Figure 14. ADP3603 Switch Configuration Charging the
Output Capacitor
During the second phase, the positive terminal of the pump
capacitor is connected to ground and the negative terminal is
connected to the output, resulting in a voltage inversion at the
output terminal. Output regulation is done by adjusting the ON
resistance of the S3 through the feedback control loop.
The ADP3603 alternately charges C
P
to the input voltage when
C
P
is switched in parallel with the input supply, and then trans-
fers charge to C
OUT
when CP is switched in parallel with C
OUT
.
Switching occurs at 120 kHz rate. During the time that C
P
is
charging, the peak current is approximately 2 times the output
current. During the time that C
P
is delivering charge to C
OUT
,
the supply current drops down to about 2 mA. An input supply
bypass capacitor will supply part of the peak input current
drawn by the ADP3603, and average out the current drawn
from the supply. A minimum input supply bypass capacitor of
1 µF, preferably a low ESR capacitor such as tantalum or multi-
layer ceramic chip capacitor, is recommended. A large capacitor
may be desirable in some cases, for example when the input
supply is connected to the ADP3603 through long leads, or
when the pulse current drawn by the device might effect other
circuitry through supply coupling.
The output capacitor, C
OUT
, is alternately charged to the C
P
voltage when CP is switched in parallel with C
OUT
. The ESR of
the C
OUT
introduces steps in the V
OUT
waveform whenever the
charge pump charges C
OUT
. This tends to increase V
OUT
ripple.
Ceramic or tantalum capacitors are recommended for C
OUT
if
minimum ripple is desired. The ADP3603 can operate with a
range of capacitors from 1 µF to 100 µF and larger without any
stability problems. However, all tested parameters are obtained
using 10 µF multilayer ceramic capacitors.
In most applications, IR drops due to printed circuit board
traces do not present a problem. In this case, V
SENSE
is tied to
the output at a convenient pcb location not far from the V
OUT
.
However, if a reduction in IR drops or improvement in load
regulation is desired, the sense line can be used to monitor the
output voltage at the load. To avoid excessive noise pickup, the
V
SENSE
line should be as short as possible and away from any
noisy line.
Capacitor Selection
While the exact values of the CIN and C
OUT
are not critical,
good quality, low ESR capacitors such as solid tantalum and
multilayer ceramic capacitors are recommended to minimize
voltage losses at high currents. For a given load current, factors
affecting the output voltage performance are in Figure 15:
• Pump (C2) and the output (C3) capacitance
• ESR of the C2 and C3.
APPLICATION INFORMATION
The ADP3603 uses a charge pump to generate a negative output voltage from a positive input supply. To understand the
operation of the ADP3603, a review of a basic switch capacitor
building block is helpful.
f
R
L
V1 V2
C1
C2
AB
Figure 11. Basic Switch Capacitor Circuit
In Figure 11, when the switch is in the A position, capacitor
C1 will be charged to voltage V1. The total charge on C1 will
be q1 = C1V1.
The switch then moves to the B position, discharging C1 to
voltage V2. After this discharge time, the charge on C1 is q2 =
C1V2. The amount of charge transferred from the source, V1,
to the output, V2, is:
∆q = q1 – q2 = C1
(V1 – V2)
If the switch is cycled f times per second, the charge transfer
per unit time (i.e., current) is:
I = f ∆q = f C1 (V1 – V2)
To obtain an equivalent resistance for the switched-capacitor
network we can rewrite this equation in terms of voltage and
impedance equivalence:
I = (V1 – V2)/(1/fC1) = (V1 – V2)/R
EQUIV
where R
EQUIV
is defined as:
R
EQUIV
= 1/f C1
Figure 11 equivalent circuit can now be drawn as shown in
Figure 12.
R
L
V1 V2
C2
R
EQUIV
R
EQUIV
=
1
fC1
Figure 12. Basic Switch Capacitor Equivalent Circuit
THEORY OF OPERATION
A switched capacitor principle is used in the ADP3603 to
generate a negative voltage from a positive input voltage. An
on-board oscillator generates two phase clocks to control a
switching network which transfers charge between the storage
capacitors. The basic principle behind the voltage inversion
scheme is illustrated in Figures 13 and 14.
V
IN
S2
C
P
S1
V
OUT
S3
S4
C
OUT
Figure 13. ADP3603 Switch Configuration Charging the
Pump Capacitor
During phase one, S1 and S2 are ON charging the pump
capacitor to the input voltage. Before the next phase begins, S1