Pin Description
Pin
1
2
3
4
5
Name
OUT
V+
CAP−
GND
CAP+
Negative voltage output.
Power supply positive input.
Connect this pin to the negative terminal of the charge-pump capacitor.
Power supply ground input.
Connect this pin to the positive terminal of the charge-pump capacitor.
a function of the ON resistance of the internal MOSFET
switches, the oscillator frequency, the capacitance and the
ESR of both C
1
and C
2
. Since the switching current charging
and discharging C
1
is approximately twice as the output cur-
rent, the effect of the ESR of the pumping capacitor C
1
will
be multiplied by four in the output resistance. The output ca-
pacitor C
2
is charging and discharging at a current approxi-
mately equal to the output current, therefore, this ESR term
only counts once in the output resistance. A good approxi-
mation of R
out
is:
Function
Circuit Description
The LM828 contains four large CMOS switches which are
switched in a sequence to invert the input supply voltage.
Energy transfer and storage are provided by external capaci-
tors.
Figure 2
illustrates the voltage conversion scheme.
When S
1
and S
3
are closed, C
1
charges to the supply volt-
age V+. During this time interval, switches S
2
and S
4
are
open. In the second time interval, S
1
and S
3
are open; at the
same time, S
2
and S
4
are closed, C
1
is charging C
2
. After a
number of cycles, the voltage across C
2
will be pumped to
V+. Since the anode of C
2
is connected to ground, the output
at the cathode of C
2
equals −(V+) when there is no load cur-
rent. The output voltage drop when a load is added is deter-
mined by the parasitic resistance (R
ds(on)
of the MOSFET
switches and the ESR of the capacitors) and the charge
transfer loss between capacitors.
where R
SW
is the sum of the ON resistance of the internal
MOSFET switches shown in
Figure 2.
High capacitance, low ESR capacitors will reduce the output
resistance.
The peak-to-peak output voltage ripple is determined by the
oscillator frequency, the capacitance and ESR of the output
capacitor C
2
:
Again, using a low ESR capacitor will result in lower ripple.
Capacitor Selection
The output resistance and ripple voltage are dependent on
the capacitance and ESR values of the external capacitors.
The output voltage drop is the load current times the output
resistance, and the power efficiency is
DS100137-26
FIGURE 2. Voltage Inverting Principle
Application Information
Simple Negative Voltage Converter
The main application of LM828 is to generate a negative
supply voltage. The voltage inverter circuit uses only two ex-
ternal capacitors as shown in the Basic Application Circuits.
The range of the input supply voltage is 1.8V to 5.5V.
The output characteristics of this circuit can be approximated
by an ideal voltage source in series with a resistance. The
voltage source equals −(V+). The output resistance, R
out
, is
Where I
Q
(V+) is the quiescent power loss of the IC device,
and I
L2
R
out
is the conversion loss associated with the switch
on-resistance, the two external capacitors and their ESRs.
The selection of capacitors is based on the specifications of
the dropout voltage (which equals I
out
R
out
), the output volt-
age ripple, and the converter efficiency. Low ESR capacitors
(following table) are recommended to maximize efficiency,
reduce the output voltage drop and voltage ripple.
Low ESR Capacitor Manufacturers
Manufacturer
Nichicon Corp.
AVX Corp.
Sprague
Sanyo
Phone
(708)-843-7500
(803)-448-9411
(207)-324-4140
(619)-661-6835
Capacitor Type
PL & PF series, through-hole aluminum electrolytic
TPS series, surface-mount tantalum
593D, 594D, 595D series, surface-mount tantalum
OS-CON series, through-hole aluminum electrolytic
5
www.national.com
NSC [ NATIONAL SEMICONDUCTOR ]
