AAT1235 Datasheet, PDF (17/21 Pages) Advanced Analogic Technologies – High Efficiency White LED Drivers for Backlight and Keypad

AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
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 temperature
rise, or thermal derating, guidelines:
I_RMS =
I_PEAK
3
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:
P_{LOSS(INDUCTOR)} = I_RMS2 · DCR
To ensure high reliability, the inductor case temper-
ature should not exceed 100ºC. In some cases,
PCB heatsinking applied to the LIN node (non-
switching) can improve the inductor's thermal capa-
bility. However, as in the case of adding extra metal
around the Schottky's anode, adding extra PCB
metal around the AAT1235's SW pin for heatsinking
may degrade EMI performance.
Shielded inductors provide decreased EMI and may
be required in noise sensitive applications.
Unshielded chip inductors provide significant space
savings at a reduced cost compared to shielded
(wound and gapped) inductors. In general, chip-
type inductors have increased winding resistance
(DCR) when compared to shielded, wound varieties.
Manufacturer
Sumida
Sumida
Sumida
Murata
Murata
Taiyo Yuden
Taiyo Yuden
Coiltronics
Coiltronics
Coiltronics
Part Number
CDRH4D22/HP-2R2
CDR4D11/HP-2R4
CDRH4D18-2R2
LQH662N2R2M03
LQH55DN2R2M03
NR4018T2R2
NR3015T2R2
SD3814-2R2
SD3114-2R2
SD3112-2R2
Inductance
(µH)
2.2
2.4
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
Max DC
Current
I(_SAAT)
2.50
1.70
1.32
3.30
3.20
2.70
1.48
1.90
1.48
1.12
DCR
(Ω)
35
105
75
19
29
60
60
77
86
140
Size (mm)
LxWxH
5.0x5.0x2.4
4.8x4.8x1.2
5.0x5.0x2.0
6.3x6.3x4.7
5.0x5.7x4.7
4.0x4.0x1.8
3.0x3.0x1.5
3.8x3.8x1.4
3.1x3.1x1.4
3.1x3.1x1.2
Type
Shielded
Shielded
Shielded
Shielded
Non-Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Table 9: Typical Surface Mount Inductors for Various Output Loads
(select I_PEAK < I_SAT).
Selecting the Boost Capacitors
The high output ripple inherent in the boost converter
necessitates the use of 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 AAT1235 boost regulator. MLC capac-
itors of type X7R or X5R are recommended to
ensure good capacitance stability over the full
operating temperature range.
The output capacitor is selected to maintain the out-
put load without significant voltage droop (ΔV_OUT)
during the power switch ON interval, when the out-
put diode is not conducting. A ceramic output
capacitor between 2.2µF and 4.7µF is recommend-
ed (see Table 8). Typically, 25V rated capacitors are
required for the 24V maximum boost output.
Ceramic capacitors selected as small as 0805 are
available which meet these requirements.
MLC capacitors exhibit significant capacitance reduc-
tion with applied voltage. Output ripple measure-
ments should confirm that output voltage droop and
operating stability are within acceptable limits. Voltage
derating can minimize this factor, but results may vary
with package size and among specific manufacturers.
1235.2007.02.1.1
17