_______________General Description
The ultra-small MAX870/MAX871 monolithic, CMOS
charge-pump inverters accept input voltages ranging
from +1.4V to +5.5V. The MAX870 operates at 125kHz,
and the MAX871 operates at 500kHz. Their high efficiency (90%) and low operating current (0.7mA for the
MAX870) make these devices ideal for both battery-powered and board-level voltage-conversion applications.
Oscillator control circuitry and four power MOSFET
switches are included on-chip. A typical MAX870/
MAX871 application is generating a -5V supply from a
+5V logic supply to power analog circuitry. Both parts
come in a 5-pin SOT23-5 package and can deliver 25mA
with a voltage drop of 500mV.
For applications requiring more power, the MAX860
delivers up to 50mA with a voltage drop of 600mV, in a
space-saving µMAX package.
________________________Applications
Local -5V Supply from 5V Logic Supply
Small LCD Panels
Cell Phones
Medical Instruments
Handy-Terminals, PDAs
Battery-Operated Equipment
____________________________Features
♦ 5-Pin SOT23-5 Package
♦ 99% Voltage Conversion Efficiency
♦ Invert Input Supply Voltage
♦ 0.7mA Quiescent Current (MAX870)
♦ +1.4V to +5.5V Input Voltage Range
♦ Require Only Two Capacitors
♦ 25mA Output Current
♦ Shutdown Control
MAX870/MAX871
Switched-Capacitor Voltage Inverters
________________________________________________________________
Maxim Integrated Products
1
TOP VIEW
IN
GND
C1-
C1+
OUT
SOT23-5
1
5
MAX870
MAX871
2
3
4
__________________Pin Configuration
NEGATIVE VOLTAGE CONVERTER
C1+
C1-
IN
OUT
GND
INPUT
SUPPLY
VOLTAGE
NEGATIVE
OUTPUT
VOLTAGE
MAX870
MAX871
4
3
52
1
__________Typical Operating Circuit
19-1240; Rev 0; 6/97
PART
MAX870C/D
MAX870EUK -40°C to +85°C
0°C to +70°C
TEMP. RANGE
PIN-
PACKAGE
Dice*
5 SOT23-5
______________Ordering Information
*
Dice are tested at TA= +25°C.
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
MAX871C/D
MAX871EUK -40°C to +85°C
0°C to +70°C Dice*
5 SOT23-5
SOT
TOP MARK
—
ABZN
—
ABZO
MAX870/MAX871
Switched-Capacitor Voltage Inverters
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN= +5V, C1 = C2 = 1µF (MAX870), C1 = C2 = 0.33µF (MAX871), TA= 0°C to +85°C, unless otherwise noted. Typical values
are at T
A
= +25°C.)
ELECTRICAL CHARACTERISTICS
(VIN= +5V, C1 = C2 = 1µF (MAX870), C1 = C2 = 0.33µF (MAX871), TA= -40°C to +85°C, unless otherwise noted.) (Note 2)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Note 1: Capacitor contribution is approximately 20% of the output impedance [ESR + 1 / (pump frequency x capacitance)].
Note 2: All -40°C to +85°C specifications are guaranteed by design.
