19-1240; Rev 1; 2/04
Switched-Capacitor Voltage Inverters
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 efficien-
cy (90%) and low operating current (0.7mA for the
MAX870) make these devices ideal for both battery-pow-
ered 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 a similar device with logic-controlled shutdown,
refer to the MAX1720/MAX1721. For applications
requiring more power, the MAX860 delivers up to 50mA
with a voltage drop of 600mV, in a space-saving µMAX
package.
o
5-Pin SOT23-5 Package
o
99% Voltage Conversion Efficiency
o
Invert Input Supply Voltage
o
0.7mA Quiescent Current (MAX870)
o
+1.4V to +5.5V Input Voltage Range
o
Require Only Two Capacitors
o
25mA Output Current
o
Shutdown Control
Features
MAX870/MAX871
Ordering Information
PART
MAX870EUK
MAX871EUK
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
PIN-
PACKAGE
5 SOT23-5
5 SOT23-5
SOT
TOP MARK
ABZN
ABZO
________________________Applications
Local -5V Supply from 5V Logic Supply
Small LCD Panels
Cell Phones
Medical Instruments
Handy-Terminals, PDAs
Battery-Operated Equipment
Typical Operating Circuit
Pin Configuration
5
C1+
IN
2
INPUT
SUPPLY
VOLTAGE
TOP VIEW
MAX870
MAX871
3
C1-
OUT
4
1
NEGATIVE
OUTPUT
VOLTAGE
OUT
1
5
C1+
IN
2
MAX870
MAX871
4
GND
C1-
3
GND
SOT23-5
NEGATIVE VOLTAGE CONVERTER
________________________________________________________________
Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
Switched-Capacitor Voltage Inverters
ABSOLUTE MAXIMUM RATINGS
MAX870/MAX871
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, 10s) .................................+300°C
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.
ELECTRICAL CHARACTERISTICS
(V
IN
= +5V, C1 = C2 = 1µF (MAX870), C1 = C2 = 0.33µF (MAX871),
T
A
= 0°C to +85°C,
unless otherwise noted. Typical values
are at T
A
= +25°C.)
PARAMETER
Supply Current
Minimum Supply Voltage
Maximum Supply Voltage
Oscillator Frequency
Power Efficiency
Voltage Conversion Efficiency
CONDITIONS
T
A
= +25°C (Note 3)
R
LOAD
= 10kΩ
R
LOAD
= 10kΩ
T
A
= +25°C
R
LOAD
= 500kΩ,
T
A
=+25°C
R
LOAD
=
∞,
T
A
=+25°C
MAX870
MAX871
MAX870
MAX871
MAX870
MAX871
C1 = C2 = 1µF
C1 = C2 = 0.47µF
C1 = C2 = 0.33µF
MAX871
C1 = C2 = 0.22µF
C1 = C2 = 0.1µF
T
A
= 0°C to + 85°C
98
96
81
325
125
500
90
75
99.3
99
20
25
20
25
35
65
50
Ω
50
MAX870
MAX871
T
A
= +25°C
T
A
= 0°C to + 85°C
1.4
1.5
5.5
169
675
MIN
TYP
0.7
2.7
1.0
MAX
1.0
3.8
UNITS
mA
V
V
kHz
%
%
MAX870
Output Resistance (Note 1)
I
OUT
= T
A
= +25°C
5mA
Note 1:
Capacitor contribution is approximately 20% of the output impedance [ESR + 1 / (pump frequency x capacitance)].
ELECTRICAL CHARACTERISTICS
(V
IN
= +5V, C1 = C2 = 1µF (MAX870), C1 = C2 = 0.33µF (MAX871),
T
A
= -40°C to +85°C,
unless otherwise noted.) (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Supply Current (Note 3)
Minimum Supply-Voltage Range
Maximum Supply-Voltage Range
Oscillator Frequency
Output Resistance
Voltage Conversion Efficiency
MAX870
MAX871
R
LOAD
= 10kΩ
R
LOAD
= 10kΩ
MAX870
MAX871
I
OUT
= 5mA
R
LOAD
=
∞
MAX870
MAX871
97
95
56
225
1.6
5.5
194
775
65
1.3
4.4
mA
V
V
kHz
Ω
%
Note 2:
All -40°C to +85°C specifications are guaranteed by design.
