an9620

Using the HI7188 with a Single Supply
Application Note
May 1996
AN9620
Author: John D. Norris
Introduction
ICL7660S Voltage Converter
The HI7188 is an easy-to-use, 16-bit, 8-channel, sigma-delta
A/D subsystem ideal for low frequency physical and electrical
measurements in scientific, medical, and industrial applications. The device has sufficient on-chip capabilities to support
moving the intelligence away from the system controller and
towards the sensors for faster and more flexible configurability
without the traditional drawbacks of low throughput per channel, higher power and cost per channel, extreme design complexity or excessive software overhead.
The ICL7660S Voltage Converter is a monolithic CMOS
voltage conversion IC. The device performs supply voltage
conversion from positive to negative for an input range of
1.5V to 12V, resulting in complementary output voltages of
-1.5V to -12V. Only 2 noncritical external capacitors are
needed for the charge pump and charge reservoir functions.
The conversion chip contains a series DC power supply
regulator, RC oscillator, voltage level translator, and four
output power MOS switches. For detailed chip information
please refer to the ICL7660S datasheet [1].
The purpose of this application note is to inform the system
designer how to use the HI7188 with a positive analog supply
if a negative analog supply is not available. The purpose of
the negative supply is bias the analog section of the HI7188.
There are two approaches to accomplish the single supply
task. Each approach will maintain full operation of the HI7188.
The first approach would involve using the ICL7660S voltage
converter to derive an inexpensive low current negative power
supply. The second approach involves offsetting the positive
analog supply (AVDD) to 10V, providing +5V to VCM, and
simply grounding the negative supply (AVSS).
ICL7660S VOLTAGE CONVERTER
Functional Description
The ICL7660S single supply circuit is shown in Figure 1. This
includes the HI7188, an ICL7660 voltage converter, and two
10µF capacitors. The HI7188 datasheet [2] contains
information on the HI7188, therefore, this discussion will focus
on the voltage conversion circuit.
29
+5V
0.1µF
+
10µF
- C1
1 NC
V+ 8
2 CAP+
NC 7
3 GND
LV 6
AVDD
+
4.7µF
HI7188
26
VCM
-5V
4 CAP-
VOUT 5
10µF
+ C
2
9, 30
AVSS
0.1µF
FIGURE 1A. VOLTAGE CONVERSION
CIRCUIT
+
FIGURE 1. ICL7660S SINGLE SUPPLY CIRCUIT
1
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Application Note 9620
ICL7660 Theory of Operation [1]
ICL7660 Application Discussion
The ICL7660S contains all the necessary circuitry to
complete a negative voltage converter, with the exception of
2 external capacitors which may be inexpensive 10µF
polarized electrolytic types. The mode of operation of the
device may be best understood by considering Figure 2,
which shows an idealized negative voltage converter.
Capacitor C1 is charged to a voltage, V+, for the half cycle
when switches S1 and S3 are closed. (Note: Switches S2
and S4 are open during this half cycle.) During the second
half cycle of operation, switches S2 and S4 are closed, with
S1 and S3 open, thereby shifting capacitor C1 to C2 such
that the voltage on C2 is exactly V+, assuming ideal switches
and no load on C2. The ICL7660S approaches this ideal situation more closely than existing non-mechanical circuits.
The output characteristics of the voltage converter (Figure 1A)
can be approximated by an ideal voltage source in series
with a resistance as shown in Figure 3. The voltage source
has a value of -(V+) with a slight ripple voltage due to the
ICL7660S switching between C1 and C2. The output impedance (RO) is a function of the ON resistance of the internal
MOS switches (shown in Figure 2), switching frequency, the
value of C1 and C2, and the equivalent series resistance
(ESR) of C1 and C2.
8
S1
2
RO
VOUT
V+
+
S2
FIGURE 3. ICL7660S EQUIVALENT CIRCUIT
VIN
C1
3
3
C2
S4
S3
5
VOUT = -VIN
4
7
FIGURE 2. IDEALIZED NEGATIVE VOLTAGE CONVERTER
In the ICL7660S, the 4 switches of Figure 2 are MOS power
switches; S1 is a P-Channel devices and S2, S3 and S4 are
N-Channel devices. The main difficulty with this approach is
that in integrating the switches, the substrates of S3 and S4
must always remain reverse biased with respect to their
sources, but not so much as to degrade their “ON”
resistances. In addition, at circuit start up, and under output
short circuit conditions (VOUT = V+), the output voltage must
be sensed and the substrate bias adjusted accordingly.
Failure to accomplish this would result in high power losses
and probable device latchup.
This problem is eliminated in the ICL7660S by a logic network
which senses the output voltage (VOUT) together with the
level translators, and switches the substrates of S3 and S4 to
the correct level to maintain necessary reverse bias.
The voltage regulator portion of the ICL7660S is an integral
part of the anti-latchup circuitry, however its inherent voltage
drop can degrade operation at low voltages. Therefore, to
improve low voltage operation “LV” pin should be connected
to GND, disabling the regulator. For supply voltages greater
than 3.5V the LV terminal must be left open to insure latchup
proof operation, and prevent device damage.
2
Below is an analysis of this application at 25oC. From the
datasheet the MOS switch resistance is typically 23Ω,
switching frequency is 10kHz. The capacitors C1 and C2 are
10µF, 16V Kemet Solid Tantalum capacitors type number
“T350106K016AS” with ESR of 1Ω at 10kHz. The typical
supply current into the HI7188 negative supply is 1mA. The
low current requirement of the HI7188 is critical since the
conversion chip is ONLY designed for low current applications. As defined in the ICL7660S datasheet, the inverted
output voltage drops significantly from -5V to -4.25V when
10mA of current is required. If additional current is needed to
drive supplementary devices, multiple ICL7660S units can
be placed in parallel [1]. Below are the theoretical calculations for output impedance and ripple voltage.
1
R O ≅ 2 × R SW + ------------------------------------ + 4 × ESR C1 + ESR C2 Ω
0.5f OSC × C 1
1
R O ≅ 2 × 23 + -------------------------------------------------------- + 4 × ESR C1 + ESR C2
3
–6
( 5 × 10 × 10 × 10 )
R O ≅ 46 + 20 + 5 × 1Ω ≅ 71Ω
1
V RIPPLE ≅ I OUT  ----------------------------------------- + 2ESRC 2
2 × f

