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參數(shù)資料
| 型號(hào)： | NCV494 |
| 廠商： | ON SEMICONDUCTOR |
| 英文描述： | SWITCHMODE Pulse Width Modulation Control Circuit(開關(guān)模式PWM控制器) |
| 中文描述： | 開關(guān)模式脈寬調(diào)制控制電路（開關(guān)模式的PWM控制器） |
| 文件頁數(shù)： | 6/14頁 |
| 文件大?。?/td> | 144K |
| 代理商： | NCV494 |
TL494, NCV494
http://onsemi.com
6
APPLICATIONS INFORMATION
Description
The TL494 is a fixedfrequency pulse width modulation
control circuit, incorporating the primary building blocks
required for the control of a switching power supply. (See
Figure 1.) An internallinear sawtooth oscillator is
frequency programmable by two external components, R
T
and C
T
. The approximate oscillator frequency is determined
by:
f
osc
≈
1.1
R
T
C
T
For more information refer to Figure 3.
Output pulse width modulation is accomplished by
comparison of the positive sawtooth waveform across
capacitor C
T
to either of two control signals. The NOR gates,
which drive output transistors Q1 and Q2, are enabled only
when the flipflop clockinput line is in its low state. This
happens only during that portion of time when the sawtooth
voltage is greater than the control signals. Therefore, an
increase in controlsignal amplitude causes a corresponding
linear decrease of output pulse width. (Refer to the Timing
Diagram shown in Figure 2.)
The control signals are external inputs that can be fed into
the deadtime control, the error amplifier inputs, or the
feedback input. The deadtime control comparator has an
effective 120 mV input offset which limits the minimum
output deadtime to approximately the first 4% of the
sawtoothcycle time. This would result in a maximum duty
cycle on a given output of 96% with the output control
grounded, and 48% with it connected to the reference line.
Additional deadtime may be imposed on the output by
setting the deadtimecontrol input to a fixed voltage,
ranging between 0 V to 3.3 V.
Functional Table
Input/Output
Controls
Output Function
f
out
f
osc
=
Grounded
Singleended PWM @ Q1 and Q2
1.0
@ V
ref
Pushpull Operation
0.5
The pulse width modulator comparator provides a means
for the error amplifiers to adjust the output pulse width from
the maximum percent ontime, established by the deadtime
control input, down to zero, as the voltage at the feedback
pin varies from 0.5 V to 3.5 V. Both error amplifiers have a
common mode input range from 0.3 V to (V
CC
2V), and
may be used to sense powersupply output voltage and
current. The erroramplifier outputs are active high and are
ORed together at the noninverting input of the pulsewidth
modulator comparator. With this configuration, the
amplifier that demands minimum output on time, dominates
control of the loop.
When capacitor C
T
is discharged, a positive pulse is
generated on the output of the deadtime comparator, which
clocks the pulsesteering flipflop and inhibits the output
transistors, Q1 and Q2. With the outputcontrol connected
to the reference line, the pulsesteering flipflop directs the
modulated pulses to each of the two output transistors
alternately for pushpull operation. The output frequency is
equal to half that of the oscillator. Output drive can also be
taken from Q1 or Q2, when singleended operation with a
maximum ontime of less than 50% is required. This is
desirable when the output transformer has a ringback
winding with a catch diode used for snubbing. When higher
outputdrive currents are required for singleended
operation, Q1 and Q2 may be connected in parallel, and the
outputmode pin must be tied to ground to disable the
flipflop. The output frequency will now be equal to that of
the oscillator.
The TL494 has an internal 5.0 V reference capable of
sourcing up to 10 mA of load current for external bias
circuits. The reference has an internal accuracy of
with a typical thermal drift of less than 50 mV over an
operating temperature range of 0
°
to 70
°
C.
5.0%
Figure 3. Oscillator Frequency versus
Timing Resistance
500 k
,
fo
100 k
10 k
1.0 k
500
1.0 k 2.0 k 5.0 k
10 k
R
T,
TIMING RESISTANCE ( )
20 k
50 k
100 k 200 k
500 k 1.0 M
V
CC
= 15 V
0.01 F
0.1 F
C
T
= 0.001 F
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| NCV4949ADR2G | 功能描述:低壓差穩(wěn)壓器 - LDO 5.0V 100 MA LDO RoHS:否 制造商:Texas Instruments 最大輸入電壓:36 V 輸出電壓:1.4 V to 20.5 V 回動(dòng)電壓（最大值）:307 mV 輸出電流:1 A 負(fù)載調(diào)節(jié):0.3 % 輸出端數(shù)量: 輸出類型:Fixed 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-20 |
| NCV4949ADWR2G | 功能描述:低壓差穩(wěn)壓器 - LDO 5.0V 100 MA LDO RoHS:否 制造商:Texas Instruments 最大輸入電壓:36 V 輸出電壓:1.4 V to 20.5 V 回動(dòng)電壓（最大值）:307 mV 輸出電流:1 A 負(fù)載調(diào)節(jié):0.3 % 輸出端數(shù)量: 輸出類型:Fixed 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-20 |
| NCV4949APDR2G | 功能描述:低壓差穩(wěn)壓器 - LDO 5.0V 100 MA LDO RoHS:否 制造商:Texas Instruments 最大輸入電壓:36 V 輸出電壓:1.4 V to 20.5 V 回動(dòng)電壓（最大值）:307 mV 輸出電流:1 A 負(fù)載調(diào)節(jié):0.3 % 輸出端數(shù)量: 輸出類型:Fixed 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-20 |
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