TPS61096A: Maximum Switching Frequency

Hi John,

The device operates at DCM mode. it turns on the low side MOSFET firstly until the inductor reach the current limit, and then turn off the MOSFET until the inductor current is zero, then the device wait 100ns level and start the next cycle.

the output current capability is dozens milli-Amp level, 150mA is too high for this device. I would suggest a device like TPS61372.

Hi Jasper,

Thanks for the quick response!

The datasheet and parametric tables mention the peak inductor current can be set for 250mA or 500mA. From your explanation it looks like the output current will have to be less than half the peak current since there is also some off time (100ns) during each cycle that the device waits before turning back on the low-side MOSFET. 

Is there a way that we can more accurately estimate the maximum average load current that a given inductor value with the TPS61096A can support? With a larger inductor the switching frequency will be lower, which means that the off time will be a lower percentage of each cycle and I am guessing you can have a larger average load current.

I wanted to try and give the customer a better estimate on what load current the device can support with different inductor values. They might not need the full 150mA they were using in the calculation.

Thanks again,

John

Hi John,

with large inductor, the 100ns can be ignored. so the maximum input current is half of the peak current. then the output capacitor can be estimated through VIN, VOUT and efficiency.

Hi Jasper,

Thanks again for the quick reply. The customer revisited their load list and thinks that 25mA will be the maximum load current but they want to know if we think the device can safely support 30mA of load current.

Just to make sure, the ILIM in the equation in section 8.2.2.4 Inductor Selection is either 250mA or 500mA based on the state of the ILIM pin, correct?

To estimate the output current I did the following. Can you please let me know if you think I made any mistakes?

• Use fsmax equation in 8.2.2.4 to calculate the frequency
• Take 1/fsmax to estimate the period
• Estimate that the device is off for 100ns of each period and use that that to come up with a "duty_cycle" estimation (how much time per period there is inductor current)
• Output current will be ratio of the input voltage / output voltage (calling this boost_ratio)
• Estimate the output current as the (0.5)*(current_limit)*(duty_cycle)*(efficiency)*(boost_ratio)

Putting everything together with a 2µH inductor and 500mA ILIM I get a fsmaz of 3.19MHz and I used a 80% for the estimation of efficiency 

0.5 * 0.5 * 0.68 * 0.8 * 0.3 = 40.85mA of output current

Thanks again for the quick help and let me know if you think my estimation is correct.

John

Hi Jasper,

Thanks for all the help, I think we are all set! 

Have a good weekend,

John