AAT1235IRN-T1 AnalogicTech, AAT1235IRN-T1 Datasheet - Page 13

• Manufacturer Part Number: AAT1235IRN-T1
• Description: High Efficiency White LED Drivers for Backlight and Keypad
• Manufacturer: AnalogicTech

SwitchReg
Over-Voltage Protection
The over-voltage protection circuit consists of a resistor
network connected from the output voltage to the OV pin
(see Figure 2). This over voltage protection circuit pre-
vents damage to the device when one of the five chan-
nels has an open LED circuit. The AAT1235 continues to
operate; however, the LED current in the remaining
channels is no longer regulated and the actual LED cur-
rent will be determined 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
10kΩ to 20kΩ to minimize switching losses without
degrading noise immunity:
If four LEDs are connected in series on one channel, the
total V
Therefore, using R3 = 12.1kΩ and setting V
= 20V is recommended. Selecting a 1% resistor, this
results in R2 = 187kΩ (rounded to the nearest standard
1% value).
1235.2009.09.1.2
Number of WLEDs on Each Channel
Figure 2: Over-Voltage Protection Circuit.
OUT(MAX)
F
from the WLEDs could be as high as 18.8V.
AAT1235
GND
). The value of R3 should be selected from
TM
R
OV
2
= R
3
·
4
3
2
⎛
⎝
V
OUT(PROTECTION)
VOUT
V
OV
R2
R3
Table 7: Over-Voltage Protection Settings.
High Efficiency White LED Drivers for Backlight and Keypad
- 1
⎞
⎠
C
OUT
OUT(PROTECTION)
w w w . a n a l o g i c t e c h . c o m
Total Maximum V
18.8
14.1
9.4
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 settings for various strings of
series-connected LEDs are shown in Table 7.
LED Brightness Control
The AAT1235 uses AS
brightness. The output current of the AAT1235 can be
changed successively to brighten or dim the LEDs in
smooth transitions (i.e., to fade in or fade out) or in dis-
crete 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 recov-
ery, high voltage Schottky diodes are recommended for
the AAT1235 boost converter. The output diode is select-
ed to maintain acceptable efficiency and reasonable
operating junction temperature under full load operating
conditions. Forward voltage (V
resistance (θ
diode. The diode’s non-repetitive peak forward surge
current rating (I
load applications, such as camera flash. I
with increasing conduction period. Manufacturers’
datasheets should be reviewed carefully to verify reli-
ability under peak loading conditions. The diode’s pub-
lished current rating may not reflect actual operating
conditions and should be used only as a comparative
measure between similarly rated devices.
20V rated Schottky diodes are recommended for output
voltages less than 15V, while 30V rated Schottky diodes
are recommended for output voltages higher than 15V.
F
(V)
JA
) are the dominant factors in selecting a
V
FSM
OUT(PROTECTION)
) should be considered for high pulsed
PRODUCT DATASHEET
2
Cwire programming to control LED
20
15
10
AAT1235
(V)
F
) and package thermal
R3 = 12.1kΩ
FSM
R2 (kΩ)
rating drops
88.7
187
140
13