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參數(shù)資料
| 型號(hào)： | LM3101 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Secondary-Side PWM Controller |
| 中文描述： | 二次側(cè)PWM控制器 |
| 文件頁(yè)數(shù)： | 9/20頁(yè) |
| 文件大?。?/td> | 319K |
| 代理商： | LM3101 |
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Functional Description
(Continued)
Frequency-Shift Circuit
The LM3101 has the ability to gradually reduce its operating
frequency during an output short circuit. The amount that
the frequency shifts and the output voltage threshold deter-
mining where the frequency starts to shift are both pro-
grammed by two external resistors, R
SC1
and R
SC2
, con-
nected to the pin R
SC
(pin 5).
A simplified internal schematic of the Frequency Shift Circuit
is shown in Figure 2. The oscillator operates at its nominal
frequency as long as the voltage at the emitter of the tran-
sistor Q2 is higher than the internal reference voltage, V
REF
.
Q2 emitter voltage is the output voltage, V
OUT
, scaled down
by the resistor divider:
V
RSC
e
V
Q2E
e
V
OUT
#
R
SC2
/(R
SC1
a
R
SC2
)
where V
Q2
l
V
REF
(1.24V) for normal operation.
If V
OUT
drops, due to an overload, a current starts to flow
through Q2. A cascoded current mirror causes one-tenth of
this current to be subtracted from the timing capacitor
charge current. Reducing the timing capacitor charge cur-
rent results in decreasing the oscillator frequency. The
breakpoint where the frequency-shift starts is programmed
by the ratio of the two resistors:
V
OUT(SC)
e
1.24V
#
[
1
a
(R
SC1
/R
SC2
)
]
The typical short circuit frequency is set by the following
equations:
F
SC
e
[
I
OSC
b
0.1
#
à
((1.24V
b
V
OUT(SC)
)/R
SC1
)
a
(1.24V/R
SC2
)
ó
] [
1/(20 pF
#
1.24V)
where
I
OSC
e
0.25
#
(1.24V/R
T
).
For example, say 140 k
X
and 100 k
X
were selected for
R
SC1
and R
SC2
, respectively, with R
T
set to 25 k
X
. Then
the output voltage level where the frequency starts to decay
is:
V
OUT(CL)
e
1.24V
#
[
1
a
(140 k
X
/100 k
X
)
]
j
3.0V,
and the short circuit frequency is (I
OSC
e
12.4
m
A and as-
suming V
OUT
is 0V during a short circuit):
F
SC
e
[
12.4
m
A
b
0.1
#
à
(1.24V/140 k
X
)
a
(1.24V/100
k
X
)
ó
] [
1/(20 pF
#
1.24V)
]
e
414.3 kHz
j
415 kHz.
[
V,
X
]
[
V,
X
]
.
[
kHz,
m
A, V,
X
]
If R
SC2
is omitted, the frequency starts to shift when V
OUT
drops below V
REF
. The short circuit frequency equation
then becomes:
F
SC
e
(1/20 pF)
#
[
(0.267/R
T
)
b
(0.09/R
SC1
)
]
v
R
SC1
e
0.09/
[
(0.267/R
T
)
b
(F
SC
#
20 pF)
]
Selecting a short-circuit frequency that is greater than one-
third the operating frequency or 188 kHz leads to a resistor
value of:
R
SC1
e
0.09/
[
(0.267/25k
X
)
b
(188kHz
#
20pF)
]
e
13k
X
.
[
kHz,
X
]
[
X
, kHz
]
.
Mode Control
The LM3101 can operate in voltage mode, current mode, or
charge mode control. Two multi-function pins are involved in
setting the operating mode, the Mode Control pin (MC - pin
2) and the Current Mode Input pin (CMI - pin 6). Figure 3
shows the simplified schematic diagram of the mode control
circuit. To operate with voltage mode control, the MC pin is
pulled high with a resistor (typically 3 k
X
), and the CMI pin is
connected to ground. The mode comparator senses the MC
pin voltage and sets the mode control multiplexer to voltage
mode control. Notice that there is a 5.6V zener diode clamp-
ing the MC pin voltage.
TL/H/11436–8
FIGURE 3. Simplified Version
of the Mode Control Circuit
TL/H/11436–9
FIGURE 4. Current Mode Sense Circuit
9
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