AAT1235 Datasheet, PDF (13/21 Pages) Advanced Analogic Technologies – High Efficiency White LED Drivers for Backlight and Keypad

Application Information
Channel Disable
Tie all unused channels to AGND. On start-up,
these channels will be automatically disabled.
LED Selection
Although the AAT1235 is specifically designed to
drive white LEDs, the device can also be used to
drive most types of LEDs with forward voltages
ranging between 2.0V and 4.7V. Since the A1, A2,
A3, and B1, B2 input current sinks are matched
with low voltage dependence, the LED-to-LED
brightness will be matched regardless of the indi-
vidual LED forward voltage (V_F) levels. In some
instances, it may be necessary to drive high-V_F
type LEDs. The low dropout (~0.1V @ 20mA I_LED)
current sinks in the AAT1235 make it capable of
driving LEDs with forward voltages as high as 4.7V
from an input supply as low as 3.0V. LED outputs
A1-A3 and B1-B2 can be combined to drive high-
current LEDs without complication, making the
AAT1235 a perfect application for large LCD dis-
play backlighting and keypad LED applications.
Constant Current Setting
The LED current is controlled by the R_SET resistor.
For maximum accuracy, a 1% tolerance resistor is
recommended. Table 6 shows the R_SET resistor
value for AAT1235 for various LED full-scale cur-
rent levels.
I_LED (mA)
30
25
20
15
10
5
R_SET (kΩ)
14.7
17.4
22.6
29.4
44.2
93.1
Table 6: Maximum LED Current and R_SET
Resistor Values (1% Resistor Tolerance).
Maximum LED current per channel versus R_SET
value is shown in Figure 1.
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
35
30
25
20
15
10
5
0
10 36 62 88 114 140 166 192 218 244 270
R_SET (kΩ)
Figure 1: LED Current vs. R_SET Values.
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 pro-
tection circuit prevents damage to the device when
one of the five channels 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 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 10kΩ to 20kΩ to minimize switch-
ing losses without degrading noise immunity:
R₂
=
R₃
·
⎛
⎝
V_{OUT(PROTECTION)}
V_OV
-
1^⎞⎠
AAT1235
OV
GND
VOUT
R2
R3
C_OUT
Figure 2: Over-Voltage Protection Circuit.
1235.2007.02.1.1
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