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# TPS40140: Issue with output voltage

Part Number: TPS40140
Other Parts Discussed in Thread: TPS40180

I have a three channel buck converter design having TPS40140 and TPS40180 (12V to 1V2). First channel in TPS40140 is master and others are in slave mode. output voltage measured (after inductor) is 0.47V. We have measured the PWM at each channel (before inductor). They are,

1. Channel 1 (Master) -> Ton = 0.3us and Tperiod = 3us (10% duty cycle)

2. Channel 2 (Slave1) -> Ton = 0.2us and Tperiod = 2.8us (<10% duty cycle)

3. Channel 3 (Slave2, tps40180) -> Ton = 0.2us and Tperiod = 2.8us (<10% duty cycle)

We have few questions,

1. What will be the expected output at COMP pin and how can we calculate the same?

2. What can be the possible reason for less duty cycle in slaves? (This circuit was working earlier)

Thanks

Midhun MK

• Hi Midhun,

The output voltage of the comp pin determines the duty cycle of the pwm. Page 7 of datasheet shows that the Vcomp is 1.1 V. You can use the equations on page 35 to calculate the desired values.

https://www.ti.com/lit/gpn/tps40140

Can you send waveform captures of the outputs?
How does the switch node look?

What are your resistors for output voltage?

Best,

Ryan