be deter-
mined by the externally programmed over-voltage
protection threshold, the inductor value, and the
switching frequency.
The resistor divider can be selected such that the
over-voltage threshold occurs prior to the output
reaching 24V (V
OUT(MAX)
). The value of R3 should
be selected from 10k
Ω
to 20k
Ω
to minimize switch-
ing losses without degrading noise immunity.
Figure 9: Over-Voltage Protection Circuit.
If four LEDs are connected in series on one channel,
the total V
F
from the WLEDs could be as high
as 18.8V. Therefore, using R3 = 12.1k
Ω
and setting
V
OUT(PROTECTION)
= 20V is recommended. Selecting
a 1% resistor, this results in R2 = 187k
Ω
(rounded to
the nearest standard 1% value).
It is always recommended to use the same number
of WLEDs on each channel and set the appropriate
over-voltage protection. Failure to do so may cause
any one of the (5) sink pins to exceed the absolute
maximum rating voltage and permanently damage
the device in case the channel is disconnected
(open circuit failure). Examples of over voltage set-
tings for various strings of series-connected LEDs
are shown in Table 3.
LED Brightness Control
The AAT1236 uses the I
2
C interface to program
and control LED brightness. The output current of
the AAT1236 can be changed successively to
brighten or dim the LEDs in smooth transitions (i.e.,
to fade in or fade out) or in discrete steps, giving
the user complete programmability and real-time
control of LED brightness.
Selecting the Schottky Diode
To ensure minimum forward voltage drop and no
recovery, high voltage Schottky diodes are recom-
mended for the AAT1236 boost converter. The out-
put diode is selected to maintain acceptable effi-
ciency and reasonable operating junction tempera-
ture under full load operating conditions. Forward
voltage (V
F
) and package thermal resistance (
θ
JA
)
are the dominant factors in selecting a diode. The
diode non-repetitive peak forward surge current
rating (I
FSM
) should be considered for high pulsed
load applications, such as camera flash. I
FSM
rating
drops
with
increasing
Manufacturers’ datasheets should be reviewed
carefully to verify reliability under peak loading con-
ditions. The diode's published current rating may
not reflect actual operating conditions and should
be used only as a comparative measure between
similarly rated devices.
conduction
period.
R2
R3
C
OUT
VOUT
AAT1236
OV
GND
R
2
= R
3
·
- 1
V
OUT(PROTECTION)
V
OV
Table 3: Over-Voltage Protection Settings.
Number of WLEDs
on Each Channel
R3 = 12.1k
Ω
R2 (k
Ω
)
187
140
88.7
Total Maximum V
F
(V)
18.8
14.1
9.4
V
OUT(PROTECTION)
(V)
20
15
10
4
3
2
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