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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的背光和鍵盤驅(qū)動(dòng)程序 |
| 文件頁數(shù)： | 11/24頁 |
| 文件大?。?/td> | 1009K |
| 代理商： | AAT1236IRN-T1 |
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
1236.2007.02.1.1
11
vides direct feedback in order to maintain the
desired LED currents.
The switching cycle initiates when the N-channel
MOSFET is turned ON and current ramps up in the
inductor. The ON interval is terminated when the
inductor current reaches the programmed peak
current level. During the OFF interval, the input
current decays until the lower threshold, or zero
inductor current, is reached. The lower current is
equal to the peak current minus a preset hysteresis
threshold, which determines the inductor ripple cur-
rent. Peak current is adjusted by the controller until
the desired LED output current level is met.
The magnitude of the feedback error signal deter-
mines the average input current. Therefore, the
AAT1236 controller implements a programmed cur-
rent source connected to the output capacitor, par-
allel with the LED channels. There is no right-half
plane zero, and loop stability is achieved with no
additional compensation components. The con-
troller 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 inter-
val current goes below zero and the boost convert-
er enters discontinuous mode operation. Further
reduction in the load current results in a correspon-
ding reduction in the switching frequency. The
AAT1236 provides pulsed frequency operation
which reduces switching losses and maintains high
efficiency under light load conditions.
Operating frequency varies with changes in the
input voltage, output voltage, and inductor size.
Once the boost converter has reached continuous
mode, further increases in the LED current will not
significantly change the operating frequency. A
small 2.2μH (±20%) inductor is selected to main-
tain 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 dis-
connect 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 guaranteed by the built-in
soft-start circuitry, which eliminates output current
overshoot across the full input voltage range and
over all load conditions.
Current Limit and Over-Temperature
Protection
The switching of the N-channel MOSFET termi-
nates when a current limit of 1.5A (typical) is
exceeded. This minimizes power dissipation and
component stresses under overload and short-cir-
cuit conditions. Switching resumes when the cur-
rent decays below the current limit.
Thermal protection disables the AAT1236 when inter-
nal power dissipation becomes excessive, as it dis-
ables both MOSFETs. The junction over-temperature
threshold is 140°C with 15°C of temperature hystere-
sis. The output voltage automatically recovers when
the over-temperature fault condition is removed.
Over-Voltage Protection
Over-voltage protection prevents damage to the
AAT1236 during open-circuit on any LED channel
causing high output voltage conditions. An over-
voltage event is defined as a condition where the
voltage on the OV pin exceeds the over-voltage
threshold limit (V
OV
= 1.2V typical). When the volt-
age on the OV pin has reached the threshold limit,
the converter stops switching and the output volt-
age 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 thresholds multi-
plied by the resistor divider scaling factor.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the V
IN
input. Under-voltage lockout (UVLO) guarantees
sufficient V
IN
bias and proper operation of all inter-
nal circuitry prior to soft start.
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