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# TPS7A80: VDO question for TPS7A8001

Part Number: TPS7A80
Other Parts Discussed in Thread: TPS74401

Hi team,

For the LDO dropout, we can find the Max dropout spec from datasheet.

The max dropout spec is different according to actual load current. I have a question about the LDO Vin dropout calculation. For our actual design which is shown as follow:

Vin=5.5V±3%， Vout=4.984±3.92% （the accuracy of output is calculated according to IC, external resistor accuracy）Imax of load =0.75A;

So the min Vin dropout of our design should meet: Vin dropout>350mV according to datasheet. But I have a question about the Vin dropout calculation. Please see the following calculation method:

Cal Method 1:  Vin dropout=Vin(Typ)-Vout(Typ)=5.5-4.984=516mV>350mV;

Cal Method 2:  Vin dropout=Vin(Typ)-Vout(Max)=5.5-5.18=320mV<350mV;

Cal Method 3:  Vin dropout=Vin(Min)-Vout(Typ)=5.5*(1-3%)-4.984=351mV>350mV;

Cal Method 4:  Vin dropout=Vin(Min)-Vout(Max)=5.5*(1-3%)-4.984*(1+3.92%)=155mV<350mV;

Please help to confirm which cal method is true because it is really important for our circuit design. Thanks a lot!

• Hi Dane,

The dropout voltage of your application would be around the 350mV range because of your load current. The closest calculation you have is Method 2.

Best,

Juliette

• Hi Juliette,

1. If the calculation method is the method 2, the dropout is 320mV@0.75A load will  be lower than 350mV. So what is the influence?

2. In my opinion, the Vin spec is 5.5V±3%， it also can be 5.5V*(1-3%)=5.335V at the worst case. So I want to know does LDO test case of the factory do not care this Vin accuracy?

3. The LDO dropout test definition of TI is as follow(it is defined in TPS74401 datasheet):

The dropout definition shows that when Vin-Vout=195mV, the Vout will be 2% below nominal. But if the designed accuracy of Vout 1.5%, even though we can meet the maximum dropout 195mV, the LDO also can not guarantee the design accuracy. Is it right?

• Hi Dane,

The LDO dropout voltage follows a linear scale, not necessarily your calculation.

An explanation can be seen on

What do you mean about the test case of the factory?

Best,

Juliette