TPS2583xQ1: Using integrated DC/DC to power VCONN
Detailed DCDC design considerations of the TPS2583x-Q1 family are shared, in addition to R, C calculations and performance test results.
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Welcome back. In session II, I will introduce the details of how to use the TPS25830 built-in DCDC to power the VCONN.
First, I will introduce the design consideration with using the DCDC power VCONN. In this solution, the resistor R2, which is between the P-MOSFET source and gate, is used to hold the P-FET VGS voltage when the load detect pin is not asserted. Make sure the P-FET is turned off, because if the P-FET is not off before DCDC is working, the VCC will charge the large caps on the TPS25830 output.
Since the VCC has very limited power capability, the VCC voltage will be pulled below the UVLO threshold, and that will cause the TPS25830 to get stuck in reset cycle. The P-MOSFET GATE resistor R3 is an option. It can be removed.
And for the R1 and C, they are very important for this solution, because they compose the RC delay circuit when the P-MOSFET is intended to turn on by the load detect pin. If the R1 is too small, then when Rd is attached, and after that deglitch time, the load detect pin is asserted, and pulls the P-FET GATE to ground too early, and that will cause issues.
Since the VCC voltage is 5 volts, but DCDC output voltage hasn't ramped up to 5 volts yet, VCC LDO will charge the DCDC output caps. This results in the VCC entering hiccup mode due to large charging current, and the TPS25830 cannot start up.
The P-MOSFET must be connected before the RSNS resistor. If collecting P-MOSFET to the CSN/OUT pin, which is after the RSNS resistor, then VCONN current is accounted in the current limit calculation. So that will cause some problems.
And the Type-C timing for VCONN power also needs attention. You can refer to the below Type-C spec. According to the table, the VCONN needs to be present within a maximum 2 milliseconds after the TPS25830 VBUS is valid, so the P-FET turn-on time needs to be designed carefully.
A good design is to turn-on the P-MOSFET once the TPS25830 DCDC output voltage ramps up to its nominal value. This time is the DCDC soft-start timing. Below is the timing from the TPS25830 datasheet. We can say the typical value of the soft-start time is 25 milliseconds.
The P-MOSFET must be turned on with a soft-start delay time after the load detect pin is asserted. Since is the MOSFET turn-on speed is very fast, therefore it can ensure that the VCONN turn-on synchronizes with the DCDC output.
With this special P-MOSFET turn-on timing design, this solution can prevent the VCC to charge the TPS25830 output caps. And it also doesn't delay too much on the VCONN power when the VBUS is valid in order to meet the maximum 2 milliseconds VCONN delay timing restriction from the USB Type-C specification.
For this solution, R3 doesn't affect the P-MOSFET turn-on time, and only the R1 and C values need to be determined. The R1 and C can be determined by following guidelines from the analysis discussed above. First, the P-MOSFET must be turned on after the soft-start time, and the P-MOSFET VGS needs to drop below the P-MOSFET VTH threshold after the soft-start time.
Per the application block on the right, the P-MOSFET GATE voltage can be deduced with the below formulas. These are the linear first-order inhomogeneous differential equations. The solution for the gate voltage is this, and the t is the TPS25830 soft-start time. VGATE is the P-MOSFET GATE voltage during the DCDC soft-start.
Here is the design example. This design is based on the below conditions that assume the R2 is 100 kiloohms, and choose C as 470 nanofarads. The R1 value can be determined based on the given R2 and C values. The VCC is the TPS25830 output voltage, and the VCC typical value is 5 volts. t is the soft-start time, and the typical value is 5 milliseconds.
The first is choose the P-MOSFET and determine VGATE. The VGATE voltage can be calculated by the below equation. In this design, select this type P-MOSFET to compose the solution. Below is the VTH parameter from this P-MOSFET according to its datasheet. It doesn't give the typical value of the VTH, but this can be measured by oscilloscope.
According to the waveform, it shows when the P-MOSFET source wattage is 5 volts, which comes from the VCC. Once GATE voltage approaches 3.54 volts, the P-MOSFET is turned on. So the VGATE is 3.54 volts.
And then we can calculate the R1 value with the conditions above. That is, R2 is 100 kiloohms, C is 470 nanofarads, VCC is 5 volts, VGATE is 3.54 volts, and t is 5 millisecond. Then the resistor R1 can be calculated, and the value is 30 kiloohms.
The below waveforms are tested based on the R and C parameters which are calculated above. The results show the TPS25 can start up normally, and the VCC no voltage drop-- we can say the VCC curve is flat. The CC pin can source 300 milliamp current. Please note in this testing we used the electronic to sink 300 milliamp current, so the design has very good performance.
The last slide will introduce limitations of this solution. In this solution, it needs to connect the VCC pin to the TPS25830 DCDC output through an external P-MOSFET. Since the TPS25830 has cable compensation function, the DCDC output voltage rises along with the increased loading at the USB port. The maximum cable compensation voltage is 1.8 volts per the TPS25830 datasheet, so the VCC pin will face a high voltage when the cable compensation function is enabled. The risk is the VCC may be damaged.
Below is the EC table from the TPS25830 datasheet. We can say the maximum voltage on the VCC pin is only 6 volts. So in the design, the cable compensation voltage must be designed below 0.9 volts at its maximum loading to guarantee the applied voltage on the VCC pin will drop into the safe range.
Another limitation is the short-to-battery protection function. If the VBUS shorts to battery, the TPS25830 output voltage can be as high as 18 volts, so the VCC will face this high voltage because it is connected to the TPS25830 output through the external P-MOSFET. Therefore, this solution cannot support BUS short-to-battery protection.