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| 型号： | A8515GLPTR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 31/34页 |
| 文件大小： | 0K |
| 描述： | IC LED DVR WHT/RGB BKLGT 16TSSOP |
| 标准包装： | 8,000 |
| 拓扑： | PWM，升压（升压） |
| 输出数： | 2 |
| 内部驱动器： | 是 |
| 类型 - 主要： | 背光 |
| 类型 - 次要： | 白色 LED |
| 频率： | 600kHz ~ 2.5MHz |
| 电源电压： | 5 V ~ 40 V |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-TSSOP（0.173"，4.40mm）裸露焊盘 |
| 供应商设备封装： | 16-TSSOP-EP |
| 包装： | 带卷 (TR) |
| 工作温度： | -40°C ~ 105°C |
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A8515
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)
(44)
4.7 μ F capacitor for this application.
> 15.9 (V) + 16 (V) = 31.9 V
because the maximum output voltage in this case is V OUT(OVP) .
The rms current through the capacitor is given by:
The peak current through the diode is calculated as:
I dp = I IN (max) + 1 / 2 Δ I Lused
(45)
I COUT rms = I OUT
D (max)
1 – D (max)
(47)
= 0.240 (A)
= 0.433 A
= 0.848 (A) + 0.096 (A) = 0.944A
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
0.765
1 – 0.765
The output capacitor must have a ripple current rating of at least
500 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:
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
C IN =
=
8
8
? I Lused
f SW ? V IN
0.191 (A)
2 (MHz) 0.05 (V)
= 0.24 μ F
(48)
1 – D (min)
C OUT = I LK
f PWM(dimming)
C IN rms =
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:
(46)
V COUT
The rms current through the capacitor is given by:
? I Lused
12
0.191 (A)
= = 0.055 A
12
(49)
= 200 ( μ A)
1 – 0.01
200 (Hz) 0.250 (V)
= 3.96 μ F
A good ceramic input capacitor with a rating of 2.2 μ F 25 V will
suffice for this application.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
31
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