Note 3:
The MAX870/MAX871 may draw high supply current during startup, up to the minimum operating supply voltage. To guarantee
proper startup, the input supply must be capable of delivering 90mA more than the maximum load current.
2
_______________________________________________________________________________________
Switched-Capacitor Voltage Inverters
MAX870/MAX871
__________________________________________Typical Operating Characteristics
(Circuit of Figure 1, V
IN
= +5V, C1 = C2 = C3, T
A
= +25°C, unless otherwise noted.)
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX870/71-TOC01
OUTPUT RESISTANCE
vs. SUPPLY VOLTAGE
MAX828/829-02
MAX870
OUTPUT RESISTANCE vs. TEMPERATURE
45
OUTPUT RESISTANCE (Ω)
40
35
30
25
20
15
10
5
V
IN
= 5.0V
V
IN
= 3.3V
V
IN
= 1.5V
MAX870/71 ROC3
3.0
2.5
SUPPLY CURRENT (mA)
2.0
1.5
1.0
MAX870
0.5
0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
MAX871
60
50
OUTPUT RESISTANCE (Ω)
50
40
MAX871
30
MAX870
20
10
5.5
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
0
-40
-15
10
35
60
85
TEMPERATURE (°C)
MAX870
OUTPUT CURRENT vs. CAPACITANCE
40
OUTPUT CURRENT (mA)
35
30
25
20
15
10
5
0
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
CAPACITANCE (µF)
V
IN
= 1.9V, V
OUT
= -1.5V
V
IN
= 3.15V, V
OUT
= -2.5V
V
IN
= 4.75V, V
OUT
= -4.0V
MAX870/871-04
MAX870
OUTPUT VOLTAGE RIPPLE
vs. CAPACITANCE
400
350
300
250
200
150
100
50
0
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
CAPACITANCE (µF)
10
0
V
IN
= 4.75V, V
OUT
= -4.0V
V
IN
= 3.15V, V
OUT
= -2.5V
V
IN
= 1.9V, V
OUT
= -1.5V
MAX870/871-05
MAX871
OUTPUT RESISTANCE vs. TEMPERATURE
MAX870/71-TOC06
45
450
OUTPUT VOLTAGE RIPPLE (mVp-p)
70
60
OUTPUT RESISTANCE (Ω)
V
IN
= 1.5V
50
40
V
IN
= 3.3V
30
20
V
IN
= 5.0V
-40
-15
10
35
60
85
TEMPERATURE (°C)
MAX871
OUTPUT CURRENT vs. CAPACITANCE
MAX870/871-07
MAX871
OUTPUT VOLTAGE RIPPLE
vs. CAPACITANCE
MAX870/71 TOC08
MAX870
OUTPUT VOLTAGE
vs. OUTPUT CURRENT
-0.5
-1.0
OUTPUT VOLTAGE (V)
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
V
IN
= 5.0V
V
IN
= 3.3V
V
IN
= 2.0V
MAX870/871-TOC9
35
30
OUTPUT CURRENT (mA)
25
20
15
10
5
0
0
500
450
OUTPUT VOLTAGE RIPPLE (mVp-p)
400
350
300
250
200
150
100
50
0
V
IN
= 3.15V, V
OUT
= -2.5V
V
IN
= 1.9V, V
OUT
= -1.5V
V
IN
= 4.75V, V
OUT
= -4.0V
0
V
IN
= 4.75V, V
OUT
= -4.0V
V
IN
= 3.15V, V
OUT
= -2.5V
V
IN
= 1.9V, V
OUT
= -1.5V
0.5
1.0
1.5
2.0
2.5
0
0.5
1.0
1.5
2.0
2.5
0
5
10
15
20
25
30
35
40
45
CAPACITANCE (µF)
CAPACITANCE (µF)
OUTPUT CURRENT (mA)
_______________________________________________________________________________________
3
Switched-Capacitor Voltage Inverters
MAX870/MAX871
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 1, V
IN
= +5V, C1 = C2 = C3, T
A
= +25°C, unless otherwise noted.)
