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參數(shù)資料
| 型號： | LT3575IFE#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 4.2 A SWITCHING REGULATOR, 1000 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 4.40 MM, LEAD FREE, PLASTIC, TSSOP-16 |
| 文件頁數(shù)： | 8/24頁 |
| 文件大?。?/td> | 240K |
| 代理商： | LT3575IFE#PBF |
LT3575
16
3575f
APPLICATIONS INFORMATION
The transformer turns ratio is selected such that the
converter has adequate current capability and a switch
stress below 50V. Table 6 shows the switch voltage stress
and output current capability at different transformer
turns ratio.
Table 6. Switch Voltage Stress and Output Current Capability vs
Turns-Ratio
N
VSW(MAX) AT VIN(MAX)
(V)
IOUT(MAX) AT VIN(MIN)
(A)
DUTY CYCLE
(%)
1:1
33.5
1.26
16~22
2:1
39
2.07
28~35
3:1
44.5
2.63
37~45
4:1
50
3.05
44~52
BIAS winding turns ratio is selected to program the BIAS
voltage to 3V~5V. The BIAS voltage shall not exceed the
input voltage.
The turns ratio is then selected as primary: secondary:
BIAS = 3:1:1.
2. Select the transformer primary inductance for target
switching frequency.
The LT3575 requires a minimum amount of time to sample
the output voltage during the off-time. This off-time,
tOFF(MIN), shall be greater than 350ns over all operating
conditions. The converter also has a minimum current limit,
IMIN, of 400mA to help create this off-time. This denes
the minimum required inductance as dened as:
L
NV
V
I
t
MIN
OUT
F
MIN
OFF MIN
=
+
()
()
The transformer primary inductance also affects the
switching frequency which is related to the output ripple. If
above the minimum inductance, the transformer’s primary
inductance may be selected for a target switching frequency
range in order to minimize the output ripple.
The following equation estimates the switching frequency.
f
tt
I
V
L
I
NV
V
L
SW
ON
OFF
PK
IN
PK
PS
OUT
F
=
+
=
+
11
()
Table 7.Switching Frequency at Different Primary
Inductance at IPK
L (μH)
fSW AT VIN(MIN)
(kHz)
fSW AT VIN(MAX)
(kHz)
15
174
205
30
87
103
60
44
51
Note: The switching frequency is calculated at maximum output.
In this design example, the minimum primary inductance is
used to achieve a nominal switching frequency of 200kHz at
full load. The 750311458 from Würth Elektronik is chosen
as the yback transformer.
Given the turns ratio and primary inductance, a custom-
ized transformer can be designed by magnetic component
manufacturer or a multi-winding transformer such as a
Coiltronics Versa-Pac may be used.
3. Select the output diodes and output capacitor.
The output diode voltage stress VD is the summation of
the output voltage and reection of input voltage to the
secondary side. The average diode current is the load
current.
VV
V
N
D
OUT
IN
=+
The output capacitor should be chosen to minimize the
output voltage ripple while considering the increase in
size and cost of a larger capacitor. The following equation
calculates the output voltage ripple.
ΔV
LI
CV
MAX
PK
OUT
=
2
4. Select the snubber circuit to clamp the switch
voltage spike.
A yback converter generates a voltage spike during switch
turn-off due to the leakage inductance of the transformer.
In order to clamp the voltage spike below the maximum
rating of the switch, a snubber circuit is used. There are
many types of snubber circuits, for example R-C, R-C-D and
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