runpatrick/sa-sim-pcb

sim/pdn.spice

* SA-SIM v1 — PDN Impedance Analysis
* Target: Z(VBAT rail at BG95 VBAT pin) < 50mΩ from DC to 100MHz
* Methodology: inject 1A AC current at modem_vbat node;
*              V(modem_vbat) in AC analysis = Z(f) in Ohms.
* FAB_REPORT.md risk #6: added 100µF tantalum per BG95 HW Design Guide §3.3.
*
* Signal path:
*   bat_pos → R_batt_int → batt_node
*           → R_jst      → jst_out
*           → L_wire     → fet_in
*           → R_fet      → rail_mid
*           → R_trace    → rail_int
*           → L_trace    → modem_vbat   ← observation / injection node
*
* All caps hang off modem_vbat to GND.

* --- Battery source ---
* Ideal 3.7V DC, AC=0: in AC analysis this is a short to GND (ideal V source)
V_BAT bat_pos 0 DC 3.7 AC 0

* Li-ion internal resistance ~50mΩ
R_batt_int bat_pos batt_node 0.050

* --- JST connector + wire parasitics ---
R_jst  batt_node jst_out 0.030
L_wire jst_out   fet_in  5n

* --- Reverse-polarity FET (series protection) ---
* Rds_on = 25mΩ; parasitic Cds = 5pF (referenced to drain/source nodes)
R_fet     fet_in  rail_mid 0.025
C_fet_par fet_in  0        5p

* --- PCB trace BAT_P → modem VBAT rail ---
* 5mm, 0.5mm width, 2-layer FR4: R≈5mΩ, L≈2nH
R_trace rail_mid rail_int 0.005
L_trace rail_int modem_vbat 2n

* =============================================================
* Decoupling network at modem_vbat node (all caps to GND)
* =============================================================

* Bulk caps on BG95 VBAT (3× 10µF X5R 0603, LCSC C19702)
* Model: series ESR + ESL to GND
C_BG95_BAT1 modem_vbat c1_n 10u
R_C1        c1_n       c1_r 0.010
L_C1        c1_r       0    1n

C_BG95_BAT2 modem_vbat c2_n 10u
R_C2        c2_n       c2_r 0.010
L_C2        c2_r       0    1n

C_BG95_BAT3 modem_vbat c3_n 10u
R_C3        c3_n       c3_r 0.010
L_C3        c3_r       0    1n

* Decoupling caps close to BG95 VBAT pins (4× 100nF 0402, LCSC C1525)
* Model: series ESR + ESL to GND
C_d1 modem_vbat cd1_n 100n
R_d1 cd1_n      cd1_r 0.020
L_d1 cd1_r      0     0.5n

C_d2 modem_vbat cd2_n 100n
R_d2 cd2_n      cd2_r 0.020
L_d2 cd2_r      0     0.5n

C_d3 modem_vbat cd3_n 100n
R_d3 cd3_n      cd3_r 0.020
L_d3 cd3_r      0     0.5n

C_d4 modem_vbat cd4_n 100n
R_d4 cd4_n      cd4_r 0.020
L_d4 cd4_r      0     0.5n

* Tantalum bulk 100µF / 10V — FAB_REPORT.md risk #6 recommended fix
* LCSC C7171 (TAJB107K010RNJ), ESR=200mΩ, ESL=2nH
C_bulk modem_vbat cb_n 100u
R_bulk cb_n       cb_r 0.200
L_bulk cb_r       0    2n

* =============================================================
* AC test stimulus: 1A AC current source at modem_vbat
* I=1A → V(modem_vbat) = Z(f)  [Ohm's law, AC analysis]
* =============================================================
I_test 0 modem_vbat AC 1

* AC decade sweep: 1Hz to 100MHz, 20 points per decade
.ac dec 20 1 100MEG

* --- Impedance measurements at key PDN frequencies ---
* vm() = magnitude of voltage phasor = |Z(f)| in Ohms
.meas ac z_dc      FIND vm(modem_vbat) AT=1
.meas ac z_1k      FIND vm(modem_vbat) AT=1k
.meas ac z_100k    FIND vm(modem_vbat) AT=100k
.meas ac z_10meg   FIND vm(modem_vbat) AT=10MEG
.meas ac z_100meg  FIND vm(modem_vbat) AT=100MEG
.meas ac z_max     MAX vm(modem_vbat) FROM=1 TO=100MEG

* Print full impedance sweep to output file
.print ac vm(modem_vbat)

.end