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參數(shù)資料
| 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)： | 19/24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
19
Selecting the Boost Inductor
The AAT1236 controllers utilize hysteretic control
and the switching frequency varies with output load
and input voltage. The value of the inductor deter-
mines the maximum switching frequency of the
boost converter. Increased output inductance
decreases the switching frequency, resulting in high-
er peak currents and increased output voltage ripple.
To maintain 2MHz maximum switching frequency
and stable operation, an output inductor selected
between 1.5μH and 2.7μH is recommended.
A better estimate of D
MAX
is possible once V
F
is
known:
Where V
F
is the Schottky diode forward voltage. If
not known or not provided by the manufacturer, a
starting value of 0.5V can be used.
Manufacturer’s specifications list both the inductor
DC current rating, which is a thermal limitation, and
peak inductor current rating, which is determined
by the saturation characteristics. Measurements at
full load and high ambient temperature should be
performed to ensure that the inductor does not sat-
urate or exhibit excessive temperature rise.
The output inductor (L) is selected to avoid saturation
at minimum input voltage and maximum output load
conditions. Peak current may be estimated using the
following equation, assuming continuous conduction
mode. Worst-case peak current occurs at minimum
input voltage (maximum duty cycle) and maximum
load. Switching frequency (F
S
) can be estimated at
500kHz with a 2.2μH inductor:
At light load and low output voltage, the controller
reduces the operating frequency to maintain maxi-
mum operating efficiency. As a result, further
reduction in output load does not reduce the peak
current. Minimum peak current can be estimated
between 0.5A and 0.75A.
At high load and high output voltages, the switch-
ing frequency is somewhat diminished, resulting in
higher I
PEAK
. Bench measurements are recom-
mended to confirm actual I
PEAK
and to ensure that
the inductor does not saturate at maximum LED
current and minimum input supply voltage.
The RMS current flowing through the boost induc-
tor is equal to the DC plus AC ripple components.
Under worst-case RMS conditions, the current
waveform is critically continuous. The resulting
RMS calculation yields worst-case inductor loss.
The RMS current value should be compared
against the inductor manufacturer's temperature
rise, or thermal derating, guidelines:
For a given inductor type, smaller inductor size leads
to an increase in DCR winding resistance and, in
most cases, increased thermal impedance. Winding
resistance degrades boost converter efficiency and
increases the inductor’s operating temperature:
To ensure high reliability, the inductor case tempera-
ture should not exceed 100oC. In some cases, PCB
heatsinking applied to the LIN node (non-switching)
can improve the inductor's thermal capability.
However, as in the case of adding extra metal
around the Schottky's anode, adding extra PCB
metal around the AAT1236's SW pin for heatsinking
may degrade EMI performance.
Shielded inductors provide decreased EMI and may
be required in noise sensitive applications.
Unshielded chip inductors provide significant space
savings at a reduced cost compared to shielded
(wound and gapped) inductors. In general, chip-
type inductors have increased winding resistance
(DCR) when compared to shielded, wound varieties.
Selecting the Boost Capacitors
The high output ripple inherent in the boost converter
necessitates the use of low impedance output filtering.
Multi-layer ceramic (MLC) capacitors provide small
size and adequate capacitance, low parasitic
equivalent series resistance (ESR) and equivalent
series inductance (ESL), and are well suited for
use with the AAT1236 boost regulator. MLC capac-
itors of type X7R or X5R are recommended to
ensure good capacitance stability over the full
operating temperature range.
P
LOSS(INDUCTOR)
= I
RMS2
· DCR
I
PEAK
3
I
RMS
=
I
OUT
(1 - D
MAX
)
D
MAX
·
V
IN(MIN)
(2
·
F
S
·
L)
I
PEAK
=
+
(V
OUT
+ V
F
- V
IN(MIN)
)
(V
OUT
+ V
F
)
D
MAX
=
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