AAT1235 Datasheet, PDF (10/21 Pages) Advanced Analogic Technologies – High Efficiency White LED Drivers for Backlight and Keypad

AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
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 cur-
rent 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 current. 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
AAT1235 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
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 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
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 AAT1235 when
internal power dissipation becomes excessive, as it
disables both MOSFETs. The junction over-tem-
perature threshold is 140°C with 15°C of tempera-
ture hysteresis. The output voltage automatically
recovers when the over-temperature fault condition
is removed.
Over-Voltage Protection
Over-voltage protection prevents damage to the
AAT1235 during open-circuit 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 voltage 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 thresholds 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_IN bias and proper operation of all inter-
nal circuitry prior to soft start.
10
1235.2007.02.1.1