AAT1235 Datasheet, PDF (16/21 Pages) Advanced Analogic Technologies – High Efficiency White LED Drivers for Backlight and Keypad

AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
For continuous LED currents, the diode junction
temperature can be estimated:
T_J(DIODE) = T_AMB + θ_JA · P_LOSS(DIODE)
External Schottky diode junction temperature
should be below 110ºC, and may vary depending
on application and/or system guidelines. The diode
θ_JA 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.
Manufacturer
Diodes, Inc.
ON Semi
ON Semi
Part Number
B0520WS
MBR130LSFT
MBR0530T
Rated I_F(AV)
Current (A)¹
0.50
1.00
0.50
Rated
Voltage (V)
20
30
30
Thermal
Resistance
(θ_JA, °C/W)¹
426
325
206
Case
SOD-323
SOD-123
SOD-123
Table 8: Typical Surface Mount Schottky Rectifiers for Various Output Loads.
(select T_J < 110°C in application circuit).
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.
A better estimate of D_MAX is possible once V_F is
known:
D_MAX
=
(V_OUT + V_F -
(V_OUT +
V_IN(MIN))
V_F)
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.
16
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:
I_PEAK =
I_OUT
(1 - D_MAX)
+
D_MAX · V_IN(MIN)
(2 · F_S · L)
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
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