AAT1235IRN-T1 AnalogicTech, AAT1235IRN-T1 Datasheet - Page 10

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

current is equal to the peak current minus a preset hys-
teresis threshold, which determines the inductor ripple
current. Peak current is adjusted by the controller until
the desired LED output current level is met.
The magnitude of the feedback error signal determines
the average input current. Therefore, the AAT1235 con-
troller implements a programmed current source con-
nected to the output capacitor, parallel with the LED
channels. There is no right-half plane zero, and loop sta-
bility is achieved with no additional compensation com-
ponents. The controller responds by increasing the peak
inductor current, resulting in higher average current in
the inductor and LED channels.
Under light load conditions, the inductor OFF interval
current goes below zero and the boost converter enters
discontinuous mode operation. Further reduction in the
load current results in a corresponding reduction in the
switching frequency. The AAT1235 provides a pulsed
frequency operation which reduces switching losses and
maintains high efficiency under light load conditions.
Operating frequency varies with changes in the input volt-
age, output voltage, and inductor size. Once the boost
converter has reached continuous mode, further increases
in the LED current will not significantly change the operat-
ing frequency. A small 2.2μH (±20%) inductor is selected
to maintain high frequency switching (up to 2MHz) and
high efficiency operation for outputs up to 24V.
Soft Start / Enable
The input disconnect switch is activated when a valid
supply voltage is present and the EN/SET pin is strobed
high. Slew rate control on the input disconnect switch
ensures minimal inrush current as the output voltage is
charged to the input voltage, prior to switching of the
N-channel power MOSFET. A monotonic turn-on is guar-
anteed by the built-in soft-start circuitry, which elimi-
nates output current overshoot across the full input volt-
age range and all load conditions.
Current Limit and
Over-Temperature Protection
The switching of the N-channel MOSFET terminates when
a current limit of 1.5A (typical) is exceeded. This mini-
mizes power dissipation and component stresses under
overload and short-circuit conditions. Switching resumes
when the current decays below the current limit.
Thermal protection disables the AAT1235 when internal
power dissipation becomes excessive, as it disables both
10
SwitchReg
TM
High Efficiency White LED Drivers for Backlight and Keypad
w w w . a n a l o g i c t e c h . c o m
MOSFETs. The junction over-temperature threshold is
140°C with 15°C of temperature hysteresis. The output
voltage automatically recovers when the over-tempera-
ture fault condition is removed.
Over-Voltage Protection
Over-voltage protection prevents damage to the AAT1235
during open-circuit causing high output voltage condi-
tions. An over-voltage event is defined as a condition
where the voltage on the OV pin exceeds the over-volt-
age threshold limit (V
age on the OV pin has reached the threshold limit, the
converter stops switching and the output voltage decays.
Switching resumes when the voltage on the OV pin drops
below the lower hysteresis limit, maintaining an average
output voltage between the upper and lower OV thresh-
olds multiplied by the resistor divider scaling factor.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN input.
Under-voltage lockout (UVLO) guarantees sufficient V
bias and proper operation of all internal circuitry prior to
soft start.
Constant Current Output Level Settings
and AS
The LED current sink level of each group and the on/off
status of each channel is controlled by AnalogicTech’s
AS
programmable, no PWM or additional control circuitry is
needed to control LED brightness. This feature greatly
reduces the burden on a microcontroller or system IC to
manage LED or display brightness, allowing the user to
“set it and forget it.” With its high-speed serial interface
(1MHz data rate), the input sink current can be changed
quickly and easily. Also the non-pulsating LED current
reduces system noise and improves LED reliability.
AS
SET pin to address and load the registers. AS2Cwire
latches data or address after the EN/SET pin has been
held high for time T
ferentiated by the number of EN/SET rising edges. Since
the data registers are 4 bits each, the differentiating
number of pulses is 24 or 16, so that Address 0 is signi-
fied by 17 rising edges, Address 1 by 18 rising edges,
and so forth. Data is set to any number of rising edges
between 1 and including 16. A typical write protocol is a
burst of EN/SET rising edges, signifying a particular
address, followed by a pause with EN/SET held high for
2
2
Cwire serial digital input. Since each current sink is
Cwire relies on the number of rising edges of the EN/
2
Cwire Serial Interface
LAT
PRODUCT DATASHEET
OV
(500μs). Address or data is dif-
= 1.2V typical). When the volt-
AAT1235
1235.2009.09.1.2
IN