AAT1405 Advanced Analogic Technologies, Inc., AAT1405 Datasheet - Page 13

• Manufacturer Part Number: AAT1405
• Description: 4 Channel Led Backlight Driver With Integrated Boost And High Frequency Direct Pwm Dimming
• Manufacturer: Advanced Analogic Technologies, Inc.

Output diode junction temperature should be maintained
below 110°C, but may vary depending on application
and/or system guidelines. The diode θ
with additional PCB area on the cathode. PCB heat-sink-
ing the anode may degrade EMI performance. The
reverse leakage current of the rectifier must be consid-
ered to maintain low quiescent (input) current and high
efficiency under light load. The rectifier reversed current
increases dramatically at elevated temperatures.
Inductor Selection
The white LED boost (step-up) converter is designed to
operate with an inductor with a minimum value of 4.7μH
for all input and output voltage combinations. The induc-
tor saturation current rating should be greater than the
NMOS current at maximum duty cycle.
The inductor (L) is selected to avoid saturation at mini-
mum input voltage, maximum output load conditions.
Peak current may be calculated from the following equa-
tion, again assuming continuous conduction mode.
Worst-case peak current occurs at minimum input volt-
age (maximum duty cycle) and maximum load.
Output Capacitor
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 white LED boost
regulator. MLC capacitors of type X7R or X5R are recom-
mended to ensure good capacitance stability over the full
operating temperature range.
1405.2010.06.1.0
SwitchReg
TM
I
PEAK
4 Channel LED Backlight Driver with Integrated Boost and High Frequency Direct PWM Dimming
D
MAX
=
1 - D
=
I
OUT
(V
MAX
OUT
V
+ V
+
OUT
D
F
+ V
- V
MAX
2 · F
F
IN(MIN)
· V
JA
S
IN(MIN)
can be minimized
· L
)
w w w . a n a l o g i c t e c h . c o m
The output capacitor is sized to maintain the output load
without significant voltage droop (ΔV
power switch ON interval, when the output diode is not
conducting. A ceramic output capacitor from 2.2μF to
4.7μF is recommended. Typically, 50V rated capacitors
are required for the 42V 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.
The output capacitor size can be estimated using the
equation:
To maintain stable operation at full load, the output
capacitor should be sized to maintain ΔV
100mV and 200mV.
The WLED 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.
Compensation Component Selection
The AAT1405 Boost architecture uses peak current mode
control to eliminate the double pole effect of the output
L&C filter and simplifies the compensation loop design.
The current mode control architecture simplifies the
transfer function of the control loop to be a one-pole,
one left plane zero and one right half plane (RHP) system
in frequency domain. The dominant pole can be calcu-
lated by:
C
f
PRODUCT DATASHEET
OUT
P
=
=
2π · R
I
F
OUT
S
· ∆V
1
AAT1405
· D
0
· C
MAX
OUT
4
OUT
) during the
OUT
between
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