TRS3238 Clock

PAN-M

Dear all.

TR3238 switching frequency is about 500KHz.
The switching cycle is about 40KHz when RS232C line connected.
Andf, when RS232C line non connected, it is about 4KHz.

How much this time change?
(ex. 4KHz - 50KHz?)
Our customer would like to design the filter which cancels this noise.

Regards,
PAN-M

6175.TRS.pdf

over 9 years ago

0 Ron Michallick over 9 years ago

TI__Guru*** 140166 points

Pan,

The main clock is not calibrated so it will vary some. The charge pumps run on demand to keep V+ and V- near 5.5V. Therefore the rate varies a lot.

Is the goal to keep this noise out DOUT lines or VCC line?

0 PAN-M over 9 years ago in reply to Ron Michallick

Mastermind 8375 points

Hi Ron-san

This goal is 450mV at 43KHz on VCC line.
Our customer would like to make a filter, so would like to set up cut-off frequency.
Can you decide cut-off frequency?

Best Regards,
PAN-M

0 Ron Michallick over 9 years ago in reply to PAN-M

TI__Guru*** 140166 points

PAN-M,

What are the capacitor values currently being used? Are the caps ceramic or some other type? Is VCC 3.3V or 5V?
Please provide the VCC waveform.

0 PAN-M over 9 years ago in reply to Ron Michallick

Mastermind 8375 points

Ron-san

These are ceramic capaccitors.
And VCC is 3.3V, It is generated using LDO from 5V.
I will request to customer the waveform of 3.3V.

I attach a customer's measurement result.
3730.TRS3238_ChargePump.pdf
Can't we get the max./min. value of the frequency for filter design?

Regards,
PAN-M

0 PAN-M over 9 years ago in reply to PAN-M

Mastermind 8375 points

Ron-san

VCC waveform is attached.

Regards,
PAN-M

0 Ron Michallick over 9 years ago in reply to PAN-M

TI__Guru*** 140166 points

PAN-M,

The goal was 450mV ripple on VCC and the current result is 40mV. The.The goal is already met, correct?

The V- discharge rate is directly proportional to the DOUT load current and inversely to V- capacitance. V/t = I/C.
The amplitude is the voltage rise of one charge pump cycle (on V+ which is mirrored to V-).
Both of these effects set the frequency.

The device variance shouldn't cause more than -30%/+40% change in frequency.

0 PAN-M over 9 years ago in reply to Ron Michallick

Mastermind 8375 points

Ron-san

Our customer was wrong.
When the filter was inserted, it was 40 mV, but it was 450mV before that.
They inserted pi type filter of 22uH and 10uF. (3.3V-pi-VCC)
They want to check whether this constant is suitable.

Regards,
PAN-M

0 PAN-M over 9 years ago in reply to Ron Michallick

Mastermind 8375 points

Ron-san

Where is the center of device variance?
Is it a design value?
Is a temperature coefficient also included?

Regards,
PAN-M

0 Ron Michallick over 9 years ago in reply to PAN-M

TI__Guru*** 140166 points

PAN-M,

For most of the questions, the customer should use their frequency evaluation data. They have preformed a more in depth study than we have internally.

There are no device specifications for the charge pump frequency.

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TRS3238 Clock