AAT1239-1_0810 Datasheet, PDF (18/23 Pages) Advanced Analogic Technologies – 40V Step-Up Converter for 4 to 10 White LEDs

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PRODUCT DATASHEET
AAT1239-1
40V Step-Up Converter for 4 to 10 White LEDs
Inductor Efficiency Considerations
The efficiency for different inductors is shown in Figure 8
for ten white LEDs in series. Smaller inductors yield
increased DCR and reduced operating efficiency.
75
CDRH5D16F-2R2 (29mΩ)
72
SD3814-2R2 (77mΩ)
69
66
63
2
5
8
11
14
17
20
LED Current (mA)
Figure 8: AAT1239-1 Efficiency for
Different Inductor Types (V_IN = 3.6V;
Ten White LEDs in Series).
Selecting the Boost Capacitors
The high output ripple inherent in the boost converter
necessitates 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 AAT1239-1
boost regulator. MLC capacitors of type X7R or X5R are
recommended to ensure good capacitance stability over
the full operating temperature range.
The output capacitor is sized to maintain the output load
without significant voltage droop (ΔV_OUT) during the
power switch ON interval, when the output diode is not
conducting. A ceramic output capacitor from 2.2μF to
4.7μF is recommended (see Table 5). Typically, 50V
rated capacitors are required for the 40V maximum
boost output. Ceramic capacitors sized as small as 0805
or 1206 are available which meet these requirements.
MLC capacitors exhibit significant capacitance reduction
with applied voltage. Output ripple measurements should
confirm that output voltage droop and operating stability
are acceptable. Voltage derating can minimize this fac-
tor, but results may vary with package size and among
specific manufacturers.
Output capacitor size can be estimated at a switching
frequency (F_S) of 500kHz (worst case).
C_OUT =
I_OUT · D_MAX
F_S · ΔV_OUT
To maintain stable operation at full load, the output
capacitor should be sized to maintain ΔV_OUT between
100mV and 200mV.
The boost converter input current flows during both ON
and OFF switching intervals. The input ripple current is
less than the output ripple and, as a result, less input
capacitance is required.
Manufacturer
Murata
Murata
Murata
Murata
Murata
Part Number
GRM188R60J225KE19
GRM188R61A225KE34
GRM21BR71E225KA73L
GRM31CR71H225KA88
GRM31CR71H475K
Value (μF)
2.2
2.2
2.2
2.2
4.7
Voltage Rating
6.3
10
25
50
50
Temp Co
X5R
X5R
X7R
X7R
X7R
Table 5: Recommended Ceramic Capacitors.
Case Size
0603
0603
0805
1206
1206
18
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1239-1.2008.10.1.2