MAX3232E: MAX3232IPWR-TSSOP-16- not work properly at recommended application circuit

Kartavya Vaghela

Part Number: MAX3232E

Tool/software:

Hello TI Team,

We are using the MAX3232IPWR (TSSOP-16) to convert UART signals to RS232 for interfacing with an RS232-based touch display.

However, during operation, we observe garbage data on the display. Upon checking the RS232 TX and RX signals using a digital oscilloscope, the signals appear distorted and do not represent proper logic levels (not clearly 0 or 1).

We followed the schematic recommendations from the datasheet. Initially, we used 0.1 µF charge pump capacitors as recommended, but the issue persisted. After replacing the capacitors with 1 µF values, the RS232 signals stabilized and the display started functioning correctly and did not observe issue while operating the display.

Could you please help us understand why the recommended 0.1 µF capacitors did not work in our case? Is there any known issue or layout consideration that could lead to this behavior?

For your reference, I’ve attached our schematic.

24 days ago

0 Clemens Ladisch 24 days ago

Guru 317370 points

DIN2 must not be left floating.

The charge pumps require a good board layout. Please show the board layout.

0 Kartavya Vaghela 24 days ago in reply to Clemens Ladisch

Prodigy 40 points

Hello Chemes Ladisch,

Find below a screenshot of our layout,

It is a 4-layer board and we provide the inner two layers. We take the GND plan,

C75,C76,C77,C79 is charge pump capacitors and C78 is input filter capacitor.

0 BOBBY 23 days ago

TI__Guru 73971 points

Please post waveform scopeshots.

Also remove d2 and D96.

-Bobby

0 Kartavya Vaghela 18 days ago in reply to BOBBY

Prodigy 40 points

Hello BOBBY,

Find below waveform snapshots,

MCU TX signals,

MCU RX signals,

RS232-TX signals

RS232-RX signals

0 Clemens Ladisch 18 days ago in reply to Kartavya Vaghela

Guru 317370 points

Is this with or without D2/D96? Are there any other components between the RS-232 bus lines and GND?

0 Kartavya Vaghela 18 days ago in reply to Clemens Ladisch

Prodigy 40 points

D2 and D96 are the only components connected in parallel across the signal line.

0 BOBBY 17 days ago in reply to Kartavya Vaghela

TI__Guru 73971 points

Your RS232 signals are all clamping when the signal goes negative. This is likely due to the D2/D96. I recommend you remove these diodes and replace them with bi-directional TVS diodes.

Please remove the D2 and D96 and verify if the clamp still occurs when the signal goes to the negative output swing.

-Bobby