TPS562209: When the inductance is too low, Vin is equal Vout

Bryan Huang

Part Number: TPS562209
Other Parts Discussed in Thread: TPS562208, , TPS562201

Hi Team

When the inductance is too low, high side mode will always turn on and low side MOS always turn off , and this time , Vin will equal Vout

Test condition:

Vin=12V

Vout set to 3.3V

L was originally 2.2uH, but replace wrong value to 0.7uH,

find out Vin is equal Vout,

my customer would like to know what is reason?

By the way

TPS562208 and TPS562209 both has the same situation

TPS562201 is still normal if L replace to under 0.5uH.

over 6 years ago

0 Shawn Nie over 6 years ago

TI__Genius 10070 points

Hi Bryan,

Could you please share the waveform?(Vin, Vout,SW, IL)

Shawn

0 Bryan Huang over 6 years ago in reply to Shawn Nie

Intellectual 445 points

Normal waveform

Abnormal Waveform

0 Shawn Nie over 6 years ago in reply to Bryan Huang

TI__Genius 10070 points

Hi Bryan,

TPS562208 and TPS562209 work at FCCM when load is light, but TPS562201 works at Eco-mode. For FCCM part, the inductor current will be negative with light load, if the ripple of inductor current is too large, it will reach to the valley current limit, the low-side FET will turn off in advance, the increase of the duty cycle will increase the output voltage, and Ton will increase, the output voltage will increase further. Finally, the device will let valley value of inductor current ripple is lower than the current limit threshold.

So you should choose the appropriate indctor.

Shawn

0 Bryan Huang over 6 years ago in reply to Shawn Nie

Intellectual 445 points

Hi Shawn:
Thanks for your reply.
I have some question.
Has the valley current limit actual value?
Assume vendor offer wrong parts to end customer, the inductance value is too low,
but I don't want the output voltage rise up, what is the improvement? (the output will damage device.)

0 Shawn Nie over 6 years ago in reply to Bryan Huang

TI__Genius 10070 points

Hi Bryan,

Yes, it is a constant value.
It is better to increase the inductance, choosing the correct inductor is very important.

Shawn

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TPS562209: When the inductance is too low, Vin is equal Vout