Datasheet

I = f Âq = f C
FLY
(V1 ± V2)
Âq = q1 -q2 = C
FLY
(V1 ± V2)
V
1
V
2
C
RES
R
L
C
FLY
BA
C
FLY
CHARGE
PUMP
INVERTOR
C
RESERVE
V
REF1
PRE
REGULATOR
V
CP,IN
f
osc
V
OUT
V
CP,OUT
POST
REGULATOR
C
out
V
DD
V
SS
V
SS
V
REF2
LM7705
SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 www.ti.com
• Controlling the power supplied from the power supply to the charge pump input by the pre-regulator
• The power supplied from the charge pump output to the load by the post-regulator.
A more detailed block diagram of the negative bias generator is given in Figure 24. The control of the pre-
regulator is based on measuring the output voltage of the charge pump. The goal of the post-regulator is to
provide an accurate controlled negative voltage at the output, and acts as a low pass filter to attenuate the output
voltage ripple. The voltage ripple is a result of the switching behavior of the charge pump and is dependent of the
output current and the values of the used capacitors.
Figure 24. Charge Pump Inverter with Input/Output Control
In the next section a simple equation will be derived, that shows the relation between the ripple of the output
current, the frequency of the internal clock generator and the value of the capacitor placed at the output of the
LM7705.
Charge Pump Theory
This section uses a simplified but realistic equivalent circuit that represents the basic function of the charge
pump. The schematic is given in Figure 25.
Figure 25. Charge Pump
When the switch is in position A, capacitor C
FLY
will charge to voltage V
1
. The total charge on capacitor C
FLY
is
Q
1
= C
FLY
x V
1
. The switch then moves to position B, discharging C
FLY
to voltage V
2
. After this discharge, the
charge on C
FLY
will be Q
2
= C
FLY
x V
2
. Note that the charge has been transferred from the source V
1
to the
output V
2
. The amount of charge transferred is:
(1)
When the switch changes between A and B at a frequency f, the charge transfer per unit time, or current is:
(2)
The switched capacitor network can be replaced by an equivalent resistor, as indicated in Figure 26.
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