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AAT2824 参数 Datasheet PDF下载

AAT2824图片预览
型号: AAT2824
PDF下载: 下载PDF文件 查看货源
内容描述: [TFT-LCD DC/DC Converter with WLED Driver and VCOM Buffer]
分类和应用:
文件页数/大小: 28 页 / 3442 K
品牌: AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
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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()  
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  
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