Boost电路的反馈电路上的两个MOS管有什么用

lingking

Boost电路的反馈电路上的两个MOS管有什么用 [复制链接]

如图所示，TI的一款Boost开关电源芯片提供了外围电路和内部框图。比较明显的是LX管脚上连接的电感、肖特基二极管就对应基本Boost的电感和二极管，LX内部的MOS管对应开关。一眼看上去大概知道BOOST管脚相关的电路是反馈作用，但是肖特基二极管接入BOOST管脚之后为什么还要经过Q1、Q2两个MOS管，如果Q2导通那不就对地短路了吗，他们的作用是什么？

kimoye

正常工作时是Q1导通 Q2不导通的 你逆向分析一下 就很简单了
事实也是如你所说 Q2一直导通输出电压就是0了 那是接地的点
你按照Q1导通 Q2导通就能还原出boost的原理图
还原的过程中你就大概搞明白哪个模块有什么作用了 以及是怎么起作用的了
至于你说的Q1 Q2都导通 的情况(不正常工况)  应该重点分析什么时候会出现以及是否会出现才是关键的个人认为
正常工作Q2肯定不导通
我也是刚刚接触 要是错了多多指教

kimoye

Q1导通 Q2不导通的时候是：
BOOST端口进来 一是为了pwm调节，这一支线进入到pwm模块
二是输出端口 VLNB 这一支线和GND端口构成输出两端接上负载使用

kimoye

我查了手册：
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The charge pump circuitry generates a voltage to drive the NMOS of the linear regulator. The voltage across the charge pump capacitor between VLNB and VCP is about 5.4 V, so the absolute value of the VCP voltage will be VLNB + 5.4 V
也就是说Q1是确保输出电压为5.4V的
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7.3.6 Short Circuit Protection, Hiccup and Overtemperature Protection
The LNB output current limit can be set by an external resistor. When short circuit conditions occur or current limit is triggered, the output current is clamped at the current limit for 4 ms with LDO on. If the condition retains, the converter will shut down for 128 ms and then restart. This hiccup behavior prevents IC from being overheat. The hiccup ON/OFF time can be set by I2C register. Refer to Control Register 1 for detail.

The low side MOSFET of the boost converter has a peak current limit threshold which serves as the secondary protection. If boost converter’s peak current limit is triggered, the peak current will be clamped as high as 3.8 A when setting ISW default and LNB current limit up to 1 A. If loading current continues to increase, output voltage starts to drop and output power drops.

Thermal shutdown prevents the chip from operating at exceedingly high temperatures. When the junction temperature exceeds 160°C, the output shuts down. When the die temperature drops below its lower threshold typically 140°C, the output is enabled.

When the chip is in overcurrent protection or thermal shutdown, the I2C interface and logic are still active. The Fault pin is pulled down to signal the processor. The Fault pin signal remains low unless the following action is taken:

If I2C interface is not used to control, EN pin must be recycled in order to pull Fault pin back to high.
If I2C interface is used, the I2C master need to read the status Control Register 2, then the Fault pin will be back to high.

Q2用于短路保护 过热保护时候起作用 正常情况下不用管他

lingking

kimoye 发表于 2019-4-8 22:14
我查了手册：
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The charge pump circuitry ge ...

Q1是类似LDO的调整管，这个我知道。Q2的作用一直不明显

lingking

kimoye 发表于 2019-4-8 22:14
我查了手册：
--------------------------------------------------------
The charge pump circuitry ge ...

输出短路的情况下，Q2起不到作用吧

kimoye

你看手册 短路的时候电流过大 这个mos管的作用就是  把输出电压降下来

qiushenghua

有没有考虑输出过压的时候？

lingking

qiushenghua 发表于 2019-4-9 11:39
有没有考虑输出过压的时候？

Q1是调整管，应该够了。感觉不太可能把Q2当成一个可调电阻来用