AAT1235 Analogic Corporation, AAT1235 Datasheet - Page 13

• Manufacturer Part Number: AAT1235
• Manufacturer: Analogic Corporation

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
instances, it may be necessary to drive high-V
type LEDs. The low dropout (~0.1V @ 20mA I
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
For maximum accuracy, a 1% tolerance resistor is
recommended. Table 6 shows the R
value for AAT1235 for various LED full-scale cur-
rent levels.
Maximum LED current per channel versus R
value is shown in Figure 1.
1235.2007.01.1.0
Resistor Values (1% Resistor Tolerance).
Table 6: Maximum LED Current and R
I
LED
30
25
20
15
10
5
(mA)
F
) levels. In some
R
SET
14.7
17.4
22.6
29.4
44.2
93.1
(kΩ Ω )
SET
SET
resistor.
resistor
SET
LED
SET
F
)
High Efficiency White LED Drivers
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
be selected from 10kΩ to 20kΩ to minimize switch-
ing losses without degrading noise immunity:
Figure 2: Over-Voltage Protection Circuit.
Figure 1: LED Current vs. R
35
30
25
20
15
10
5
0
10
R
for Backlight and Keypad
2
36
AAT1235
= R
GND
OUT(MAX)
62
OV
3
·
88
⎛
⎝
V
OUT(PROTECTION)
114
). The value of R3 should
R
VOUT
V
140
SET
OV
(kΩ Ω )
R2
R3
166
SET
192
- 1
Values.
218
⎞
⎠
C
OUT
244
270
13