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參數(shù)資料
| 型號： | LT3512HMS#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 0.8 A SWITCHING REGULATOR, 650 kHz SWITCHING FREQ-MAX, PDSO12 |
| 封裝： | LEAD FREE, PLASTIC, MSOP-16/12 |
| 文件頁數(shù)： | 9/26頁 |
| 文件大?。?/td> | 244K |
| 代理商： | LT3512HMS#TRPBF |
LT3512
17
3512fa
APPLICATIONS INFORMATION
In addition, primary inductance will determine switching
frequency.
f
SW =
1
t
ON + tOFF
=
1
L
PRI IPEAK
V
IN
+
L
PRI IPEAK
N
PS VOUT + VF
()
I
PEAK =
V
OUT IOUT 2
ηV
IN D
Example:
Let’s calculate switching frequency at our nominal VIN
of 48V.
D
=
15
+ 0.5
()2
15
+ 0.5
()2+48
= 0.39
I
PEAK =
15V 0.2A 2
0.83 48V 0.39
= 0.39A
Let’s choose LPRI = 200μH. Remember, most transform-
ers specify primary inductance with a tolerance of ±20%.
fSW = 240kHz
Finally, the transformer needs to be rated for the correct
saturation current level across line and load conditions.
In the given example, the worst-case condition for switch
current is at minimum VIN and maximum load.
I
PEAK =
V
OUT IOUT 2
ηV
IN D
I
PEAK =
15V 0.2A 2
0.83 36V 0.46
= 0.44A
Ensure that the saturation current covers steady-state
operation, start-up and transient conditions. To satisfy
these conditions, choose a saturation current 50% or more
higher than the steady-state calculation. In this example, a
saturation current between 700mA and 800mA is chosen.
Table 1 presents a list of pre-designed flyback transformers.
For this application, the Sumida 10396-T023 transformer
will be used.
Step 4: Choose the correct output diode.
The two main criteria for choosing the output diode include
forward current rating and reverse voltage rating. The
maximum load requirement is a good first-order guess
at the average current requirement for the output diode.
A better metric is RMS current.
I
RMS = IPEAK(VIN(MIN)) NPS
1– D
VIN(MIN)
3
Example:
I
RMS = 0.44 2
1– 0.46
3
= 0.37A
Next calculate reverse voltage requirement using maxi-
mum VIN:
V
REVERSE = VOUT +
V
IN(MAX)
N
PS
Example:
V
REVERSE = 15V +
72V
2
= 51V
A 1.0A, 60V diode from Diodes Inc. (DFLS160) will be used.
Step 5: Choose an output capacitor.
The output capacitor choice should minimize output voltage
ripple and balance the trade-off between size and cost for
a larger capacitor. Use the equation below at nominal VIN:
C
=
I
OUT D
ΔV
OUT fSW
Example:
Design for ripple levels below 50mV.
C
=
0.2A 0.39
0.05V 240kHz
= 6.5F
A 22μF, 25V output capacitor is chosen. Remember ce-
ramic capacitors lose capacitance with applied voltage.
The capacitance can drop to 40% of quoted capacitance
at the max voltage rating.
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