13.24.1.6 Calculate the Flying Capacitor Voltage Drop in One Cycle While Supplying 20 mA

  • dV = IOUT × dt/C
  • dV = 20 mA × 6.67 µs/180 nF
  • dV = 0.741V @ 20 mA

The second and subsequent transfer cycles will have a higher voltage available for transfer, since the capacitor is not completely depleted with each cycle. VCAP will then be VCAP – dV after the first transfer, plus VDDH – (VCAP – dV) times the RC constant. This repeats for each subsequent cycle, allowing a larger charge pump capacitor to be used if the system will tolerate several charge transfers before requiring full output voltage and current.

Repeating Charging Path (Flying Capacitor Across CAP1 and CAP2) for the second cycle (and subsequent by recalculating for each new value of VCAP after each transfer):

  • VCAP = (VCAP – dV) + (VDDH – (VCAP – dV)) (1 – e-T/t)
  • VCAP = (5.31V – 0.741V) + (5.5V – (5.31V – 0.741V)) ×

    (1 – e-[6.67 µs/([7.5W + 3.5W + 20 mW] × 180 nF)])

  • VCAP = 4.567V + 0.934V × 0.96535

VCAP = 5.468V is available for transfer on the second cycle.