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| 型号： | MIC3230YML TR |
| 厂商： | Micrel Inc |
| 文件页数： | 16/19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： | 无 |
| 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： | 576-3401-6 |
Eq. (26) t transition _ max ≈
Micrel, Inc.
Qg
Igatedrv
where Qg is the total gate charge of the external
MOSFET provided by the MOSFET manufacturer and
MIC3230/1/2
voltage stress on the diode is the max V OUT and therefore a
diode with a higher rating than max V OUT should be used. An
80% de-rating is recommended here as well.
Eq (28) P diode ≈V SCHOTTKY ×I OUT_ max
the Qg should chosen at a V GS ≈ 10V. This is not an
exact value, but is more of an estimate of t transition _ max .
The FET manufacturers’ provide a gate charge at a
specified V GS voltage:
Eq. (29)
P diode ≈ V SCHOTTKY × I LED _ max
P diode ≈ 0 . 25 W
C In _ FET =
Q G
@ V GS
MIC3230 Power Losses
The power losses in the MIC3230are:
This is the FET’s input capacitance. Select a FET with
Eq.(30)
P MIC 3230 = Q gate × V gate × F + I Q × Vin
the R DS ( on ) ( 25 C ) for R DS ( on ) ( 125 C ) .
R DS(on) and Q G such that the external power is below
about 0.7W for a SO-8 or about 1W for a PowerPak
(FET package). The Vishay Siliconix Si7148DP in a
PowerPak SO-8 package is one good choice. The
internal gate driver in the MIC3230/1/2 is 2A. From the
Si7148DP data sheet:
R DS(on)_25°C =0.0145 ?
Total gate Charge=68nC (typical)
The R DS ( on ) ( temp ) is a function of temperature. As the
temperature in the FET increases so does the R DS(on) .
To find R DS ( on ) ( temp ) use Equation 27, or simply double
o o
where Q gate is the total gate charge of the external
MOSFET. V gate is the gate drive voltage of the MIC3230.
F is the switching frequency. I Q is the quiescent current of
the MIC3230 found in the electrical characterization table.
I Q = 3 . 2 mA . V IN is the voltage at the V IN pin of the MIC3230 .
From Eq.(30)
P MIC 3230 = 68 nF × 12 × 500 kHz + 3 . 2 mA × 14 = 0 . 45 W
OVP - Over voltage protection
Set OVP higher than the maximum output voltage by at least
one volt. To find the resistor divider values for OVP use
Equation 3 and set the OVP=30V and R8=100k ? :
Eq. (27) R DS ( on ) ( temp ) = R DS ( on ) ( 25 o C ) × ( 1 . 007 ( Temp ? 25 ) )
The R DS ( on ) ( temp ) at 125°C is:
o
R 9 =
100 k Ω × 1 . 245
30 ? 1 . 245
= 4 . 33 k Ω
? 25 o )
R DSon ( 125 o C ) = 0 . 0145 × ( 1 . 007 ( 125
) ≈ 30 m Ω
From Equation 26: t transition ≈
=
= 34 ns
?
From Equation 23: P FET _ COND = 1 . 64 2 × 30 m Ω = 62 mW
Qg 68 nC
Igatedrv 2 A
I FET _ AVE _ max = 1 . 64 A
V OUT _ max = 28 V
From Equation 25:
P FET _ SWITCH _ max = 1 . 64 A × 28 V × 34 ns × 500 kHz = 0 . 78 Watts
From Equation 22
P FET = 62 mW + 0 . 78 W = 0 . 84 W
This is about the limit for a part on a circuit board without
having to use any additional heat sinks.
Rectifier Diode
A Schottky Diode is best used here because of the lower
forward voltage and the low reverse recovery time. The
PCB Layout
1. All typologies of DC-to-DC converters have a reverse
recovery current (RRC) of the flyback or (freewheeling)
diode. Even a Schottky diode, which is advertised as having
zero RRC, it really is not zero. The RRC of the freewheeling
diode in a boost converter is even greater than in the Buck
converter. This is because the output voltage is higher than
the input voltage and the diode has to charge up to –V OUT
during each on-time pulse and then discharge to V F during
the off-time.
2. Even though the RRC is very short (tens of nanoseconds)
the peak currents are high (multiple amperes). The high
RRC causes a voltage drop on the ground trace of the PCB
and if the converter control IC is referenced to this voltage
drop, the output regulation will suffer.
3. It is important to connect the IC’s reference to the same
point as the output capacitors to avoid the voltage drop
caused by RRC. This is also called a star connection or
single point grounding.
4. Feedback trace: The high impedance traces of the FB
should be short.
March 2011
16
M9999-030311-D
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