IN to GND..............................................................+6.0V to -0.3V
OUT to GND..........................................................-6.0V to +0.3V
C1+..............................................................(V
IN
+ 0.3V) to -0.3V
C1-............................................................(V
OUT
- 0.3V) to +0.3V
OUT Output Current ...........................................................50mA
OUT Short Circuit to GND .............................................Indefinite
Continuous Power Dissipation (T
A
= +70°C)
SOT23-5 (derate 7.1mW/°C above +70°C)...................571mW
Operating Temperature Range
MAX870EUK/MAX871EUK ...............................-40°C to +85°C
Storage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10sec).............................+300°C
MAX871
MAX870
R
LOAD
= 500kΩ,
TA=+25°C
R
LOAD
= 10kΩ
R
LOAD
= 10kΩ
TA= +25°C
CONDITIONS
mA
2.7 3.8
0.7 1.0
Supply Current
96 99
%
90
Power Efficiency
kHz
325 500 675
1.4 1.0
V
1.5
Minimum Supply Voltage
V5.5Maximum Supply Voltage
81 125 169
UNITSMIN TYP MAXPARAMETER
TA= +25°C
TA= 0°C to + 85°C
25
MAX871
MAX870
I
OUT
= 5mA
MAX871
R
LOAD
= 10kΩ
MAX870
CONDITIONS
mA
4.4
1.3
Supply Current
Ω65Output Resistance
kHz
225 775
Oscillator Frequency
V1.6Minimum Supply-Voltage Range
56 194
UNITSMIN TYP MAXPARAMETER
TA= +25°C
Oscillator Frequency
C1 = C2 = 0.47µF
I
OUT
=
5mA
C1 = C2 = 1µF
ΩOutput Resistance (Note 1)
20 50
MAX870
TA= +25°C C1 = C2 = 0.33µF 20 50
C1 = C2 = 0.22µF 25MAX871
C1 = C2 = 0.1µF 35
65TA= 0°C to + 85°C
75
MAX870
MAX871
MAX870
MAX871
R
LOAD
= ∞, TA=+25°C
%
98 99.3
Voltage Conversion Efficiency
MAX870
MAX871
R
LOAD
= ∞
%
97
Voltage Conversion Efficiency
95
MAX870
MAX871
R
LOAD
= 10kΩ V5.5Maximum Supply-Voltage Range
MAX870/MAX871
Switched-Capacitor Voltage Inverters
_______________________________________________________________________________________
3
0
0.5
1.0
1.5
2.0
2.5
3.0
1.5 2.52.0 3.0 3.5 4.0 4.5 5.0 5.5
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX870/71-TOC01
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)
MAX870
MAX871
60
10
1.5 2.5 3.02.0
OUTPUT RESISTANCE
vs. SUPPLY VOLTAGE
50
MAX828/829-02
SUPPLY VOLTAGE (V)
OUTPUT RESISTANCE (Ω)
3.5 4.0 5.54.5 5.0
40
20
30
MAX870
MAX871
0
15
10
5
20
25
30
35
40
45
50
-40 10-15 35 60 85
MAX870
OUTPUT RESISTANCE vs. TEMPERATURE
MAX870/71 ROC3
TEMPERATURE (°C)
OUTPUT RESISTANCE (Ω)
VIN = 1.5V
VIN = 3.3V
VIN = 5.0V
45
0
0 0.5 3.02.5
MAX870
OUTPUT CURRENT vs. CAPACITANCE
35
40
MAX870/871-04
CAPACITANCE (µF)
OUTPUT CURRENT (mA)
1.0 1.5 2.0 3.5
30
25
20
10
5
15
VIN = 3.15V, V
OUT
= -2.5V
VIN = 1.9V, V
OUT
= -1.5V
VIN = 4.75V, V
OUT
= -4.0V
35
0
0 0.5 2.0
MAX871
OUTPUT CURRENT vs. CAPACITANCE
25
30
MAX870/871-07
CAPACITANCE (µF)
OUTPUT CURRENT (mA)
1.0 1.5 2.5
20
15
10
5
VIN = 3.15V, V
OUT
= -2.5V
VIN = 1.9V, V
OUT
= -1.5V
VIN = 4.75V, V
OUT
= -4.0V
450
0
0 0.5 2.0 2.5 3.0 4.03.5 4.5
MAX870
OUTPUT VOLTAGE RIPPLE
vs. CAPACITANCE
50
350
400
MAX870/871-05
CAPACITANCE (µF)
OUTPUT VOLTAGE RIPPLE (mVp-p)
1.51.0 5.0
300
250
200
150
100
VIN = 4.75V, V
OUT
= -4.0V
V
IN
= 3.15V, V
OUT
= -2.5V
V
IN
= 1.9V, V
OUT
= -1.5V
0
20
10
40
30
60
50
70
-40 10-15 35 60 85
MAX871
OUTPUT RESISTANCE vs. TEMPERATURE
MAX870/71-TOC06
TEMPERATURE (°C)
OUTPUT RESISTANCE (Ω)
VIN = 1.5V
VIN = 3.3V
VIN = 5.0V
0
150
100
50
200
250
300
350
400
450
500
0 1.00.5 1.5 2.0 2.5
MAX871
OUTPUT VOLTAGE RIPPLE
vs. CAPACITANCE
MAX870/71 TOC08
CAPACITANCE (µF)
OUTPUT VOLTAGE RIPPLE (mVp-p)
VIN = 4.75V, V
OUT
= -4.0V
V
IN
= 3.15V, V
OUT
= -2.5V
V
IN
= 1.9V, V
OUT
= -1.5V
__________________________________________Typical Operating Characteristics
(Circuit of Figure 1, VIN= +5V, C1 = C2 = C3, TA= +25°C, unless otherwise noted.)
