
Demystifying Switched-Capacitor Circuits
228
IN I
in l
≅+
(
)
⋅1
(6.3)
From Equations (6.2) and (6.3), we can obtain the approximate expression of the
charge-pump’s conversion effi ciency, which is as follows:
η
=≅
+
+
+
−
+
⋅
+
⋅
VI
VI
NC
CC
N
N
N
IT
CC
out l
dd in
p
lclk
p
1
11
11
1V
N
N
V
V
dd
thn
dd
−
+
⋅
(6.4)
The second item in the right part of the foregoing is the effi ciency loss due to output
ripples, which is determined by the number of power stages, load current, switching
frequency, parasitic capacitance, and power supply voltage.
The last item of Equation (6.4) refl ects a major drawback of the Dickson charge-
pump—that is, the threshold voltage drop across each di ...