TPS92691 OVP circuit for buck boost mode

Mahendra Patel1

Other Parts Discussed in Thread: TPS92691

Hello Team,

As per figure 42 in the TPS92691 datasheet, for Buck-Boost mode OVP circuit, base of PNP transistor connects to LED+ and Emitter is pulled to VIN through a resistor.

On the other hand, as per figure 26, base of PNP transistor is connected to VIN and emitter is pulled to LED+ through a resistor.

Which circuit is correct? Formula 43 is applicable for which configuration?

Regards,

Mahendra

over 9 years ago

0 Clinton Jensen over 9 years ago

TI__Mastermind 25500 points

I will double check with the engineer who wrote the datasheet, but I believe there is an error in the schematic of figure 41 so the other schematic and equation should be correct.

0 Ruslan Gim over 8 years ago in reply to Clinton Jensen

Prodigy 150 points

Hello.

Could you tell me why I have to use PNP transistor for OVP? And how I have to select this transistor?

TPS92691 Boost and Boost-to-Battery LED Driver Evaluation Board doesn't use this transistor and I check it work correctly.

Thank you, And sorry for my English.

0 Clinton Jensen over 8 years ago in reply to Ruslan Gim

TI__Mastermind 25500 points

Hello Ruslan,

In boost configuration the transistor is not needed since your output voltage (LED+) is the LED string voltage.

In buck-boost/boost-to-batter configuration the LED+ terminal voltage will be VIN plus the LED string voltage. So on the EVM you can actually trip OVP by increasing the input voltage even though the LED string voltage is fine.

The transistor allows you to monitor only the LED string voltage in buck-boost configuration so that it will be independent of VIN. Any small PNP will work. Referring to figure 26 in the datasheet you set the OVP by setting a current using the PNP. The voltage across Rov2 is the LED string voltage. That sets the PNP current to VLED/Rov2. Then you can set Rov1 to set the OVP pin threshold with that current.

Regards,

Clint

0 Ruslan Gimaz over 8 years ago in reply to Clinton Jensen

Prodigy 20 points

So is it mean that Formula 43 doesn't work for this application? I have to calculate it throw current across PNP transistor, don't I?

0 Ruslan Gimaz over 8 years ago in reply to Clinton Jensen

Prodigy 20 points

So It is mean that Formula 43 is doesn't work for this application? I have to calculate it throw current across PNP transistor, don't I?

0 Clinton Jensen over 8 years ago in reply to Ruslan Gimaz

TI__Mastermind 25500 points

Hello Ruslan,

Formula 43 should be correct for using the PNP transistor. The 0.7 term comes from the Vbe.

Regards,

Clint

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TPS92691 OVP circuit for buck boost mode