-5.0
-4.0
-4.5
-3.0
-3.5
-2.0
-2.5
-1.5
-0.5
-1.0
0
0 10 15 205 25 30 35 40 45
MAX870
OUTPUT VOLTAGE
vs. OUTPUT CURRENT
MAX870/871-TOC9
OUTPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
V
IN
= 2.0V
V
IN
= 3.3V
V
IN
= 5.0V
_____________________Pin Description
MAX870/MAX871
Switched-Capacitor Voltage Inverters
4 _______________________________________________________________________________________
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN= +5V, C1 = C2 = C3, TA= +25°C, unless otherwise noted.)
0
20
10
40
30
60
50
70
90
80
100
0 10 15 205 25 30 35 4540 50
MAX870
EFFICIENCY vs. OUTPUT CURRENT
MAX870/71-TOC10
OUTPUT CURRENT (mA)
EFFICIENCY (%)
VIN = 2.0V
VIN = 3.3V
VIN = 5.0V
0
20
10
50
40
30
80
70
60
90
0 15 205 10 25 30 35 40
MAX871
EFFICIENCY vs. OUTPUT CURRENT
MAX870/71 TOC11
OUTPUT CURRENT (mA)
EFFICIENCY (%)
VIN = 2.0V
VIN = 3.3V
VIN = 5.0V
100
250
200
150
300
350
400
450
500
550
600
-40 10-15 35 60 85
PUMP FREQUENCY vs. TEMPERATURE
MAX870/71-TOC12
TEMPERATURE (°C)
PUMP FREQUENCY (kHz)
VIN = 1.5V, MAX871
VIN = 1.5V, MAX870
VIN = 3.3V OR 5.0V, MAX870
VIN = 3.3V OR 5.0V, MAX871
MAX870
OUTPUT NOISE AND RIPPLE
MAX870/71-TCC13
2µs/div
V
IN
= 3.3V, V
OUT
= -3.18V, I
OUT
= 5mA,
20mV/div, AC COUPLED
Flying Capacitor’s Positive TerminalC1+5
GroundGND4
Flying Capacitor’s Negative TerminalC1-3
PIN
Positive Power-Supply InputIN2
Inverting Charge-Pump OutputOUT
1
FUNCTIONNAME
VOLTAGE INVERTER
OUT
IN
C1+
V
IN
R
L
C1
0.33µF*
*1µF
(MAX870)
C2
0.33µF*
C3
0.33µF*
5
1
2
3
4
V
OUT
GNDC1-
MAX870
MAX871
Figure 1. Test Circuit
MAX871
OUTPUT NOISE AND RIPPLE
MAX870/71-TCC14
1µs/div
V
IN
= 3.3V, V
OUT
= -3.14V, I
OUT
= 5mA,
20mV/div, AC COUPLED
_______________Detailed Description
The MAX870/MAX871 capacitive charge pumps invert
the voltage applied to their input. For highest performance, use low equivalent series resistance (ESR)
capacitors (e.g., ceramic).
During the first half-cycle, switches S2 and S4 open,
switches S1 and S3 close, and capacitor C1 charges to
the voltage at IN (Figure 2). During the second halfcycle, S1 and S3 open, S2 and S4 close, and C1 is level
shifted downward by VINvolts. This connects C1 in parallel with the reservoir capacitor C2. If the voltage across
C2 is smaller than the voltage across C1, then charge
flows from C1 to C2 until the voltage across C2 reaches
-VIN. The actual voltage at the output is more positive
than -VIN, since switches S1–S4 have resistance and the
load drains charge from C2.
