TPS7B7702-Q1: [TPS7B7702QPWPRQ1] Circuit review and inquiries

Hello Jeon,

                 The excel sheet you attached had three sheets and , as you mention, the questions seemed to be identical. Please find below my responses to the questions:

(1) Range of Voltages on SENSE pin:

(a) For over current fault condition, as mentioned in the datasheet snapshot you attached in the Excel file, the SENSE pin voltage varies from 2.4V (Min) to 2.65V (Max) and would typically latch to 2.55V

(b) For Normal load currents, as mentioned in the datasheet snapshot, the V_SENSE pin assumes a voltage of (I_O*R_SENSE)/CURRENT_RATIO, where CURRENT_RATIO is the ratio of I_OUT and I_SENSE and is typically 198 in value. The Electrical Characteristics table (6.5) of the datasheet in page 6, provides the accuracy of the CURRENT_RATIO (OUTx to SENSEx current ratio accuracy) for various load currents. For example, a load current of 60mA would result in CURRENT_RATIO having an accuracy of ±5%. The accuracy of CURRENT_RATIO along with the accuracy of the resistor can be used to determine the range of voltage on the SENSE pin. 

(c) For no load current, I am running a simulation to determine the CURRENT_RATIO accuracy for your specific choice of resistor divider values (22KOhm, 5.6KOhm). I will provide you the results by the end of the day tomorrow.   

(2) Diode requirement: The device has reverse current (short to battery) protection built-in. Hence strictly speaking, the external diode is not required. 

(3) LDO1 & LDO2 Interaction:

(a) LDO1 and LDO2 are operate fairly independent of each other (except for thermal shutdown). 

(b) If output of LDO1 results in Thermal shutdown, then both outputs (that of LDO1 and LDO2) will shut down. But for other faults (Current limit, short to Battery), fault on one LDO output will not impact the output of the second LDO.

(c) The governing thermal equation remains : T_J = T_A + θ_JA* (P₁ + P₂), where T_J is the junction temperature, T_A is the ambient temperature, θ_JA is the thermal resistance and P₁ and P2 are the power dissipated in LDO1 and LDO2 respectively. To prevent thermal shutdown T_J needs to be less than 175°C. Short circuit/Over Current in one LDO output does not necessarily result in thermal shutdown, but if it does, disabling LDO1 would remove P1 and help bring the part out of Thermal Shutdown. It would not be required to disable LDO2.  As the equation suggests it depends on the values of P₁, P₂, T_A and θ_JA.    

(4) Fault Conditions: In case of thermal shutdown, the output of both LDO channels will disable once TJ exceeds ~175°C. When TJ reduces in temperature by ~15°C, the outputs will turn on again. Here toggling of EN pin is not required. When reverse current fault occurs however the ERR_N asserts Low. To remove this latch condition, the fault (reverse current) condition must be removed and the EN pin must be toggled. 

(5) Overall Circuit review: I think the values of the resistors and capacitors chosen look good. I do not understand what empty terminal treatment means, could you please explain what that term means?

I hope this helps with what you were looking for. Let me know if you have more questions. Thanks!

Regards,

Srikanth

Hello Jeon,

                   You mentioned that there is a question on the NC pin, but I could not find it. Regarding setting the voltage level on the SENSE pin, as was mentioned, it depends on the value of the load current. V_SENSE pin assumes a voltage of (I_OUT*R_SENSE)/CURRENT_RATIO. In sheet 1 &2 the typical value of I_OUT is given to be 60mA and R_SENSE has been selected to be 2KΩ. For this magnitude of I_OUT, the typical value of CURRENT_RATIO is 198 and it varies by 5% (provided in table 4 of Datasheet). Hence for 60mA of load current, the voltage on the sense pin would typically be ~600mV (with max and min values of ~640mV and ~580mV respectively). In sheet 3 the typical I_OUT is 8mA and 49mA for LDO1 and LDO2 respectively. The corresponding sense voltage would typically be ~80mV (Max ~100mV, Min ~68mV) and ~500mV (Max 550mV, Min 450mV) respectively. 

                   Regarding the No-load condition, there is still an output current given by V_FB/R_FB. For the circuit in Sheet 1 for example, R_FB is selected to be 5.6kΩ and since V_FB is 1.2V, the load current is ~220uA. For this load current we simulated the SENSE pin output current to typically be ~1uA. Hence the voltage on the SENSE pin would be ~2mV (for R_SENSE of 2KΩ).

                    Regarding the question on Page 3, apologies for my oversight. Yes, even if the load current on the LDO is 1mA, we can accurately detect the fault conditions. The current limit can be set externally using R_LIM = (1.233*198)/I_{CURRENT LIM}. In sheets 1-3, it looks like 910Ω and 1.2KΩ have been selected which result in current limits of ~270mA and 200mA respectively. To maintain the accuracy, it would be best to set this current limit to at least 50mA. The resistors and capacitor values selected for all three circuits look good. Thanks!

Regards,

Srikanth