PUMP × C 2
1
V RIPPLE ≅ I OUT  ------------------------------------------------- + 2
 2 × 10kHz × 10µF

V RIPPLE ≅ 7mV
Measured Performance
The circuit in Figure 1A was added to the evaluation platform
to determine performance. The testing consisted of both
noise and linearity with the standard -5V supply and a -5V
supply derived from the ICL7660S. Noise data was derived
from 100 readings while linearity involved single measurements in 0.5V increments. Comparison of the performance
data showed no degradation of either noise or linearity while
using the ICL7660S voltage converter.
Application Note 9620
+10V Supply Operation
Measured Performance
The application circuit is shown in Figure 4. The positive
analog supply (AVDD) is tied to 10V while the negative
supply (AVSS) is analog ground. To ensure proper operation,
the virtual ground pin (VCM) must be set midway between
the positive and negative supplies. In this case +5V. This is
critical to ensure the chopper stabilized operational amplifier
is biased correctly otherwise performance would be severely
degraded.
The circuit in Figure 4 was added to the evaluation platform
to determine performance. The testing consisted of both
noise and linearity with the standard +-5V supplies and the
single 10V supply as shown in Figure 4. Noise data was
derived from 100 readings while linearity involved single
measurements in 0.5V increments. Comparison of the performance data showed no degradation of either noise or linearity while using the single 10V supply circuit.
Conclusion
29
+10V
AVDD
+
0.1µF
4.7µF
26
+5V
9, 30
HI7188
VCM
This application note described two methods of using the
HI7188 with a single positive analog supply if a negative
analog supply was not available. The first method involved
the ICL7660S voltage converter to derive an inexpensive low
current negative power supply. The second method involved
offsetting the positive analog supply (AVDD) to 10V,
grounding the negative supply (AVSS) and biasing the virtual
ground pin to +5V.
References
AVSS
[1] ICL7660S Datasheet, File Number 3179, Intersil
Corporation, Melbourne, Florida, 1994.
FIGURE 4. 10V SUPPLY CIRCUIT
[2] HI7188 Datasheet, File Number 4016, Intersil
Corporation, Melbourne, Florida, 1995.
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