Charge-Pump Output
The MAX870/MAX871 are not voltage regulators: the
charge pump’s output source resistance is approximately 20Ω at room temperature (with VIN= +5V), and
V
OUT
approaches -5V when lightly loaded. V
OUT
will
droop toward GND as load current increases. The
droop of the negative supply (V
DROOP-
) equals the cur-
rent draw from OUT (I
OUT
) times the negative convert-
er’s source resistance (RS-):
V
DROOP-
= I
OUT
x RS-
The negative output voltage will be:
V
OUT
= -(VIN– V
DROOP-
)
Efficiency Considerations
The power efficiency of a switched-capacitor voltage
converter is affected by three factors: the internal losses in the converter IC, the resistive losses of the pump
capacitors, and the conversion losses during charge
transfer between the capacitors. The total power loss is:
The internal losses are associated with the IC’s internal
functions, such as driving the switches, oscillator, etc.
These losses are affected by operating conditions such
as input voltage, temperature, and frequency.
The next two losses are associated with the voltage
converter circuit’s output resistance. Switch losses
occur because of the on-resistance of the MOSFET
switches in the IC. Charge-pump capacitor losses
occur because of their ESR. The relationship between
these losses and the output resistance is as follows:
where f
OSC
is the oscillator frequency. The first term is
the effective resistance from an ideal switchedcapacitor circuit. See Figures 3a and 3b.
ΣP = P + P
+ P
+ P
LOSS INTERNAL LOSSES SWITCH LOSSES
PUMP CAPACITOR LOSSES
CONVERSION LOSSES
MAX870/MAX871
Switched-Capacitor Voltage Inverters
_______________________________________________________________________________________ 5
S1
IN
S2
S3 S4
C1
C2
V
OUT
= -(VIN)
Figure 2. Ideal Voltage Inverter
V+
C1
f
C2 R
L
V
OUT
Figure 3a. Switched-Capacitor Model
R
EQUIV
=
R
EQUIV
V
OUT
R
L
1
V+
f × C1
C2
Figure 3b. Equivalent Circuit
P +P
= I x R
PUMP CAPACITOR LOSSES CONVERSION LOSSES
OUT
2
OUT
R
f x C
R ESR ESR
OUT
OSC
SWITCHES C C
≅
( )
+ + +
1
1
2 4
1 2
MAX870/MAX871
Switched-Capacitor Voltage Inverters
6 _______________________________________________________________________________________
Conversion losses occur during the charge transfer
between C1 and C2 when there is a voltage difference
between them. The power loss is:
__________Applications Information
Capacitor Selection
To maintain the lowest output resistance, use capacitors with low ESR (Table 1). The charge-pump output
resistance is a function of C1’s and C2’s ESR.
Therefore, minimizing the charge-pump capacitor’s
ESR minimizes the total output resistance.
Flying Capacitor (C1)
Increasing the flying capacitor’s size reduces the output resistance. Small C1 values increase the output
resistance. Above a certain point, increasing C1’s
capacitance has a negligible effect, because the output resistance becomes dominated by the internal
switch resistance and capacitor ESR.
Output Capacitor (C2)
Increasing the output capacitor’s size reduces the output ripple voltage. Decreasing its ESR reduces both
output resistance and ripple. Smaller capacitance values can be used with light loads if higher output ripple
can be tolerated. Use the following equation to calculate the peak-to-peak ripple:
Input Bypass Capacitor
Bypass the incoming supply to reduce its AC impedance
and the impact of the MAX870/MAX871’s switching
noise. The recommended bypassing depends on the circuit configuration and on where the load is connected.
When the inverter is loaded from OUT to GND, current
from the supply switches between 2 x I
OUT
and zero.
Therefore, use a large bypass capacitor (e.g., equal to
the value of C1) if the supply has a high AC impedance.
When the inverter is loaded from IN to OUT, the circuit
draws 2 x I
OUT
constantly, except for short switching
spikes. A 0.1µF bypass capacitor is sufficient.
Voltage Inverter
The most common application for these devices is a
charge-pump voltage inverter (Figure 1). This application requires only two external components—capacitors
C1 and C2—plus a bypass capacitor, if necessary.
Refer to the
Capacitor Selection
section for suggested
capacitor types.
Cascading Devices
Two devices can be cascaded to produce an even
larger negative voltage (Figure 4). The unloaded output
voltage is normally -2 x VIN, but this is reduced slightly
by the output resistance of the first device multiplied by
the quiescent current of the second. When cascading
more than two devices, the output resistance rises dramatically. For applications requiring larger negative
voltages, see the MAX864 and MAX865 data sheets.
