AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
For continuous LED currents, the diode junction
temperature can be estimated:
on application and/or system guidelines. The diode
θJA can be minimized 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 efficiency under
light load. The rectifier reverse current increases
dramatically at elevated temperatures.
TJ(DIODE) = TAMB + θJA · PLOSS(DIODE)
External Schottky diode junction temperature
should be below 110ºC, and may vary depending
Thermal
Rated IF(AV)
Rated
Voltage (V)
Resistance
Manufacturer
Part Number
Current (A)1
JA, °C/W)1
θ
(
Case
Diodes, Inc.
ON Semi
ON Semi
B0520WS
0.50
1.00
0.50
20
30
30
426
325
206
SOD-323
SOD-123
SOD-123
MBR130LSFT
MBR0530T
Table 8: Typical Surface Mount Schottky Rectifiers for Various Output Loads.
(select TJ < 110°C in application circuit).
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 (FS) can be estimated at
500kHz with a 2.2µH inductor:
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 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 rip-
ple. To maintain 2MHz maximum switching frequen-
cy and stable operation, an output inductor selected
between 1.5µH and 2.7µH is recommended.
IOUT
(1 - DMAX
DMAX
·
· VIN(MIN)
FS · L)
IPEAK
=
+
)
(2
A better estimate of DMAX is possible once VF is
known:
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.
(VOUT + VF - VIN(MIN)
(VOUT + VF)
)
DMAX
=
Where VF is the Schottky diode forward voltage. If
not known or not provided by the manufacturer, a
starting value of 0.5V can be used.
At high load and high output voltages, the switch-
ing frequency is somewhat diminished, resulting in
higher IPEAK. Bench measurements are recom-
mended to confirm actual IPEAK and to ensure that
the inductor does not saturate at maximum LED
current and minimum input supply voltage.
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 RMS current flowing through the boost induc-
tor is equal to the DC plus AC ripple components.
Under worst-case RMS conditions, the current
16
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