MAX870
EFFICIENCY vs. OUTPUT CURRENT
MAX870/71-TOC10
MAX871
EFFICIENCY vs. OUTPUT CURRENT
80
70
EFFICIENCY (%)
60
50
40
30
20
10
0
V
IN
= 2.0V
V
IN
= 3.3V
V
IN
= 5.0V
MAX870/71 TOC11
PUMP FREQUENCY vs. TEMPERATURE
550
500
PUMP FREQUENCY (kHz)
450
400
350
300
250
200
150
100
V
IN
= 1.5V, MAX870
V
IN
= 3.3V OR 5.0V, MAX870
V
IN
= 1.5V, MAX871
V
IN
= 3.3V OR 5.0V, MAX871
MAX870/71-TOC12
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0
5
V
IN
= 2.0V
V
IN
= 3.3V
V
IN
= 5.0V
90
600
10 15 20 25 30 35 40 45 50
OUTPUT CURRENT (mA)
0
5
10
15
20
25
30
35
40
-40
-15
10
35
60
85
OUTPUT CURRENT (mA)
TEMPERATURE (°C)
MAX870
OUTPUT NOISE AND RIPPLE
MAX870/71-TCC13
MAX871
OUTPUT NOISE AND RIPPLE
MAX870/71-TCC14
2µs/div
V
IN
= 3.3V, V
OUT
= -3.18V, I
OUT
= 5mA,
20mV/div, AC COUPLED
1µs/div
V
IN
= 3.3V, V
OUT
= -3.14V, I
OUT
= 5mA,
20mV/div, AC COUPLED
_____________________Pin Description
PIN
NAME
OUT
IN
C1-
GND
C1+
FUNCTION
Inverting Charge-Pump Output
Positive Power-Supply Input
Flying Capacitor’s Negative Terminal
Ground
Flying Capacitor’s Positive Terminal
*1µF
(MAX870)
2
3
1
OUT
IN
MAX870
MAX871
C1-
GND
4
C3
0.33µF*
R
L
V
IN
1
2
3
4
5
V
OUT
C1+
5
C2
0.33µF*
C1
0.33µF*
VOLTAGE INVERTER
Figure 1. Test Circuit
4
_______________________________________________________________________________________
Switched-Capacitor Voltage Inverters
_______________Detailed Description
The MAX870/MAX871 capacitive charge pumps invert
the voltage applied to their input. For highest perfor-
mance, 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 half-
cycle, S1 and S3 open, S2 and S4 close, and C1 is level
shifted downward by V
IN
volts. This connects C1 in par-
allel 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
-V
IN
. The actual voltage at the output is more positive
than -V
IN
, since switches S1–S4 have resistance and the
load drains charge from C2.
S1
IN
C1
S2
MAX870/MAX871
S3
S4
C2
V
OUT
= -(V
IN
)
Charge-Pump Output
The MAX870/MAX871 are not voltage regulators: the
charge pump’s output source resistance is approxi-
mately 20Ω at room temperature (with V
IN
= +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
= -(V
IN
– V
DROOP-
)
Figure 2. Ideal Voltage Inverter
where the output impedance is roughly approximated
by:
R
OUT
≅
(
f
OSC
)
1
x C1
+
2R
SW
+
4ESR
C1
+
ESR
C2
Efficiency Considerations
The efficiency of the MAX870/MAX871 is dominated by
its quiescent supply current (I
Q
) at low output current
and by its output impedance (R
OUT
) at higher output
current; it is given by:
×
R
OUT
I
OUT
I
η≅
1
−
OUT
I
OUT
+
I
Q
V
IN
The first term is the effective resistance of an ideal
switched-capacitor circuit (Figures 3a and 3b), and
R
SW
is the sum of the charge pump’s internal switch
resistances (typically 8Ω to 9Ω at V
IN
= +5V). The typical
output impedance is more accurately determined from
the
Typical Operating Characteristics.
__________Applications Information
Capacitor Selection
To maintain the lowest output resistance, use capacitors
with low ESR (Table 1). The charge-pump output resis-
tance is a function of C1’s and C2’s ESR. Therefore,
minimizing the charge-pump capacitor’s ESR minimizes
the total output resistance.
f
V+
V
OUT
V+
R
EQUIV
V
OUT
R
EQUIV
=
1
f
×
C1
C2
R
L
C1
C2
R
L
Figure 3a. Switched-Capacitor Model
Figure 3b. Equivalent Circuit
5
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