AAT1409
Eight-Channel LED Backlight Driver
with Integrated Boost and High Frequency Direct PWM Dimming
13
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The approximate power loss on the Schottky diode can
be determined:
LOSS-DIODE
AVG
F
OUT
F
Diode junction temperature can be estimated.
J
AMB
JA
LOSS-DIODE
Output diode junction temperature should be maintained
below 110°C, but may vary depending on application
and/or system guidelines. The diode θJA can be minimized
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 reverse current
increases dramatically at elevated temperatures.
Inductor Selection
The white LED boost (step-up) converter is designed to
operate with a minimum inductor value of 4.7μH for all
input and output voltage combinations. The inductor
saturation current rating should be greater than the
NMOS current at maximum duty cycle.
DMAX =
OUT
F
IN(MIN)
VOUT + VF
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.
IPEAK = +
OUT
1 - DMAX
MAX
IN(MIN)
2 · FS · L
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 (ΔVOUT) 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. 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 factor, but results may vary with package size and
among specific manufacturers.
The output capacitor size can be estimated using the
equation:
COUT =
OUT
MAX
FS · ∆VOUT
To maintain stable operation at full load, the output
capacitor should be sized to maintain ΔVOUT between
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 AAT1409 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 the frequency domain. The dominant pole can be cal-
culated by:
fP =
2π · R0 · C4