TPS7B770x-Q1 Vdrop and Vsense

Hello Yamashiro-san,

I've assigned your post to the appropriate apps engineer, and he will respond to your question.

Regards,
Karl

Hi Yamashiro-san,

Thanks for the antenna LDO opportunities. Please find my answer below:
- Is there Dropout Voltage vs Output Current Characteristic at Tj=85C?
<Jason L>: Please refer to Figure 7 in datasheet Page 9.
Dropout Voltage vs Output Current Characteristic is proportional to the temperature, the value at Tj=85C is between the value at Tj=25C and Tj=125C.

- If my customer sets Vsense=2.4V that is the maximum possible output current under the normal operation, how much is the Vsense voltage between normal current and over currnet?
<Jason L>: There is almost no gap of VSENSE voltage between normal current and overcurrent, as customer already sets VSENSE=2.4V at the maximum output current.
Typical VSENSE value is 2.55V at overcurrent/current limit condition, however, it would varies from 2.4V to 2.65V for different devices.

- How much is the delay time of Vsense output after detecting overcurrent?
<Jason L>: This delay time is related with the capacitor(s) and resistor(s) connected at the SENSE pin, it equals to the sense capacitor charging up time.
For quick calculation, the time is longer than 3*R(SENSE)*C(SENSE).

Is customer evaluating this device now, do they have any comments? Have they build PCB boards?

Best regards,
Jason LIU

Thank you for your reply.
My customer want to know more detail.

- If my customer sets Vsense=2.4V at Iout=300mA and LIMx shorted to GND(Ilim=340mA min).
  How much is the Vsense voltage between maximum normal current (300mA) and over current (340mA min)?

- If my customer sets Rsense=1.5k and Csense=1uF. The delay time is 4.5msec?

Best Regards,
tateo

Hi Yamashiro-san,

1. Same answer. There is almost no gap of VSENSE voltage between normal current and overcurrent, as customer already sets VSENSE=2.4V at 300mA current and there is about +/- 3% current ratio accuracy for when IO = 100 to 300 mA.
May I know why customer is concern about this spec?
2. 3*R(SENSE)*C(SENSE) is calculated according to the capacitor charging time constant, considering the resistor/capacitor tolerance, please choose the delay time longer than this value, .
If customer is debugging the program, suggest their software engineers work together with the hardware engineers to determine the delay time based on customer's circuit board.

Best regards,
Jason Liu

Hi Yamashiro-san,

One more, if customer connected LIMx to GND directly, they are using the device internal current limit.
The minimum value of the internal current limit is 340mA, but not all the device have the same internal current limit value.
For most devices, the internal current limit are different, higher than 340mA and lower than 550mA.

If customer want to set this value accurately, please use external current limit resistor.

Best regards,
Jason Liu

Thank you for your support.
I talked with my customer.
My customer uses this device under the following condition.

 Vin=8V
 Vout=7.4V
 The maximum possible output current under the normal operation=100mA
 Current limit=119mA to 140mA

- My customer concerns about dropout voltage. Now there is small margin.
  My customer wants to know dropout voltage at Tj=85C. 85C is maximum temperture at customer's condition.
  Is there Dropout Voltage vs Output Current Characteristic at Tj=85C?

- My customer considers Rsense and Rlim. What is example value at customer's condition?
  I think datasheet Figure 19 is the best. Current limit value is less than Vsense=2.4V.
  ex. Rsense=3.3k, Rlim=2k

- There is Vsense equation(4) in datasheet P18.
  In this case, which is better value of Iomax, the maximum possible output current under the normal operation or current limit?

Best Regards,
tateo

Hi Yamashiro-san,

1.
Dropout voltage is related with the device output current and the device working temperature. As you know, the dropout voltage value increases with device output current increases. The dropout voltage value also increases with device working temperature rises.
You could find the Dropout voltage vs Output Current figure in the TPS7B7702-Q1 datasheet. When Temperature=125 degreeC and Iload=120mA, dropout voltage is about 500mV.
Also there is spec defined in the Electrical Characteristics table, the maximum dropout voltage is 500mV with 100mA load under TJ = –40ºC to +150ºC conditions.

In customer's case, load current is smaller than 100mA under normal conditions and TJ=85 degreeC, and the dropout voltage should be smaller than 500mV.
So if customer could guarantee the input voltage is no smaller than 7.9V, device would not enter the dropout region.

2.
With customer's setup, Rsense=3.3k, Rlim=2k, then current limit threshold is set to about 122mA, and Vsense is 1.67V when load is 100mA.
If customer prefers to have enough margin of sense voltage between normal current and current limit, this is a good example.
If customer does not need any margin, they could use the equation (4), and set the Iomax to current limit (122mA).
I agree with you, the previous one is better.

Best regards,
Jason Liu

Thank you for your support.

My customer wants to know dropout voltage at Tj=85 degreeC.
Worst case is Vin=7.9V and Vout=7.67V. There is no margin.
Now my customer's phase is final prototype. They should change Vout voltage.
But they can't greatly change Vout voltage.
So they need dropout voltage at Tj=85 degreeC.
They don't want guaranteed value. They will only use it as reference.

Best Regards,
tateo

Hi Yamashiro-san,

Please use: the maximum dropout voltage is 500mV with 100mA load under TJ = –40ºC to +150ºC conditions.

As in your previous reply, customer will set Vout=7.4V while Vin=8V, there is enough margin. Even Vin dropped down to 7.9V, it still meet the requirement.

Best regards,
Jason

Thank you for your support.

My customer has one more question.
In maximum dropout condition(ex. Vin=7.9V, Vout=7.4V, Iout=100mA), will the device have any adverse effects?
My customer concerns about current sense accuracy, current limit accuracy and load transiet.

Best Regards,
tateo

Hi Yamashiro-san,

Device could keep the same performance when device is working in normal mode or dropout mode for current sense accuracy, current limit accuracy.

For the load transient, that depends on the TPS7B7702-Q1 working conditions:
If the power supply VIN is strong enough, which would not drop down when LDO load change from light to heavy, device keep the same good load transient performance.
If the power supply VIN is weak, which would drop down when LDO load change from light to heavy, device may enter dropout region, and output voltage would follow VIN.

Best regards,
Jason