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| 型号： | A8510GECTR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 30/33页 |
| 文件大小： | 0K |
| 描述： | IC LED DRVR BCKLGHT/WHT 26QFN-EC |
| 标准包装： | 1 |
| 拓扑： | PWM，升压（升压） |
| 输出数： | 8 |
| 内部驱动器： | 是 |
| 类型 - 主要： | 背光 |
| 类型 - 次要： | 白色 LED |
| 频率： | 600kHz ~ 2.25MHz |
| 电源电压： | 5 V ~ 40 V |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 26-WFQFN 裸露焊盘 |
| 供应商设备封装： | 26-QFN/MLP-EP（4x4） |
| 包装： | 标准包装 |
| 工作温度： | -40°C ~ 105°C |
| 其它名称： | 620-1385-6 |
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A8510
Wide Input Voltage Range, High Efficiency
Fault Tolerant LED Driver
The reverse breakdown voltage rating for the output diode in a
SEPIC circuit should be:
A capacitor larger than 3.96 μ F should be selected due to degra-
dation of capacitance at high voltages on the capacitor. Select a
V BD > V OUT(OVP) (max) + V IN (max)
> 15.9 (V) + 16 (V) = 31.9 V
(43)
4.7 μ F capacitor for this application.
The rms current through the capacitor is given by:
because the maximum output voltage in this case is V OUT(OVP) .
The peak current through the diode is calculated as:
I COUT rms = I OUT
D (max)
1 – D (max)
(46)
I dp = I IN (max) + 1 / 2 Δ I Lused
= 1.131 (A) + 0.160 (A) = 1.291 A
(44)
= 0.320 (A)
0.765
1 – 0.765
= 0.577 A
C IN = (47)
? V IN
= 1.00 μ F
=
1 – D (min)
C OUT = I LK
f PWM(dimming)
C IN rms =
? I Lused
The third major component in deciding the switching diode is the
reverse current, I R , characteristic of the diode. This characteristic
is especially important when PWM dimming is implemented.
During PWM off-time the boost converter is not switching. This
results in a slow bleeding off of the output voltage, due to leakage
currents. I R can be a large contributor, especially at high tempera-
tures. On the diode that was selected in this design, the current
varies between 1 and 100 μ A. It is often advantageous to pick a
diode with a much higher breakdown voltage, just to reduce the
reverse current. Therefore for this example, pick a diode rated for
a V BD of 60 V, instead of just 40 V.
Step 7 Choosing the output capacitors. The output capacitors
must be chosen such that they can provide filtering for both the
boost converter and for the PWM dimming function. The biggest
factors that contribute to the size of the output capacitor are:
PWM dimming frequency and PWM duty cycle. Another major
contributor is leakage current, I LK . This current is the combina-
tion of the OVP leakage current as well as the reverse current of
the switching diode. In this design the PWM dimming frequency
is 200 Hz and the minimum duty cycle is 1%. Typically, the volt-
age variation on the output, V COUT , during PWM dimming must
be less than 250 mV, so that no audible hum can be heard. The
capacitance can be calculated as follows:
(45)
V COUT
The output capacitor must have a ripple current rating of at least
600 mA. The capacitor selected for this design is a 4.7 μ F 50 V
capacitor with a 1.5 A current rating.
Step 8 Selecting input capacitor. The input capacitor must be
selected such that it provides a good filtering of the input voltage
waveform. A estimation rule is to set the input voltage ripple,
Δ V IN , to be 1% of the minimum input voltage. The minimum
input capacitor requirements are as follows:
? I Lused
8 f SW
0.319 (A)
8 800 (kHz) 0.05 (V)
The rms current through the capacitor is given by:
(48)
12
0.319 (A)
= = 0.092 A
12
A good ceramic input capacitor with a rating of 2.2 μ F 25 V will
suffice for this application.
= 200 ( μ A)
1 – 0.01
200 (Hz) 0.250 (V)
= 3.96 μ F
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
30
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