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TL494

PULSE-WIDTH-MODULATION CONTROL CIRCUITS

FEATURES

· complete pwm power-control circuitry

· uncommitted outputs for 200-ma sink or source current

· output control selects single-ended or push-pull operation

· internal circuitry prohibits double pulse at either output

· variable dead time provides control over total range

· internal regulator provides a stable 5-v reference supply with 5% tolerance

· circuit architecture allows easy synchronization

INSTRUCTIONS

The TL494 incorporates all the functions required in the construction of a pulse-width-modulation (PWM) control circuit on a single chip. Designed primarily for power-supply control, this device offers the flexibility to tailor the power-supply control circuitry to a specific application.

The TL494 contains two error amplifiers, an on-chip adjustable oscillator, a dead-time control (DTC) comparator, a pulse-steering control flip-flop, a 5-V, 5%-precision regulator, and output-control circuits.

The error amplifiers exhibit a common-mode voltage range from –0.3 V to VCC – 2 V. The dead-time control comparator has a fixed offset that provides approximately 5% dead time. The on-chip oscillator can be bypassed by terminating RT to the reference output and providing a sawtooth input to CT, or it can drive the common circuits in synchronous multiple-rail power supplies.

The uncommitted output transistors provide either common-emitter or emitter-follower output capability. The TL494 provides for push-pull or single-ended output operation, which can be selected through the output-control function. The architecture of this device prohibits the possibility of either output being pulsed twice during push-pull operation.

The TL494C is characterized for operation from 0°C to 70°C. The TL494I is characterized for operation from –40°C to 85°C.

ELECTRICAL CHARACTERISTICS

=15V, f =10kHz(unless over recommended operating free-air temperature range, V

cc

otherwise noted)

APPLICATIONS INFORMATION Description

The TL494 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 frequency –programmable by two external components, RT and CT. The approximate oscillator frequency is determined by :

f osc =

T

T C R 1

.1

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 sawtooth voltage is greater than the control signals. Therefore, an increase in

control –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 120 mV input offset which limits the minimum output deadtime to approximately the 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 0 V to 3.3 V.

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 0.5 V to 3.5 V. Both error amplifiers have a common mode input range from –0.3 V to (VCC – 2V), and may be used 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 two 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 or 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 used 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 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 ±5.0% with a typical thermal drift of less than 50 mV over an operating temperature range of 0° to 70°C.

TL494 脉宽调制控制电路

特征

完整的PWM(脉宽调制)电路

有200mA不受限制的输出电流下降和输入电流下降

输出可控，可选择单端或是推拉操作

内部电流禁止在任意管脚输出双脉冲

在所有范围内能够调整死区时间

内部带5V参考电压，有5％的误差

电路结构允许简单的同步

说明

TL494芯片集合了PWM所需的所有的控制电路。芯片在首要的电源控制上设计了能灵活提供外加电源的方案。

TL494芯片内涵有两个误差放大器一个自带可调振荡器一个死区控制比较器DTC，一个脉冲转向触发器，一个5 V5 %的误差的精密的输出电路。

误差放大器的共模电压可从－0.3V至－2V变化。死区控制比较器有固定的偏移量，能够提供5%的死区时间。片上的振荡器能够通过RT的外界电路控制，并可以对CT管脚输入锯齿波，还能够驱动同步电路。

芯片的三极管输出不受限制。可以是共射极输出。也可以使射极跟随器输出。TL494提供了推拉式输出或者是单端口输出，这些都能够通过控制输出端口实现。此芯片也能够防止在端口同时输出两个脉冲的可能性。

TL494C的工作温度是0至70℃。TL494I的工作温度是－40℃至85℃。

电特性

= 15V,f =10KHz下测定的(除非特殊说明)

以下参数使在环境温度V

CC

应用信息 描述

TL494是一种频率固定的脉冲调制控制电路，集成了开关电源控制所需要的主要模块。内部线性的锯齿波振荡器频率由2个外部元器件决定，RT 和CT 。近似的振荡频率可以由下面公式决定：

f osc =

T

T C R ?1

.1 输出脉冲宽度调制是通过在CT 上的正锯齿波和2个控制信号中的任意一个比较而实现的。驱动晶体管Q1和Q2的或非门，当双稳态触发器的时钟输入是低电平的时候才使能，即锯齿波电压大于控制信号时。因此，增大控制信号的幅度会相应的减少输出脉冲的宽度。

控制信号是外部输入信号，可以反馈到死区控制、误差放大器输入或者反馈输入。死区控制比较器包含有效的120mV 输入偏置能把最小输出死区时间控制在锯齿波前4%的周期左右。这样的结果是在输出控制接地的时候，输出最大占空比为96%，接参考电平时为48%。通过在死区控制输入加固定电压增加更多死区时间，电压范围在0V 到3.3V 。

功能表

脉冲宽度调制比较器器为误差放大器调节输出脉冲宽度提供一个方法，当反馈脚电压从0.5V到3.5V变化的时候，输出脉冲宽度从确定好的死区时间变化到零。每个误差放大器共模输入范围都是-0.3V到(VCC-2V),可以用来检测电源输出电压和电流。误差放大器输出端都处于高电平，在PWM调制比较器的同相输入端进行或运算。基于这种结构，放大器需要最小的的输入，来支配控制回路。

当CT放电的时候，在死区时间比较器的输出端产生一个正脉冲，对受脉冲控制的双稳态触发器计时，并且截至晶体管Q1和Q2。当输出控制端接到参考电平的时候，脉冲控制的双温暖触发器工作在推挽式，交替控制输出晶体管的开通。输出频率是振荡器频率的一半。在单端式最大占空比不超过50%的场合，输出驱动同样可以从Q1和Q2取得。这适合于在变压器有一个反馈绕组和用一个捕获二极管吸收电压的场合。当在单端模式下需要高的输出电流驱动的时候，Q1和Q2可以并联起来使用，此时输出控制管脚必须连接到地来禁止双稳触发。此时输出频率等于振荡器频率。

TL494内部有5V的参考电压，能够提供10mA负载电流供外部电路。在0～70℃范围内提供温度漂移为50mV，精确度为±5%。