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參數(shù)資料
| 型號(hào)： | MIC2171BU |
| 廠商： | MICREL INC |
| 元件分類(lèi)： | 穩(wěn)壓器 |
| 英文描述： | Isolated Flyback Switching Regulator with 9V Output |
| 中文描述： | 5.5 A SWITCHING REGULATOR, 115 kHz SWITCHING FREQ-MAX, PSSO5 |
| 封裝： | TO-263, 5 PIN |
| 文件頁(yè)數(shù)： | 7/10頁(yè) |
| 文件大?。?/td> | 80K |
| 代理商： | MIC2171BU |
1997
4-9
MIC2171
Micrel
4
mode is preferred because the feedback control of the
converter is simpler.
When L1 discharges its current completely during the MIC2171
off-time, it is operating in discontinuous mode.
L1 is operating in continuous mode if it does not discharge
completely before the MIC2171 power switch is turned on
again.
Discontinuous Mode Design
Given the maximum output current, solve equation (1) to
determine whether the device can operate in discontinuous
mode without initiating the internal device current limit.
(1)
I
I
2
V
V
OUT
CL
IN(min)
OUT
≤
δ
(1a)
δ
=
V
+ V
– V
V
OUT
+V
OUT
F
IN(min)
F
Where:
I
CL
= internal switch current limit
I
CL
= 2.5A when
δ
< 50%
I
CL
= 1.67 (2 –
δ
) when
δ
≥
50%
(Refer to Electrical Characteristics.)
I
OUT
= maximum output current
V
IN(min)
= minimum input voltage = V
IN
– V
SW
δ
= duty cycle
V
OUT
= required output voltage
V
F
= D1 forward voltage drop
For the example in Figure 1.
I
OUT
= 0.25A
I
CL
= 1.67 (2–0.662) = 2.24A
V
IN(min)
= 4.18V
δ
= 0.662
V
OUT
= 12.0V
V
F
= 0.36V (@ .26A, 70
°
C)
Then:
I
2.235
2
.178
4
0.662
OUT
≤
×
12
I
OUT
≤
0.258A
This value is greater than the 0.25A output current require-
ment, so we can proceed to find the minimum inductance
value of L1 for discontinuous operation at P
OUT
.
(2)
L1
V
2 P
OUT SW
≥
(
)
δ
2
Where:
P
OUT
= 12
×
0.25 = 3W
f
SW
= 1
×
10
5
Hz (100kHz)
The junction temperature for any semiconductor is calculated
using the following:
T
J
= T
A
+ P
(total)
θ
JA
Where:
T
J
= junction temperature
T
A
= ambient temperature (maximum)
P
(total)
= total power dissipation
θ
JA
= junction to ambient thermal resistance
For the practical example:
T
A
= 70
°
C
θ
JA
= 45
°
C/W (TO-220)
Then:
T
J
= 70 + (1.24
×
45)
T
J
= 126
°
C
This junction temperature is below the rated maximum of
150
°
C.
Grounding
Refer to Figure 5. Heavy lines indicate high current paths.
MIC2171
IN
SW
FB
COMP
V
IN
GND
Single point ground
Figure 5. Single Point Ground
A single point ground is strongly recommended for proper
operation.
The signal ground, compensation network ground, and feed-
back network connections are sensitive to minor voltage
variations. The input and output capacitor grounds and
power ground conductors will exhibit voltage drop when
carrying large currents. Keep the sensitive circuit ground
traces separate from the power ground traces. Small voltage
variations applied to the sensitive circuits can prevent the
MIC2171 or any switching regulator from functioning prop-
erly.
Boost Conversion
Refer to Figure 1 for a typical boost conversion application
where a +5V logic supply is available but +12V at 0.25A is
required.
The first step in designing a boost converter is determining
whether inductor L1 will cause the converter to operate in
either continuous or discontinuous mode. Discontinuous
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