摘要
随着便携式应用的不断深入,便携式电子设备的体积越来越小。开关电源作为便携式电子产品的主要供电模块,其所占用空间也必须越来越小。为了在有效的缩小开关电源体积的同时维持其带载能力的不变,我们需要提高开关电源的工作频率,这就对其内部集成的基准电压模块的高频电源抑制比(PSRR)性能提出了更高的要求。
除了电源抑制比之外,输出电压的温度稳定性也是基准电压源一个重要的性能指标。当前很多实现了二阶温度曲率补偿的带隙基准结构非常复杂,这会引入较大的器件失配引起的输出失调电压。这个失调电压会使得基准的输出电压偏离原始设计的中心值,从而使得基准输出电压的温度系数达不到设计目标,甚至是比没有补偿时更差。
针对以上两个问题,本文在分析对比了两种结构的带隙基准之后,结合它们的各自优点,确定了所设计的带隙基准的主体结构。同时本文从器件选型和电路结构优化这两个方面入手,提出以下三个改进方法:第一、采用负温度系数的电阻,进行二阶温度曲率补偿。第二、在Brokaw带隙基准结构的基础上增加了一个电阻和NPN三极管,用于在高温时对基准输出电压的温度系数进行直接的调整。第三、在对Brokaw结构进行深入小信号分析的基础上,提出了一种新的在Brokaw结构中引入零点的方法。通过零点的引入,有效地提高了带隙基准环路的交越频率,从而优化了带隙基准的高频PSRR性能。以上的各个改进都通过仿真验证了其有效性。
在进行了上面的分析设计之后,作者利用Hspice软件对新设计的带隙基准电路进行了整体仿真。从仿真结果可以看出,新设计的带隙基准电压源的输出电压温度系数与电源抑制比性能都得到了优化,达到了预期的目的。
Along with the continuous developing of portable applications, the size of portable electronic equipment is getting smaller and smaller. As the main power supply modules of the portable equipment, the space of switching power supply modulator needs to be reduced. For this reason, a higher switching frequency of the switching power supply modulator is required. This gives its internal voltage reference macro a higher frequency power supply rejection ratio (PSRR) performance demand.
In addition to power supply rejection ratio, the temperature stability of the output voltage reference is also an important performance specification. Currently, many second-order temperature curvature compensation bandgap references are realized with very complex structure. It may result a larger offset voltage from device mismatch. This offset voltage causes the output voltage deviate from the original typical value. Beside the temperature coefficient of the reference voltage will be affected by the temperature coefficient of the offset, maybe even worse than compensated before.
In view of the two questions above, this paper analyses and compares two different bandgap reference structures. After that the main structure of the new bandgap reference is determined by combined the respective advantages of the two structures above, research on device selection and structure optimization is following. There are mainly three improvement made in the new bandgap. First, second-order temperature curvature compensation is received by taking a resistance with negative temperature coefficient. Second, based on the Brokaw bandgap reference structure, an additional NPN transistor and resistor are used to directly adjust the temperature coefficient of output voltage in high temperature region. Third, based on the small-signal analysis of the Brokaw structure, a new method which introduces a zero into the Brokaw structure is proposed. This zero effectively improves the crossover frequency of the bandgap reference loop, and then optimizes the high frequency PSRR performance of new bandgap reference. Each of the improvement has been simulated to prove its effectiveness.
The performance of the designed bandgap reference is derived from Hspice simulation. The simulation results show that the temperature coefficient and power supply rejection ratio performance of the new bandgap reference structure have been optimized to achieve the desired results.
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