TL494: Operation of TL494

takahiko yokota

Mastermind 7135 points

Part Number: TL494

Please let me ask you the operation of TL494.

I think that TL494 doesn't start switching when DT terminal voltage is over CT terminal voltage.

But the pulse occur from E1 terminal before operating internal oscilator.

Is this operation right?

Best regards,

Yokota

over 7 years ago

0 Sonal Singh over 7 years ago

TI__Expert 6080 points

Hello Yokota,

Thank you for your question.
The DTC terminal provides control of the minimum OFF time, the controller produces switching till the time DTC voltage has not reached a threshold voltage {3V-3.3V as per the datasheet}, which is also the linear ramp voltage of the CT waveform. Once CT reaches 3V the oscillator circuit discharges the cap and the charging cycle is retained.
I am not sure about what you mean by the second part of your question but, the modulation is achieved by comparing the CT signal to one of the control input signals and will be enabled as soon as the CT voltage is greater than the input voltage.

Kind Regards,
Sonal

0 takahiko yokota over 7 years ago in reply to Sonal Singh

Mastermind 7135 points

Hello Sonal-san

The gate pulse occur from E1 terminal before operating internal oscilator.

I think this problem's reason is rising Vref after fully rised Vcc.

Is this understand correct?

If this understand correct, are there the rules about the rising time of Vref?

Best regards,
Yokota

0 Sonal Singh over 7 years ago in reply to takahiko yokota

TI__Expert 6080 points

Hello Yokota,

Do you mean to say that Vref rises after the Vin/Vcc? Can I ask you to share your schematic because it is not the right operation. Have you compared your schematic to the typical application?

Typically Vref should rise with the Vcc until Vref hits 5V.

Regards,

Sonal

0 takahiko yokota over 7 years ago in reply to Sonal Singh

Mastermind 7135 points

Hello Sonal-san

Yes, Vref rises after the Vin/Vcc.
I cann't attach the schematic this community for customer's information.
Could I send you by e-mail?

Best regards,
Yokota

0 takahiko yokota over 7 years ago in reply to takahiko yokota

Mastermind 7135 points

Hello Sonal-san

My customer used 33uF capacitor of between Vref and GND for slow start.

But the pulse didn't occur by delete this capacitor.

Should they delete it?

Best regards,

Yokota

0 Sonal Singh over 7 years ago in reply to takahiko yokota

TI__Expert 6080 points

Hello Yokota,

Yes the capacitor is causing the delay in the rise time of Vref. I suggest removing that capacitor.

Regards,

Sonal

0 takahiko yokota over 7 years ago in reply to Sonal Singh

Mastermind 7135 points

Hello Sonal-san

Thank you for the reply.
Is there any influence of noise without Vref capacitor?

Best regards,
Yokota

0 Sonal Singh over 7 years ago in reply to takahiko yokota

TI__Expert 6080 points

Hello Yokota,

Vref is not usually a noise sensitive pin. However it is very important to make sure that while pcb designing to keep the feedback traces away from the EMI inductor and high voltage traces. Also keeping traces as small as possible and with without loops would help you reduce the noise interference.

Regards,
Sonal

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TL494: Operation of TL494