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# TPS748A-Q1: Dropout voltage in the specific condition

Part Number: TPS748A-Q1
Other Parts Discussed in Thread: TPS79633-Q1, , TPS748A

We want to know the dropout voltage under the case of Vin=3.8V, Vbias = 5V, and Vout=3.3V.

Need to compare the characteristics  between TPS79633-Q1 and TPS748A-Q1 to judge the validity in our product.

• Hi Naoki,

Thank you for your question. The dropout voltage is highly affected by the load current of the application. I have attached a good graph to show the effect of different output currents. This graph is for the TPS748A-Q1 and although the conditions are not exactly the same, you will be fine with the 500 mV of dropout in your condition. I have also attached the graph for TPS79633. Even at the highest current you can expect the dropout voltage to be lower than your 700 mV.

Hope this helps, please let me know if you have any questions.

Thank you,

Josh Nachassi

• Hi Josh

As you say, dropout voltage deeply depends on load current and I want to compare the values between TPS748A-Q1 and TPS79633 at 1A as the representative value.

1. TPS79633 (TPS79630)

From the figure, VDO seems to be 250mV at 25℃ / 330mV at 125℃.

Do you know the test condition of Vin and Vout in this case?

Or is it O.K to consider the same value can be also applied to my condition?

2. TPS748A-Q1

As you say, I can estimate the dropout voltage less than 500mV from the figure.

(Around 75mV at 25℃ and 100mV at 125℃)

Is it O.K to consider that the dropout voltage characteristic under my condition has a similar one?

If it's clear, we can use TPS748A as an improved solution.

• Hi Naoki,

1. TPS79633

Thank you for specifying your load conditions. Here are the conditions for the data in the table and the data in the table for dropout voltage lines up well with the graph because on the graph at 1 A you see 250 mV and it is what is seen in the table as well.

You can see your 3.3-V condition here as well. All conditions are under the 500 mV differential you have.

2. TPS748A-q1

Here you can see the variance in dropout voltage to the output current and you can see that we are well below the 500 mV. These are the operating conditions.

Thank you,

Josh Nachassi

• Hi Josh

Sorry the inserted images are not visible...

Thanks.

N. Chiyoda

• Hi Naoki,

Here you go, this should work.

1. TPS79633

Thank you for specifying your load conditions. Here are the conditions for the data in the table and the data in the table for dropout voltage lines up well with the graph because on the graph at 1 A you see 250 mV and it is what is seen in the table as well.

You can see your 3.3-V condition here as well. All conditions are under the 500 mV differential you have.

2. TPS748A-q1

Here you can see the variance in dropout voltage to the output current and you can see that we are well below the 500 mV. These are the operating conditions.

Thank you,

Josh Nachassi

• Hi Josh

About TPS748A-q1 dropout voltage, the test condition is VBIAS - VOUT = 5 - 0.8 = 4.2V.

On the other hand, it is 1.7V in our case since VBIAS=5V and VOUT = 3.3V.

In this case, operating point of the Power MOSFET seems to be different and dropout voltage will get worse.

Do you have any estimated value under the VOUT=3.3V?

• Hi Naoki,

Hope you are doing well. You are curious about the dropout between the Vout and the Vbias? In your case of 1.7-V, it still clears the dropout range stated in the data sheet.

• Hi Josh

Sorry the inserted image seems not to visible again..

Could you try update as before?

Dropout between Vin and Vout is my concern.

Just want to compare the values between TPS79633 and TPS748A under Vout=3.3V setting.

• Hi Naoki,

Okay, I thought you were curious about Vbias because of your previous question. In terms of Vin and Vout I would still look at the previous response I wrote to you. There should no issue with you reaching dropout with this device with the 500-mV differential even at max current.

Can you not see the comparison I previously made?

Please let me know if you have more questions.

Thank you,

Josh Nachassi

• Hi Josh

Yes you already wrote the data sources about Vin-Vout dropout voltage as below.

But it has some deviation from our setting of Vout=3.3V, Vbias=5V.

For TPS79633 is O.K but for TPS748A it seems to be difficult to refer the below data since the test condition of Vbias - Vout > 3.25V doesn't meet our condition. (Vbias - Vout = 5V - 3.3V = 1.7V)

Of course, it should have some margin comparing with 500mV, but it is preferred to share the concrete value based on our setting if possible.

If you say the deviation between two conditions (datasheet and our case) is neglectable, it should be also O.K.

TPS79633

TPS748A-q1

• Hi Naoki,

Yes, I am saying that even in your case you will be fine and shouldn't have to worry about dropout. I spoke to a colleague as well and he confirmed. As long as you have enough headroom between Vbias and Vout this device will regulate properly in your condition.

Thank you,
Josh Nachassi

• Hi Josh

O.K now I understood 500mV dropout margin is enough so no need to take care about concrete dropout value in our case.

Appreciate for your support on this confirmation.