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參數(shù)資料
| 型號： | LT1683IG |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Ultralow Noise Push-Pull DC/DC Controller |
| 中文描述： | SWITCHING CONTROLLER, 250 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封裝： | 5.30 MM, PLASTIC, SSOP-20 |
| 文件頁數(shù)： | 18/24頁 |
| 文件大小： | 237K |
| 代理商： | LT1683IG |
18
LT1683
1683f
APPLICATIU
Continuous operation occurs when the current in the
inductor never goes to zero. Discontinuous operation
occurs when the inductor current drops to zero before the
start of the next cycle and can occur with small inductors
and light loads. There is nothing inherently bad about
discontinuous operation, however, converter control and
operation are somewhat different. The inductor is smaller
for discontinuous operation but the peak currents in the
switch, the transformer, the diodes, inductor and capaci-
tor will be higher which may produce greater losses.
For continuous operation the inductor ripple current must
be less than twice the output current. The worst case for
this is at maximum input (lowest duty cycle, DC
MIN
) but in
the following we will evaluate at nominal input since the
I
OUT
/4 is somewhat arbitrary.
Note when both inputs are off, the inductor current splits
between both secondary outputs and the diode common
goes to 0V.
Looking at the inductor current during off time, output
ripple current is:
W
U
U
I
OUT MIN
=
=
=
+
I
(
)
(
f
)
I
I
I
L
V
V
DC
OUT
OUT MIN
(
OUT MAX
(
OUT MIN
(
F
OUT
2
4
1 2
/
)
(
)
)
)
The inductance of the transformer primary should be such
that L, when reflected into the primary, dominates the
input current. In other words, we want the magnetizing
current of the transformer small with respect to the
current going through the transformer to L. In general,
then, the inductance of the primary should be at least five
times that of L reflected to the input. This ensures that
most of the power will be passed through the transformer
to the load. It also increases the power capability of the
converter and reduces the peak currents that the switch
will see.
L
L
N
PRI
=
5
2
If the magnetizing current is small, say below 100mA, then
a smaller L can be used with a higher percentage of the
switch current generated by the magnetizing current.
With the value of L set, the ripple in the inductor is:
(
I
L f
=
+
)
(
)
V
V
DC
L
OUT
F
1 2
However, the peak inductor current is evaluated at maxi-
mum load and maximum input voltage (minimum DC).
I
I
I
L MAX
(
L MAX
(
L MAX
(
2
)
)
)
=
+
The magnetizing ripple current can be shown to be:
=
+
I
V
N L
V
f
MAG
OUT
F
PRI
and the peak current in the switch is:
I
SW(PEAK)
= N I
L(MAX)
+
I
MAG
This current should be less than the current limit.
Worst-case switch ripple is:
I
SW(PEAK)
= N
I
L(MAX)
+
I
MAG
In the push-pull converter the maximum switch voltage
will be 2 V
IN
. Because voltage is slew-controlled, the
leakage spikes are small. So, the MOSFET should have a
maximum rated switch voltage at least 20% higher than
2 V
IN
.
So, given the turns ratio, primary inductance and current,
the transformer can be designed. The design of the trans-
former will require analyzing the power losses of the
transformer and making necessary adjustments.
Most transformer companies can assist you with design-
ing an optimal solution. For instance Midcom, Inc. (1-800-
643-2661). Linear Technology’s application group can
also help.
As an example say we are designing a 48V
±
20% to 5V
100kHz converter with 2A output and 500mA ripple. Then
starting with a guess for the on voltage of the MOSFET plus
sense resistor of 0.5V and V
F
of 0.5V:
N
=
+
(
)
=
5
48 80
0 5
.
88
0 5
.
1
.
6 1
%
%
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