AAT1239-1_0810 Datasheet, PDF (16/23 Pages) Advanced Analogic Technologies – 40V Step-Up Converter for 4 to 10 White LEDs

SwitchReg^TM
PRODUCT DATASHEET
AAT1239-1
40V Step-Up Converter for 4 to 10 White LEDs
The maximum duty cycle can be estimated from the
relationship for a continuous mode boost converter.
Maximum duty cycle (D_MAX) is the duty cycle at minimum
input voltage (V_IN(MIN)).
D_MAX =
V_OUT - V_IN(MIN)
V_OUT
The average diode current is equal to the output
current.
I_AVG(TOT) = I_OUT
The average output current multiplied by the forward
diode voltage determines the loss of the output diode.
P_LOSS(DIODE) = I_AVG(TOT) · V_F
= I_OUT · V_F
For continuous LED currents, the diode junction tem-
perature can be estimated.
T_J(DIODE) = T_AMB + θ_JA · P_LOSS(DIODE)
Output diode junction temperature should be maintained
below 110ºC, but may vary depending on application
and/or system guidelines. The diode θ_JA can be mini-
mized with additional PCB area on the cathode. PCB
heat-sinking the anode may degrade EMI performance.
The reverse leakage current of the rectifier must be con-
sidered to maintain low quiescent (input) current and
high efficiency under light load. The rectifier reverse cur-
rent increases dramatically at elevated temperatures.
Selecting the Boost Inductor
The AAT1239-1 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 fre-
quency and switching loss, but results in higher peak
currents and increased output voltage ripple. To maintain
2MHz maximum switching frequency and stable opera-
tion, an output inductor sized from 1.5μH to 2.7μH is
recommended. For higher efficiency in Li-ion battery
applications (V_IN from 3.0V to 4.2V) and stable operation,
increasing the inductor size up to 10μH is recommended.
Figure 15 and 16 show the special enhanced efficiency
application.
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, it can be estimated at 0.5V.
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 completed to
ensure that the inductor does not saturate or exhibit
excessive temperature rise.
Manufacturer
Taiwan Semiconductor
Co., Ltd.
Diodes, Inc
Zetex
Part
Number
SS16L
SS15L
SS14L
B340LA
ZHCS350
Rated
Forward
Current
(A)
1.1
3
0.35
Non-Repetitive
Peak Surge
Current (A)
30
30
30
70.0
4.2
Rated
Voltage
(V)
60
50
40
40
40
Thermal
Resistance
(θ_JA, °C/W)
45
45
45
25
330
Size (mm)
(LxWxH)
3.8x1.9x1.43
3.8x1.9x1.43
3.8x1.9x1.43
5.59x2.92x2.30
1.7x0.9x0.8
Case
Sub SMA
Sub SMA
Sub SMA
SMA
SOD523
Table 3: Typical Surface Mount Schottky Rectifiers for Various Output Levels.
16
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1239-1.2008.10.1.2