DATA SHEET
AAT2822/2823/2824/2825
TFT-LCD DC/DC Converter with WLED Driver and VCOM Buffer
Choose R21 = 4.99kΩ and VC25 = 2V, and solve for R22:
high efficiency under light load. The rectifier's reverse
current increases dramatically at elevated temperatures.
R21
R22
=
= 28kΩ
Selecting the WLED Step-Up Inductor
(0.3V - VR8) · (VC25 - 0.3V)
The WLED step-up converter has the same topology as
the main step-up converter. It is designed to operate
with a 2.2μH inductor for all input and output voltage
combinations. The inductor saturation current rating
should be greater than the NMOS current limit.
The low-pass filter should be chosen to produce an
acceptable ripple for the DC dimming voltage and a
small time constant. For application where the PWM fre-
quency is greater than 10KHz, the optimum values for
the low-pass filter are R23 = 4.99kΩ and C25 = 0.1µF.
(VOUT + VF - VIN(MIN)
)
DMAX
=
Selecting the Schottky Diode
VOUT + VF
To ensure minimum forward voltage drop and no recov-
ery, high-voltage Schottky diodes are considered the
best choice for the WLED boost converter. The output
diode is sized to maintain acceptable efficiency and rea-
sonable operating junction temperature under full load
operating conditions. Forward voltage (VF) and package
thermal resistance (θJA) are the dominant factors to con-
sider in selecting a diode. The diode's non-repetitive
peak forward surge current rating (IFSM) should be con-
sidered for high pulsed load applications such as camera
flash. The IFSM rating drops with increasing conduction
period. Manufacturers’ datasheets should be consulted
to verify reliability under peak load conditions. The
diode’s published current rating may not reflect actual
operating conditions and should be used only as a com-
parative measure between similarly rated devices.
The output inductor (L) is selected to avoid saturation at
minimum input voltage and maximum output load con-
ditions. Peak current may be calculated from the follow-
ing equation, again assuming continuous conduction
mode. Worst-case peak current occurs at minimum input
voltage (maximum duty cycle) and maximum load.
Switching frequency is estimated at 1.3MHz with a
2.2µH inductor.
IOUT
1 - DMAX
DMAX · VIN(MIN)
2 · FS · L
IPEAK
=
+
Selecting the WLED Step-Up Capacitors
The high output ripple inherent in the boost converter
necessitates low impedance output filtering.
40V rated Schottky diodes are recommended for outputs
less than 30V, while 60V rated Schottky diodes are rec-
ommended for outputs greater than 35V.
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 WLED boost
regulator. MLC capacitors of type X7R or X5R are recom-
mended to ensure good capacitance stability over the
full operating temperature range.
The average diode current is equal to the output cur-
rent:
IAVG = IOUT
The average output current multiplied by the forward
diode voltage determines the loss of the output diode.
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 with a value of
2.2μF to 4.7μF is recommended. Typically, 50V rated
capacitors are required for the 28V maximum boost out-
put. Ceramic capacitors sized as small as 0805 or 1206
are available which meet these requirements.
PLOSS_DIODE = IAVG · VF = IOUT · VF
Diode junction temperature can be estimated:
TJ = TA + θJA · PLOSS_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 mini-
mized with additional PCB area on the cathode. PCB
heat-sinking the anode may degrade EMI performance.
The reverse leakage current of the rectifier must be con-
sidered to maintain low quiescent (input) current and
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.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
19
202081B
• Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 2, 2012
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
