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TL431反馈电路

更新时间：2019-08-14 14:10:49 大小：247K 上传用户：nick7788查看TA发布的资源 标签：tl431反馈电路 下载积分：2分 评价赚积分 （如何评价?） 打赏 收藏 评论(0) 举报

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TL431反馈电路，详细介绍了反馈电路的使用等

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Implementing the TL431 feedback loop  
Christophe BASSO  
MOTOROLA SPS, Toulouse Center  
Avenue Eisenhower, BP1029, 31023 TOULOUSE Cedex France  
33 (0)5 61 19 90 12; e-mail:  
This paper details the numerous ways to implement a feedback network with an optocoupler and a  
TL431 when implementing shunt regulators. Depending on the configuration of the devices and the method used  
to measure the open-loop response, some unusual results may appear.  
The shunt regulator  
The optocoupler is used alone and delivers some current to a shunt regulator such as implemented by  
the MC3337X series or the MC44608. In these devices, the duty-cycle DC is adjusted by injecting a current into  
a feedback pin (FB). When the current is low or zero, the duty-cycle is pushed to the max (74% for the  
MC33370, 80% for the MC44608). If more current is pushed into the pin, the duty-cycle goes toward a few  
percents. The amount of current needed to go from full DC to null DC determines the PWM gain, assuming the  
measurement is carried upon a linear portion on the curve DC versus IFB. According to these remarks, there are  
two ways to model the PWM chain but only one is valid to calculate the pole and zeroes created by the primary  
compensation network (MC3337X series only). Figure 1 details how the duty-cycle conversion takes place :  
Figure 1  
The complete PWM chain in a shunt regulator  
As previously said, the FB corresponds to the input of a shunt regulator. To better understand the way it  
works, you can replace the shunt regulator by a power zener: when the voltage you apply on the FB pin is below  
the shunt breakdown level, no current flows into the pin and DC is maximum. When the FB level reaches the  
zener threshold, a current circulates in the pin and is converted into a lowering duty-cycle. First remark, in  
steady-state operation, the FB pin is at the shunt level as given in the data-sheet: 8.6V for the MC3337X series or  
5V for the MC44608. You shall then provide the feedback current through a source whose value is, at least, two  
or three volts above the shunt value. Otherwise you will not reach the appropriate level to regulate and you will  
force the optocoupler to operate in low VCEs region where the conductance dIC/dVCE is rather poor.  
For AC analysis, the FB pin can be replaced by the dynamic resistor of the power zener,  
dVzener/dIdiode: 18Ω for the MC3337X series, 20Ω for the MC44608. This value gives you the AC impedance  
seen from the FB pin. On MC3337X series, it will dictate the locations of the pole and zeroes you create by  
adding capacitors around this pin. This is NOT the PWM gain, but rather an intermediate current/voltage  
conversion gain. The complete gain, as highlighted by figure 1, depends on the internal sawtooth amplitude  
(1.6Vpp for the 44608, 1.4Vpp for the MC3337X) and the maximum duty-cycle. The calculation of G is easily  
done following the steps:  
MC44608  
MC3337X  
∆IFB of 2mA ! ∆DC of 80%  
∆IFB of 6mA!∆DC of 74%  
2mA . 20Ω = 40mV  
6mA . 18Ω = 108mV  
80% over 1.6Vpp = 1.28V  
74% over 1.4Vpp = 1.036V  
1

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