为什么BUCK控制器也有最大电流这一项？

只看该作者 · 2016-7-15 10:14

我在TI网上选DC－DC芯片时发现，内置MOS管的降压芯片有一项参数为最大输出电流，这个很好理解，里边的MOS管一定的功率限制。但是那些BUCK控制器，需要外接MOS的那种，电流又不走它身上走，为什么也有最大电流限制呢？超过它给定的电流会给芯片带来什么不可控吗？
麻烦了解的兄弟给点帮助。谢谢

4万 · 2016-7-15 10:31

那么就请你说说具体是那个控制器啊

只看该作者 · 2016-7-15 11:03

所有的。
这两天想搞个太阳能电板来玩玩，那就需要将电存起来。我需要一个支持恒流恒压的芯片来自己搭一个充电器，尝试MPPT控制。
我看中了LM25085这个芯片。
它的最大占空比100%。支持恒流恒压。但我看手册上写着最大电流10A。这个电流有点小。我预计的是20A的电流。我就在想，既然电流不走它身上走过，什么要限制最大电流呢？
这个最大电流在手册里好像没有提及，但在技术参数页面里有

4万 · 2016-7-15 11:55

以下这些英文来自LM25085的手册啊

简单地讲，

PFET比同类NFET，的内阻大一些，

受PFET内阻的影响，受LM25085门极驱动的能力限制。

所以外挂PFET的最大电流限制在10A

The LM25085 is a PFET buck (step-down) DC-DC controller
using the constant on-time (COT) control principle. The input
operating voltage range of the LM25085 is 4.5V to 42V. The
use of a PFET in a buck regulator greatly simplifies the gate
drive requirements and allows for 100% duty cycle operation
to extend the regulation range when operating at low input

voltage. However, PFET transistors typically have higher on-
      然而与相同NFET想比较， PFET有着更加高的导通电阻和门充电。

resistance and gate charge when compared to similarly rated

NFET transistors. Consideration of available PFETs, input
               考虑到可以获取的 PFET，输入电压，和 LM25085的门极驱动能力

voltage range, gate drive capability of the LM25085, and ther-

mal resistances indicate an upper limit of 10A for the load
  所以就限制10A的上限

current for LM25085 applications. Constant on-time control is

implemented using an on-time one-shot that is triggered by

the feedback signal. During the off-time, when the PFET (Q1)

is off, the load current is supplied by the inductor and the out-

put capacitor. As the output voltage falls, the voltage at the

feedback comparator input (FB) falls below the regulation

threshold. When this occurs Q1 is turned on for the one-shot
period which is determined by the input voltage (VIN) and the
RT resistor. During the on-time the increasing inductor current
increases the voltage at FB above the feedback comparator
threshold. For a buck regulator the basic relationship between
the on-time, off-time, input voltage and output voltage is:

LM25085.pdf (500.17 KB)