Paralleling Devices
Paralleling multiple MAX870s or MAX871s reduces the
output resistance. Each device requires its own pump
capacitor (C1), but the reservoir capacitor (C2) serves
all devices (Figure 5). Increase C2’s value by a factor
of n, where n is the number of parallel devices. Figure 5
shows the equation for calculating output resistance.
Combined Doubler/Inverter
In the circuit of Figure 6, capacitors C1 and C2 form the
inverter, while C3 and C4 form the doubler. C1 and C3
are the pump capacitors; C2 and C4 are the reservoir
V =
I
f x C2
RIPPLE
OUT
OSC
+ 22x I x ESR
OUT C
P C1 V V
C2 V 2V V x f
CONV.LOSS IN
2
OUT
2
RIPPLE
2
OUT RIPPLE OSC
[
]
/
/
= −
+
−
1
2
1
2
Table 1. Low-ESR Capacitor Manufacturers
Surface-Mount
Tantalum
PRODUCTION
METHOD
(714) 969-2491
(803) 946-0690
PHONE
(603) 224-1961 (603) 224-1430
(714) 960-6492
(803) 626-3123
FAXMANUFACTURER
AVX
Matsuo
Sprague
SERIES
TPS series
267 series
593D, 595D series
(714) 969-2491
(803) 946-0690AVX
Matsuo (714) 960-6492
(803) 626-3123X7R
X7R
Surface-Mount
Ceramic
MAX870/MAX871
Switched-Capacitor Voltage Inverters
_______________________________________________________________________________________ 7
capacitors. Because both the inverter and doubler use
part of the charge-pump circuit, loading either output
causes both outputs to decline toward GND. Make sure
the sum of the currents drawn from the two outputs
does not exceed 40mA.
Heavy Output Current Loads
Under heavy loads, where higher supply is sourcing current into OUT, the OUT supply must not be pulled above
ground. Applications that sink heavy current into OUT
require a Schottky diode (1N5817) between GND and
OUT, with the anode connected to OUT (Figure 7).
Layout and Grounding
Good layout is important, primarily for good noise performance. To ensure good layout, mount all components as close together as possible, keep traces short
to minimize parasitic inductance and capacitance, and
use a ground plane.
MAX870
MAX871
“n”
MAX870
MAX871
“1”
2
1
V
OUT
C2
2
+V
IN
C1
C2
C1
3 3
4 4
5 51
V
OUT
= -nV
IN
…
…
Figure 4. Cascading MAX870s or MAX871s to Increase
Output Voltage
MAX870
MAX871
“n”
MAX870
MAX871
“1”
2
1
V
OUT
C2
2
+V
IN
C1
C1
3
3
4 4
5
51
V
OUT
= -V
IN
R
OUT
=
R
OUT
OF SINGLE DEVICE
NUMBER OF DEVICES
…
…
Figure 5. Paralleling MAX870s or MAX871s to Reduce Output
Resistance
MAX870
MAX871
2
1
V
OUT
= (2VIN) -
(V
FD1
) - (V
FD2
)
C2
+V
IN
C1
3
4
5
V
OUT
= -V
IN
C4
D1
D1, D2 = 1N4148
C3
D2
Figure 6. Combined Doubler and Inverter
MAX870
MAX871
4
1
GND
OUT
Figure 7. High V- Load Current
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
8
___________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
© 1997 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
MAX870/MAX871
Switched-Capacitor Voltage Inverters
___________________Chip Information
TRANSISTOR COUNT: 58
SUBSTRATE CONNECTED TO IN
________________________________________________________Package Information
SOT5L.EPS
Shutdown Control
If shutdown control is necessary, use the circuit in
Figure 8. The output resistance of the MAX870/MAX871
will typically be 20Ω plus two times the output resistance of the buffer driving IN. The 0.1µF capacitor at
the IN pin absorbs the transient input currents of the
MAX870/MAX871.
The output resistance of the buffer driving the IN pin
can be reduced by connecting multiple buffers in parallel. The polarity of the shutdown signal can also be
changed by using a noninverting buffer to drive IN.
MAX870
MAX871
2
C1-
IN
OUT
C1+
GND
1
C2
C
IN
0.1µF
C1
3
5
4
OUTPUT
INPUT
OFF
ON
SHUTDOWN
LOGIC
SIGNAL
Figure 8. Shutdown Control
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