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Texas Instruments 如何实现比基准更低的输出电压 Application notes
Application Notes
ZHCA768 – April 2018
如何实现比基准更低的输出电压
Frank Du
Sales and Marketing/East China
Abstracts
目前 DC/DC 变换器的内部参考电压基本都是 0.6V,从反馈原理上讲,只能输出大于等于 0.6V 的
输出,如果遇到某些要求低于 0.6V 输出的应用就没有办法了,本应用报告给出了如何产生低于内
部基准输出电压的办法。
Contents
1.应用背景 ..................................................................................................................................... 2
2.TPS53355 输出固定的 0.52V/10A.............................................................................................. 2
3.TPS546C23 输出 0.57V-0.75V/15A ........................................................................................... 4
4.结论 ............................................................................................................................................ 6
5.参考文献 ..................................................................................................................................... 6
Figures
Figure 1 反馈网络示意图 ............................................................................................................... 2
Figure 2 TPS53355 EVM 原理图 .................................................................................................. 3
Figure 3 TPS53355 空载启动 ........................................................................................................ 3
Figure 4 TPS53355 满载启动 ........................................................................................................ 4
Figure 5 TPS53355 6A-12A Transinet .......................................................................................... 4
Figure 6 TPS546C23 反馈 ............................................................................................................ 5
Figure 7 Vout vs VDAC ................................................................................................................. 6
如何实现比基准更低的输出电压
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Application Notes
ZHCA768 – April 2018
1.应用背景
随着半导体工艺的不断升级,有越来越多的低电压供电芯片出现,0.8V 以下的核电压随处可见,
最低电压甚至到了 0.6V 以下,而目前的 DC/DC 的内部基准大部分都是 0.6V 或更高,所以相应
的输出电压最低是 0.6V,如何输出 0.6V 以下的电压,一方面可以选择更低基准的变换器,但选
择会很少,另一方面我们可以在现有芯片的基础上做些修改去实现。本文分别介绍固定输出和可
调输出两种应用。
2.TPS53355 输出固定的 0.52V/10A
Figure1 为典型的电压型反馈补偿网络,该直流输出传递函数是 Vout=VREF*(1+RFBT/RFBB)。这
个式子括号内的值是大于等于 1 的,所以输出是大于等于 VREF 的,TPS53355 的 VREF 是 0.6V,
那么该变换器的最低输出电压就是 0.6V.
Figure 1 反馈网络示意图
那么如何让 TPS53355 输出 0.52V,Figure2 为 TPS53355 的 EVM 图纸,在不做任何修改时,
输出电压有 R10 和 R12 决定,Vout=VREF*(1+R10/R12)=0.6×(1+0.78/10)=0.647V,通过加入
Rx 并连接到 VREG 上,见下图圈内部分,通过改变 Rx 的值可以改变输出电压,当 Rx 为 27K 时,
输出电压便为 0.52V。针对这种应用需要变换器本身有一个稳定 LDO 输出去产生额外的偏置电流,
这对大部分 DC/DC convert 来说是可以实现的,但需要注意三点:1 是需要注意 LDO 输出电流的
能力,一般仅需要 1mA 以下的偏置电流;2 是需要注意 LDO 电压的精度,LDO 电压的精度会直
接影响输出电压的精度;3 是因加入偏置电流,该电流是从 FB 往输出方向流的,该电流无法流入
变换器,需要一个相应电流(小于 1mA)的假负载形成回流路径。我们分别进行了空满载测试和
动态负载测试,测试波形见 Fig3-Fig5.
如何实现比基准更低的输出电压
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Application Notes
ZHCA768 – April 2018
Figure 2 TPS53355 EVM 原理图
Figure 3 TPS53355 空载启动
如何实现比基准更低的输出电压
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Application Notes
ZHCA768 – April 2018
Figure 4 TPS53355 满载启动
Figure 5 TPS53355 6A-12A Transient
3.TPS546C23 输出 0.57V-0.75V/15A
除了利用芯片自身的 LDO 提供偏置,还可以利用 DAC 的输出提供偏置,这样既没有前面提
到的自身 LDO 电流能力和精度问题,而且还可以实现调压。实际客户需要 TPS546C23 提供
如何实现比基准更低的输出电压
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Application Notes
ZHCA768 – April 2018
0.57V-0.75V 输出,TPS546C23 的基准是 0.6V,所以没有额外的偏置是无法输出 0.57V 的,因
为要利用 DAC 进行调压,因此利用 DAC 提供偏置,实现低于 0.6V 的输出,客户要求 DAC 的
0V-2.5V 对应 0.75V-0.57V 的调压。
Figure 6 TPS546C23 反馈
利用叠加定理得到下式:
预设 R1=1K
VFB=0.6V,
VDAC=0V 时,Vout=0.75V;
VDAC=2.5V 时。Vout=0.57V
求得:
R2=12.2K
如何实现比基准更低的输出电压
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Application Notes
ZHCA768 – April 2018
R3=13.9K
Vout 和 VDAC 的关系见下图:
Figure 7 Vout vs VDAC
4.结论
本应用报告提供了如何实现低于内部基准输出的方法,提出了两种解决思路,为此类应用场景
提供了解决方案。
5.参考文献
1. TPS546C23 datasheet,Texas Instruments Inc.
2. TPS546C23EVM2-746,Texas Instruments Inc.
3. TPS53355 datasheet,Texas Instruments Inc.
4. TPS53355EVM-743,Texas Instruments Inc.
如何实现比基准更低的输出电压
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