Datasheet

POWER
SUPPLY
PRE
REGULATOR
LOAD
CHARGE
PUMP
POST
REGULATOR
LM7705
V
+
OSCILLATOR
C
FLY
OUT=V-
C
RES
CAP
+
CAP
-
S2
S1
S4
S3
Ó1
Ó2
LM7705
www.ti.com SNVS420B –NOVEMBER 2008–REVISED MARCH 2013
APPLICATION INFORMATION
This applications section will give a description of the functionality of the LM7705. The LM7705 is a switched
capacitor voltage inverter with a low noise, −0.23V fixed negative bias output. The part will operate over a supply
voltage range of 3 to 5.25 Volt. Applying a logical low level to the SD input will activate the part, and generate a
fixed −0.23V output voltage. The part can be disabled; the output is switched to ground level, by applying a
logical high level to the SD input of the part.
FUNCTIONAL DESCRIPTION
The LM7705, low noise negative bias generator, can be used for many applications requiring a fixed negative
voltage. A key application for the LM7705 is an amplifier with a true zero output voltage using the original parts,
while not exceeding the maximum supply voltage ratings of the amplifier.
The voltage inversion in the LM7705 is achieved using a switched capacitor technique with two external
capacitors (C
FLY
and C
RES
). An internal oscillator and a switching network transfers charge between the two
storage capacitors. This switched capacitor technique is given in Figure 22.
Figure 22. Voltage Inverter
The internal oscillator generates two anti-phase clock signals. Clock 1 controls switches S1 and S2. Clock 2
controls switches S3 and S4. When Switches S1 and S2 are closed, capacitor C
FLY
is charged to V
+
. When
switches S3 and S4 are closed (S1 and S2 are open) charge from C
FLY
is transferred to C
RES
and the output
voltage OUT is equal to -V
+
.
Due to the switched capacitor technique a small ripple will be present at the output voltage, with a frequency of
the oscillator. The magnitude of this ripple will increase for increasing output currents. The magnitude of the
ripple can be influenced by changing the values of the used capacitors.
In the next section a more detailed technical description of the LM7705 will be given.
TECHNICAL DESCRIPTION
As indicated in the functional description section, the main function of the LM7705 is to supply a stabilized
negative bias voltage to a load, using only a positive supply voltage. A general block diagram for this charge
pump inverter is given in Figure 23. The external power supply and load are added in this diagram as well.
Figure 23. LM7705 Architecture
The architecture given in Figure 23 shows that the LM7705 contains 3 functional blocks:
• Pre-regulator
• Charge pump inverter
• Post-regulator
The output voltage is stabilized by:
Copyright © 2008–2013, Texas Instruments Incorporated Submit Documentation Feedback 9
Product Folder Links: LM7705