Part Number: BQ25173
Hello,
I am using the BQ25173 to charge a 0.22 F, 3.5 V supercapacitor (NEXCW224Z3.5V10.7X5.5TRF) as a backup supply for a MAX31341C RTC.
RTC timekeeping current: ~200–370 nA
Minimum timekeeping voltage: 1 V
Input to charger: 3.3 V
Target supercap voltage: ~3.3 V
Output Voltage Setting
Rt = 499 k
Rb = 160 k
Vfb ≈ 0.8 V
Vout = 0.8 × (499 + 160) / 160 ≈ 3.3 V
Charge Current Setting
Ichg = Kiset / Riset
Kiset = 330 ohm·mA
Riset = 680 Ohm
Ichg ≈ 330 / 680 ≈ 0.485 mA
Estimated Charge Time
t = C × V / I
t = 0.22 × 3.3 / 0.000485
t ≈ 1500 s
t ≈ 25 minutes
Estimated RTC Backup Time
Usable charge:
Q = 0.22 × (3.3 − 1.0) = 0.506 C
Backup time:
T = 0.506 / 370e-9
T ≈ 1.37e6 s
T ≈ 380 hours (~16 days)
Measured Behavior
Vin = 3.3 V
Measured supercap voltage = 3.1 V
Possible reasons considered
• Dropout of the internal pass element
• Supercapacitor leakage
• Feedback and reference tolerances
Questions
-
Is the charge current calculation (Ichg = 330 / Riset) correct?
-
With Vin = 3.3 V and Vout set to 3.3 V, is the lower steady-state voltage expected?
-
Is 3.1 V across the supercap normal in this setup?
-
Any recommended practices for supercap backup with this charger?
Thank you.
