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参数资料
| 型号： | PJ494CD |
| 元件分类： | 稳压器 |
| 英文描述： | 0.25 A SWITCHING CONTROLLER, 300 kHz SWITCHING FREQ-MAX, PDIP16 |
| 封装： | DIP-16 |
| 文件页数： | 7/11页 |
| 文件大小： | 688K |
| 代理商： | PJ494CD |
PJ494
Switchmode Pulse Width Modulation Control Circuit
5-11
2002/11.rev.A
Description
The PJ494 is a fixed-frequency pulse width modulation control circuit, incorporating the primary building blocks required for
the control of a switching power supply . (See Figure 1.) An internal-linear sawtooth oscillator is frequescy-programmable by two
external components, RT and CT. The approximate oscillator frequency is determined by:
……….. For more information refer to Figure 3.
Output pulse width modulation is accomplished by comparison of the positive sawtooth waveform across capacitor CT to
either of two control signals. The NOR gates, which drive output transistors Q1 and Q2, are enabled only when the flip-flop
clock-input line is in its low state. This happens only during that portion of time when the sawtoothvoltage is greater than the
control signals. Therefore, an increase in contro-signal 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 120mV input offset which limits the minimum output deadtime to
approximately tge first 4% of the sawtooth-cycle 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 deadtime-control input to a fixed voltage, ranging between 0V to 3.3V .
The pulse width modulator comparator provides a means for the error amplifiers to adjust the output pulse width from the
maximum percent on-time, established by the deadtime control input, down to zero as the voltage at the feedback pin varies from
0.5V to 3.5V. Both error amplifiers have a common mode input range from -0.3C to (Vcc – 2V), and may be ised to sense power-
supply output voltage and current. The error –amplifier outputs are active high and are ORed together at the noninverting input of
the pulse-width modulator comparator. With this configuration, the amplifier that demands minimum output on time, dominates
control of the loop.
When capacitor CT is discharged, a positive pulse is generated on the output of the deadtime comparator, which clocks the
pulse-steering flip=flop and inhibits the output transistors, Q1 and Q2. With the output-control connected to the reference line, the
pulse-steering flip-flop directs the modulated pulses to each of the rwo output transistors alternately for push-pull operation. The
output frequency is equal to half that of the oscillator. Output drive can also be taken from Q1 and Q2, when single-ended
operation with a maximum on-time of less than 50% is required. This is desirable when the output transformer has a ringback
winding with a catch diode ised for snubbing. When higher output-drive currents are required for single-ended operation, Q1 and
Q2 may be connected in parallel, and the output mode pin must be tied to ground to disable the flip-flop. The output frequency
will now be equal to that of the oscillator.
The PJ494 has an internal 5.0V reference capable of sourcing up to 10mA of laod current for external bias circuits. The
reference has an internal accuracy of ±5.0% with a typical thermal drift of less than 50mV over an operating temperature range of 0 to
70℃.
Figure 3. Oscillator Frequency versus Timing Resistance
APPLICATIONS INFORMATION
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相关代理商/技术参数 |
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| PJ494CS | 制造商:未知厂家 制造商全称:未知厂家 功能描述:Switchmode Pulse Width Modulation Control Circuit |
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| PJ4L40 | 制造商:PANJIT 制造商全称:Pan Jit International Inc. 功能描述:QUAD SCHOTTKY DATA LINE BUS TERMINATOR |
| PJ4L40_08 | 制造商:PANJIT 制造商全称:Pan Jit International Inc. 功能描述:QUAD SCHOTTKY DATA LINE BUS TERMINATOR |
| PJ4L40T/R13 | 制造商:PANJIT 制造商全称:Pan Jit International Inc. 功能描述:QUAD SCHOTTKY DATA LINE BUS TERMINATOR |
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