AAT1407 Advanced Analogic Technologies, Inc., AAT1407 Datasheet - Page 13

• Manufacturer Part Number: AAT1407
• Description: 6 Channel Led Backlight Driver With Integrated Boost And High Frequency Direct Pwm Dimming
• Manufacturer: Advanced Analogic Technologies, Inc.

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
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
switch ON interval, when the output diode is not conduct-
ing. 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
1407.2010.06.1.0
SwitchReg
TM
I
PEAK
6 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)
OUT
· V
S
) during the power
IN(MIN)
· L
)
w w w . a n a l o g i c t e c h . c o m
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 AAT1407 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:
The ESR zero of the output capacitor (see Figure 3) can
be calculated by:
Where:
C
R
R
capacitor.
4
O
ESR
is the output filter capacitor
is the equivalent load resistor value
is the equivalent series resistance of the output
f
Z_ESR
C
f
OUT
P
PRODUCT DATASHEET
=
=
=
2π · R
2π · R
I
F
OUT
S
· ∆V
1
AAT1407
· D
0
1
ESR
· C
MAX
OUT
· C
4
4
OUT
between
13