AAT1235IRN-T1 AnalogicTech, AAT1235IRN-T1 Datasheet - Page 15

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

SwitchReg
External Schottky diode junction temperature should be
below 110ºC, and may vary depending on application
and/or system guidelines. The diode θ
mized with additional metal PCB area on the cathode.
However, adding additional heat-sinking metal around
the anode may degrade EMI performance. The reverse
leakage current of the rectifier must be considered to
maintain low quiescent (input) current and high effi-
ciency under light load. The rectifier reverse current
increases dramatically at elevated temperatures.
Selecting the Boost Inductor
The AAT1235 controller utilizes hysteretic control and the
switching frequency varies with output load and input
voltage. The value of the inductor determines the maxi-
mum switching frequency of the boost converter. Increased
output inductance decreases the switching frequency,
resulting in higher peak currents and increased output
voltage ripple. To maintain 2MHz maximum switching fre-
quency and stable operation, an output inductor selected
between 1.5μH and 2.7μH is recommended.
A better estimate of D
Where V
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 satu-
ration characteristics. Measurements at full load and
high ambient temperature should be performed to
ensure that the inductor does not saturate or exhibit
excessive temperature rise.
The output inductor (L) is selected to avoid saturation at
minimum input voltage and maximum output load condi-
1235.2009.09.1.2
Manufacturer
Diodes, Inc.
ON Semi
ON Semi
F
is the Schottky diode forward voltage. If not
TM
D
Table 8: Typical Surface Mount Schottky Rectifiers for Various Output Loads.
MAX
=
(V
MAX
OUT
Part Number
is possible once V
(V
MBR130LSFT
+ V
MBR0530T
B0520WS
OUT
F
+ V
- V
F
IN(MIN)
)
(select T
High Efficiency White LED Drivers for Backlight and Keypad
)
JA
can be mini-
F
is known:
Current (A)
J
w w w . a n a l o g i c t e c h . c o m
Rated I
< 110°C in application circuit).
0.50
1.00
0.50
F(AV)
1
tions. 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
inductor:
At light load and low output voltage, the controller reduc-
es the operating frequency to maintain maximum operat-
ing 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 switching fre-
quency is somewhat diminished, resulting in higher I
Bench measurements are recommended to confirm
actual I
saturate at maximum LED current and minimum input
supply voltage.
The RMS current flowing through the boost inductor 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 tempera-
ture 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:
Voltage (V)
Rated
PEAK
20
30
30
I
and to ensure that the inductor does not
PEAK
S
) can be estimated at 500kHz with a 2.2μH
P
LOSS(INDUCTOR)
=
(1 - D
I
PRODUCT DATASHEET
RMS
I
OUT
MAX
=
(θ
Resistance
Thermal
JA
)
= I
, °C/W)
+
I
426
325
206
PEAK
RMS
AAT1235
3
D
(2 · F
MAX
2
· DCR
· V
1
S
IN(MIN)
· L)
SOD-123
SOD-323
SOD-123
Case
PEAK
15
.