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| 型号： | MIC3231YML TR |
| 厂商： | Micrel Inc |
| 文件页数： | 15/19页 |
| 文件大小： | 0K |
| 描述： | IC LED DRVR HP CONST CURR 12-MLF |
| 标准包装： | 1 |
| 恒定电流： | 是 |
| 拓扑： | PWM，升压（升压） |
| 输出数： | 1 |
| 内部驱动器： | 是 |
| 类型 - 主要： | 通用 |
| 频率： | 100kHz ~ 1MHz |
| 电源电压： | 6 V ~ 45 V |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 12-VFDFN 裸露焊盘，12-MLF? |
| 供应商设备封装： | 12-MLF?（4x4） |
| 包装： | 标准包装 |
| 工作温度： | -40°C ~ 125°C |
| 产品目录页面： | 1082 (CN2011-ZH PDF) |
| 其它名称： | 576-3403-6 |
Micrel, Inc.
Maximum Power dissipated in R CS is;
Input Capacitor
MIC3230/1/2
P R CS = I R CS _ RMS × R CS
Eq. (17)
2
The input current is shown in Figure 5. For superior
performance, ceramic capacitors should be used because of
Eq. (18)
I R CS _ RMS _ max = I FET _ RMS _ max = D ? ? I IN _ AVE _ max 2 +
0 . 26 2 ? ?
I R CS _ RMS = 0 . 78 ? 1 . 64 2 +
?
12 ? ?
?
?
?
?
?
= 1 . 44 A _ rms
?
P R CS = 1 . 25 2 × . 15 = 0 . 31 watt
I L _ PP 2 ? ?
12
their low equivalent series resistance (ESR). The input ripple
current is equal to the ripple in the inductor plus the ripple
voltage across the input capacitor, which is the ESR of C IN
times the inductor ripple. The input capacitor will also
bypass the EMI generated by the converter as well as any
voltage spikes generated by the inductance of the input line.
For a required V IN_RIPPLE :
Eq. (21)
Use a 1/2 Watt resistor for R CS .
Output Capacitor
C IN =
I IN _ PP
8 × V IN _ RIPPLE × F SW
=
( 0 . 28 A )
8 × 50 mV × 500 kHz
= 1 . 4 μ F
ILED nom * D nom * T
In this LED driver application, the ILED ripple current is a
more important factor compared to that of the output
ripple voltage (although the two are directly related). To
find the C OUT for a required ILED ripple use the following
calculation:
For an output ripple ILED ripple = 20% of ILED nom
ILED ripple = 0 . 2 × 0 . 35 = 70 mA
Eq. (19) C out =
ILED ripple * ( R adj + R LED _ total )
Find the equivalent ac resistance R LED _ ac from the
datasheet of the LED. This is the inverse slope of the
ILED vs. V F curve i.e.:
This is the minimum value that should be used. The input
capacitor should also be rated for the maximum RMS input
current. To protect the IC from inductive spikes or any
overshoot, a larger value of input capacitance may be
required and it is recommended that ceramic capacitors be
used. In this design example a value of 4.7μF ceramic
capacitor was selected.
MOSFET Selection
In this design example, the FET has to hold off an output
voltage maximum of 30V. It is recommended to use an 80%
de-rating value on switching FETs, so a minimum of a 38V
FET should be selected. In this design example, a 75V FET
has been selected.
The switching FET power losses are the sum of the
Eq. (20)
R LED _ ac =
Δ V F
Δ ILED
conduction loss and the switching loss:
Eq. (22) P FET = P FET _ COND + P FET _ SWITCH
C out =
ILED nom * D nom * T
? I L _ PP ?
= D ? I IN _ AVE +
?
I FET _ RMS
12 ? ?
?
T =
Eq. (24)
In this example, use R LED _ ac = 0 . 1 ? for each LED.
If the LEDs are connected in series, multiply
R LED _ ac = 0 . 1 ? by the total number of LEDs. In this
example of 6 LEDs, we obtain the following:
R LED _ total = 6 × 0 . 1 Ω = 0 . 6 Ω
= 4 . 1 uF
ILED ripple * ( R adj + R LED _ total )
Use the next highest standard value, which is 4.7 μ F.
There is a trade off between the output ripple and the
rising edge of the PWMD pulse. This is because
between PWM dimming pulses, the converter stops
pulsing and C OUT will start to discharge. The amount that
C OUT will discharge depends on the time between PWM
Dimming pluses. At the next PWMD pulse C OUT has to
be charged up to the full output voltage V OUT before the
desired LED current flows.
The conduction loss of the FET is when the FET is turned
on. The conduction power loss of the FET is found by the
following equation:
Eq. (23) P FET _ COND = I FET _ RMS 2 × R DSON , where
2
2
?
The switching losses occur during the switching transitions
of the FET. The transition times, t transition , are the times when
the FET is turning off and on. There are two transition times
per period, T. It is important not to confuse T (the period)
with the transition time, t transition .
1
Fsw
Eq. (25)
P FET _ SWITCH _ max = I FET _ AVE _ max × V OUT _ max × t transition _ max × F SW
To find t transition _ max :
March 2011
15
M9999-030311-D
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