SLUSF08 March 2024 TPS1213-Q1
ADVANCE INFORMATION
By following similar design procedure as outlined in Section 8.2.2, the external component values are calculated as below:
Use following equation to calculate the IINRUSH:
Use following equation to calculate the required Cg based on IINRUSH calculated in Equation 24.
Where,
I(G) is 165 µA (typical),
To set IINRUSH at 1.76 A, Cg value is calculated to be ~ 20.6 nF.
A series resistor Rg must be used in conjunction with Cg to limit the discharge current from Cg during turn-off . The chosen value of R3 is 100 Ω and Cg is 22 nF.
During normal operation, the resistor RBYPASS along with bypass FET RDSON is used to set load wakeup current threshold.
For selecting the MOSFET Q3, important electrical parameters are the maximum continuous drain current ID, the maximum drain-to-source voltage VDS(MAX), the maximum drain-to-source voltage VGS(MAX), and the drain-to-source ON resistance RDSON.
Based on the design requirements, BUK7J1R4-40H is selected and its ratings are:
40-V VDS(MAX) and ±20-V VGS(MAX)
RDS(ON) is 1.06-mΩ typical at 10-V VGS
MOSFET Qg(total) is 73 nC typical
MOSET VGS(th) is 2.4 V min
MOSFET CISS is 5.4 nF typical
The recommended range of the short-circuit threshold voltage which is same as load wakeup threshold, V(SCP/LWU), extends from 30 mV to 500 mV. Values near the low threshold of 30 mV can be affected by the system noise. Values near the upper threshold of 500 mV would result in high short-circuit current threshold. To minimize both the concerns, 50 mV is selected as the short-circuit or load wakeup threshold voltage.
The V(SCP/LWU) value can also be calculated based on selected RISCP resistor by following equation:
RBYPASS resistor value can be selected using below equation:
Refer to Equation 13 in Section 8.1.1.2 section for update in equation in final revision of IC.
To set 50 mA as load wakeup threshold, RBYPASS value is calculated to be ~ 1 Ω.
The average power rating of the bypass resistor can be calculated by following equation:
The average power dissipation of RBYPASS is calculated to be ~ 0.0025 W
The peak power dissipation in the bypass resistor is given by following equation:
The peak power dissipation of RBYPASS is calculated to be ~ 256 W
The peak power dissipation time for power-up with short into LPM can be calculated based on following equation:
where,
V(G2_GOOD) is internal threshold with 7 V (typical) value,
I(G2) is 165 μA (typical),
VGS(th) is gate to source voltage and CISS is effective input capacitance of selected bypass FET.
Based on Equation 30, TPULSE is calculated to be ~ 769 μs.
One 1-Ω, 1.5-W, 1% CRCW25121R00FKEGHP resistor is used to support both average and peak power dissipation for > TPULSE time calculated in Equation 30.
TI suggests the designer to share the entire power dissipation profile of bypass resistor with the resistor manufacturer and get their recommendation.
The peak short-circuit current in bypass path can be calculated based on following equation:
IPEAK_BYPASS is calculated to be 16-A based on RBYPASS selected in Equation 27.
TI suggest the designer to ensure that operating point (VBATT_MAX, IPEAK_BYPASS) for bypass path (Q3) is within the SOA curve for > TPULSE time calculated in Equation 30.