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TPS61199驱动4串LED灯时闪烁

Other Parts Discussed in Thread: TPS61199

板子完全按照tps61199evm评估板layout,器件、设置等都一样,输入24V,ovp为60V,电流60mA每串。每组led灯串为18颗红色灯串联,正常未使用的IFB悬空。只有一或者两组灯串时,正常,输出led+为38V,IFB电压为0.8V。四组的时候,闪烁。根据open protection描述,未使用的IFB未接地时,输出LED+会上升到OVP电压之后慢慢下降到灯串所需电压,两串灯时符合该描述,一开始使用的IFB电压会上升到OVP电压减去LED灯串电压,22V左右,和输出LED+一样慢慢下降。四串灯时,IFB电压开始也会上升到22V左右,但之后会突然跳到30V左右,这时候超过了IFB_OVP的29.5V,于是IC停止转换,这时候LED灯串的电压<38V,不能维持灯亮,于是灯熄灭,灯IFB电压下降后,IC再次转换灯亮,接着IFB电压再次跳变,如此反复,造成一亮一灭现象。
把不使用的IFB接地后,不会出现上述现象。由于是想做成通用板,不确定使用几组,每次跳接IFB比较麻烦,能否帮忙分析一下该现象和解决方法,谢谢!

tps61199芯片资料:www.ti.com/.../tps61199.pdf

评估板资料:www.ti.com/.../slvu421.pdf

  • 示波器量测的图片,蓝色是输出LED+,由于示波器问题,其真实最高点电压是59V。黄色是使用的灯串对应的IFB引脚。图中可以看出,IFB突然升到30V

  • Hi

          IFB不用需要接GND, 而不能悬空。

          If the application requires fewer than eight LED strings, the TPS61199 simply requires shorting the unused IFB pin to ground. The device detects the voltage less than 0.3 V and immediately disables the string during start-up. Refer to Figure 11.

  • Hi

       谢谢您的回复!我知道规格书要求接地,但是如果悬空,那么芯片应该认为属于open状态。根据OPNE protection所说,最后也应该正常工作。而且两串灯能够工作,为什么4串灯就不行了呢?而且IC为什么要设置为接地,我看有些厂家的悬空就好了,如RT8561。

      When one of the LED strings is open, the boost output rises to the clamp threshold voltage (see 5. Output overvoltage protection using the OVP pin ). The device detects the open string by sensing no current on the corresponding IFB pin. As a result, the device deactivates the open IFB pin and removes it from the voltage feedback loop. Afterwards, the output voltage returns to the voltage required for the connected WLED strings. The IFB pin currents of the connected strings remain in regulation during this process.
    If all the LED strings are open, the device repeatedly attempts to restart until the fault is cleared.