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TPS54540B: Foldback Frequency

Brian Wang0928
Genius 14120 points
Part Number: TPS54540B

Hello,

Customer wonder to use this device in MD sector. Here there are few questions. please help to support.

1. Is these any method to control the current limit of the device? on the datasheet, the variation of the current limit is from 6.3A to 9.5 at certain conditions. But customer would like to know if they would like to control the max. limit current from 9.5A to 8.5A for an instant. is there any way to do? how about to pull a zener diode to clamp the Voltage at VCOM pin?

2. It seems there is no over current waveform on the datasheet? In the d/s, it says it's using foldback frequency against the short and over current. But why it says the output overload conditions reset the soft-start timer? what's this purpose?

During the foldback freq. event, Is output rail able to delivery current to the output continuously? What's the difference between the Foldback and hiccup mode during short fault condition?

Regards

Brian W

  • 0
    TI__Guru 50120 points

    Hi,

    1. Looking at the EC table specifically at the current limit test, the results show that the current limit does not change between different VIN voltages (4.5V to 42V). However for different junction temperature, the current limit may widen. I think a zener diode clamp on VCOMP pin may adjust current limit given that the COMP pin voltage is compared to the  high-side switch current each cycle. However I would suggest customer test this on an EVM to verify given that adding a zener diode may impact loop response since the component might have some parasitic capacitance. If the customer is operating at room then the current limit can be expected to be within the min,typ, max of 7A/7.9A/8.8A.

    2. When the part is overloaded the soft-start timer is reset so that when the overload condition is removed, the part can slowly ramp up output voltage to reduce inrush current and output voltage overshoot. During frequency foldback, the device is still able to deliver current to the output. The frequency will foldback and increase off-time to allow more time for the inductor current to ramp down. The TPS54540B does not have hiccup mode. Please refer to this app note for details between cycle-by-cycle current limit and hiccup-mode (https://www.ti.com/lit/an/snva604/snva604.pdf#page=7).