TPS61236P: TPS61236EVM-676

Jeffrey Cohen

Part Number: TPS61236P
Other Parts Discussed in Thread: TPS61236EVM-676

Hello TI Support,

I am working on a design that has the TPS61236P. I have set the output using resistors R1/R2 to be 4.9V. The input voltage is 3.7V.

I am using an electronic load to test the regulator and I am using the Evaluation Module TPS61236EVM-676.

The maximum current will be 1.5A.

I have been asked to calculate the inrush current due to the capacitors that are in front of the other regulators fed by this booster regulator? How would I do this?

Also: I am finding that if I apply a constant current load which is greater than 250 mA on start up, the regulator voltage output goes very low, (80mV) yet the power supply that drives the electronic load is reading a current of 250 mA and the supply that sources the load is still driving the output at a voltage of 3.7V and 250mA. Why is this happening? What is the biggest load that can be placed on the regulator output on startup?

Thank you very much for your help and support.

Jeff Cohen

over 5 years ago

0 Zack Liu over 5 years ago

TI__Mastermind 34201 points

Hi Jeffrey,

What you find during startup at a 250mA load current condition is correct because TPS61236P has a soft start feature. The device integrates an internal soft start circuit which controls ramp up of the output voltage and prevents the converter from inrush current during start-up.
When the device is enabled, the rectifying switch is turned on to charge the output capacitor to the input voltage. This is called the pre-charge phase. During this phase, the output current is limited to the pre-charge current limit Ilim_pre, which is proportional to the output voltage. The pre-charge current limit increases when the output voltage gets higher. You could take a look at the Ilim_pre spec on page 5 EC tables.
It is recommended to apply no load or a light load during the startup process, and apply the full load only after the TPS6123X starts up successfully.

0 Jeffrey Cohen over 5 years ago in reply to Zack Liu

Prodigy 110 points

Hello Zack,

Thank you for your response. I see that the value for the precharge mode current limit, Ilim_Pre is 1.7A for a Vout = 3V. I have Vout = 4.9V shouldn't I be able to start up with more than a 250mA load?

Thank you very much for your help and support.

Jeff

0 Zack Liu over 5 years ago in reply to Jeffrey Cohen

TI__Mastermind 34201 points

Hi Jeff,
You could see the figure 7. It could not start up with a constant 250mA current load because at very begging the vout is only several hundred mV, the minimum DC pre-charge current is less than 100mA. Figure 8 shows the minimum load resistance at startup.

0 Jeffrey Cohen over 5 years ago in reply to Zack Liu

Prodigy 110 points

Hello Zack,

I attached a development board that is very similar to the real load to the TPS61236EVM The startup current waveform is shown below. The voltage input from the battery that will run the system is 3.7V and my supply was set at 3.6V to match Figure 7.

The data points are in the table below. Is the draw of 50mA at 0.26V something to worry about? This would be a resistance of 4 OHms which should be okay yet it is not in figure 7.

Thank you very much for all of your help.

Regards,

Jeff

Vout (Volts)	Iout (mA)
.260	50
1.26	100
1.82	343

0 Zack Liu over 5 years ago in reply to Jeffrey Cohen

TI__Mastermind 34201 points

Hi Jeffrey,

The waveform pictures you attached couldn't be shown. Can you upload them again?
If you use a 4ohm resistor as load resistor, it should start up successfully.

0 Jeffrey Cohen over 5 years ago

Prodigy 110 points

0 Jeffrey Cohen over 5 years ago in reply to Zack Liu

Prodigy 110 points

Hello Zack,

Thank you very much.

I will make sure that the ESR of the capacitors attached to VOUT is greater than 4 OHMs at A low frequency. Does the One MegOHm loading of the scope probe change the VOUT characteristic?

Thanks,

Jeff

0 Zack Liu over 5 years ago in reply to Jeffrey Cohen

TI__Mastermind 34201 points

Hi Jeffrey,

Figure 7 means load resistance in parallel with output capacitors, not the ESR of the output capacitors.
Because output voltage will ramp up from 0V to 4.9V slowly, so if vout is 0.5V, Iout is 0.5V/4=0.125A. If Vout ramps up to 1V, Iout is 1v/4=0.25A. But if you connect the Vout to electronic load, even vout is 0.5V, the e-load will change its inner resistance to make Iout=250mA. In such conditon, the load resistance is 0.5V/0.25A=2ohm.

How much startup current do you need? Maybe I can find another suitable boost converter for you.

0 Zack Liu over 5 years ago in reply to Jeffrey Cohen

TI__Mastermind 34201 points

Hi Jeff,

I'll close this thread first. If you still have any worries, please respond or open a new thread. Thanks.

0 Jeffrey Cohen over 5 years ago in reply to Zack Liu

Prodigy 110 points

Hi Zack,

Thank you very much for all of your help. Please close it out.

Regards,

Jeff

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TPS61236P: TPS61236EVM-676