DATA SHEET
AAT2822/2823/2824/2825
TFT-LCD DC/DC Converter with WLED Driver and VCOM Buffer
When the rising OVP threshold is exceeded, switching is
stopped and the output voltage decays. Switching auto-
matically restarts when the output drops below the
lower OVP hysteresis voltage (100mV typical), and as a
result the output voltage increases. The cycle repeats,
maintaining an average DC output voltage proportional
to the average of the rising and falling OVP levels (mul-
tiplied by the resistor divider scaling factor). High oper-
ating frequency and low output voltage ripple ensure DC
current and negligible flicker in the LED string(s).
Maximum ILED Current (mA)
R8(Ω)
30
25
20
15
10
5
0.768
0.909
1.15
1.54
2.32
4.64
Table 2: Maximum LED Current and Ballast
Resistor (R8) Values for 10” Panel Size.
While OVP is active, the maximum LED current program-
ming error (ΔILED) is proportional to voltage error across
an individual LED (ΔVFLED).
Typical white LEDs are driven at maximum continuous
currents of 15mA to 20mA. The maximum number of
series-connected LEDs is determined by the minimum
output voltage of the boost converter (VLED), minus the
maximum feedback voltage (VWFB(MAX)) divided by the
maximum LED forward voltage (VFLED(MAX)) which can be
estimated from the manufacturers’ datasheet at the
maximum LED operating current.
N · VFLED(TYP) - VOVP(MIN) - VWFB
∆VFLED
=
N
To minimize the ΔILED error, the minimum OVP voltage
(VOVP(MIN)) may be increased, yielding a corresponding
increase in the maximum OVP voltage (VOVP(MAX)
)
.
R5
VLED = VOVP(TYP)
·
+ 1
Measurements should confirm that the maximum switch-
ing node voltage (VWLX(MAX)) is less than 30V under worst
case operating conditions.
R4
VOVP(MIN) - VWFB(MAX)
N =
VFLED(MAX)
R2
R1
VWLX(MAX) = VOVP(MAX)
·
+ 1 + VF + VRING
For example, the typical forward voltage of the white
LED is 3.5V at 20mA.
VF is the Schottky diode D2 forward voltage at turn-OFF.
VRING is the voltage ring occurring at turn-OFF.
R5
R4
464kΩ
10kΩ
VLED = VOVP(TYP)
·
+ 1 = 0.6V ·
+ 1 = 27.8V
White LED Selection and Current Setting
VOVP(MIN) - VWFB(MAX)
27.8V - 0.6V
3.5V
The WLED current is controlled by the WFB voltage and
the ballast resistor (R8). For maximum accuracy, a 1%
tolerance resistor is recommended.
N =
=
= 7.8 LEDs
VFLED(MAX)
Therefore, under these typical operating conditions, 7
LEDs can be used in series for each string.
The ballast resistor (R8) value can be calculated as fol-
lows:
VWFB(MAX)
ILED(MAX)
PWM Dimming Control
R8 =
The dimming of the white LED can be controlled using a
PWM or a filter PWM signal. By connecting a PWM signal
to the WDIM pin and adjusting the duty cycle of the PWM
signal, the dimming of the white LED changes proportion-
ally to the percentage of the duty cycle as shown in Figure
4. However, the dimming control using PWM connected to
the WDIM pin can operate at a frequency up to 1kHz.
Where VWFB = 0.3V
For example, if the maximum current for each string of
3 series LEDs is 20mA, the maximum current for a 10
inch panel (3S13P) is 260mA (20mA x 13), which cor-
responds to a minimum resistor value of 1.15Ω
0.3V
260mA
R8 = =
1.15Ω
Skyworks Solutions, Inc.
•
Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
17
202081B
• Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 2, 2012
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
