DATA SHEET
AAT2822/2823/2824/2825
TFT-LCD DC/DC Converter with WLED Driver and VCOM Buffer
6 is 6.04kΩ. It is best to select the smallest value pos-
sible for R10, as this will keep the value of R9 to a mini-
mum. With R10 selected, R9 can be determined:
positive and negative charge pumps (regardless of the
number of stages) is:
VREVERSE = VIN - VF
VGL
VGL
R9 =
· R10
=
· R10
The BAT54SDW quad Schottky diode in a SOT363
(2x2mm) package is a good choice for multiple-stage
charge pump configuration.
VREF
1.2V
Positive Output Voltage (VGH
)
The positive output voltage is set by a resistive divider
from the output (VGH) to the FBP and ground pins.
Limiting the value of R7 to 6.04kΩ or lower reduces noise
in the feedback circuit.
White LED Driver
The white LED backlight driver can be enabled when input
supply rises above under voltage lockout threshold. To
reduce inrush current it is recommended that the main
boost and white LED driver are not enabled concurrently.
Once R7 has been determined, solve for R6:
VGH
VFBP
VGH
0.6V
R6 = R7 ·
- 1 = R7 ·
- 1
Over-Voltage Protection (OVP)
with Open Circuit Failure
The OVP protection circuit consists of a resistor network
tied from the output voltage to the OVP pin (see Figure
3). To protect the device from open circuit failure, the
resistor divider can be selected such that the over-volt-
age threshold occurs prior to the output reaching
VLED+(MAX). The value of R5 should be selected from 10kΩ
to 20kΩ to minimize losses without degrading noise
immunity.
Flying and Output Capacitors
The minimum value for the flying capacitor is limited by
the output power requirement, while the maximum value
is set by the bandwidth of the power supply. If CFLY is too
small, the output may not be able to deliver the power
demanded, while too large of a capacitor may limit the
bandwidth and time required to recover from load and
line transients. A 0.1µF X7R or X5R ceramic capacitor is
typically used. The voltage rating of the flying and reser-
voir output capacitors varies with the number of charge
pump stages. The reservoir output capacitor value should
be roughly 10 times the value of the flying capacitor. Use
larger capacitors for reduced output ripple.
VLED+(MAX)
VLED+(MAX)
0.6V
R4 = R5 ·
-1 = 10kΩ ·
-1
V
OVP
L2
D2
C5
VWLX
VLED
VIN
C4
Input Capacitor
WLX
The primary function of the input capacitor is to provide
a low impedance loop for the edges of pulsed current
drawn by the IC. A low ESL X7R or X5R type ceramic
capacitor is ideal for this function. The size required will
vary depending on the load, output voltage, and input
voltage characteristics. Typically, the input capacitor
value should be 5 to 10 times the value of the flying
capacitor. If the source impedance of the input supply is
high, a larger capacitor may be required. To minimize
stray inductance, the capacitor should be placed as
closely as possible to the IC. This keeps the high fre-
quency content of the input current localized, minimizing
radiated and conducted EMI.
R4
OVP
WFB
R8
R5
0.6V
0.3V
Figure 3: Over-Voltage Protection Circuit.
OVP Constant Voltage Operation
Under closed loop constant current conditions, the output
voltage is determined by the operating current, LED for-
ward voltage characteristics (VFLED), quantity of series
connected LEDs (N), and the feedback pin voltage (VFB).
Rectifier Diodes
For the rectifiers, use Schottky diodes with a voltage rat-
ing of 1.5 times the input voltage. The maximum steady-
state voltage seen by the rectifier diodes for both the
VOUT = VFB + N · VFLED
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
16
202081B
• Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 2, 2012
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
