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19-1240; Rev 0; 6/97
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 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 MAX package.
____________________________Features
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
MAX870/MAX871
______________Ordering Information
PART MAX870C/D MAX870EUK MAX871C/D MAX871EUK TEMP. RANGE 0C to +70C -40C to +85C 0C to +70C -40C to +85C PINPACKAGE Dice* 5 SOT23-5 Dice* 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
* Dice are tested at TA = +25C.
__________Typical Operating Circuit
__________________Pin Configuration
5
C1+
IN
2
INPUT SUPPLY VOLTAGE
TOP VIEW
MAX870 MAX871
3 C1OUT 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
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Switched-Capacitor Voltage Inverters MAX870/MAX871
ABSOLUTE MAXIMUM RATINGS
IN to GND ..............................................................+6.0V to -0.3V OUT to GND ..........................................................-6.0V to +0.3V C1+ ..............................................................(VIN + 0.3V) to -0.3V C1-............................................................(VOUT - 0.3V) to +0.3V OUT Output Current ...........................................................50mA OUT Short Circuit to GND .............................................Indefinite Continuous Power Dissipation (TA = +70C) SOT23-5 (derate 7.1mW/C above +70C)...................571mW Operating Temperature Range MAX870EUK/MAX871EUK ...............................-40C to +85C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+300C
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
(VIN = +5V, C1 = C2 = 1F (MAX870), C1 = C2 = 0.33F (MAX871), TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER CONDITIONS MIN TYP MAX UNITS Supply Current Minimum Supply Voltage Maximum Supply Voltage Oscillator Frequency Power Efficiency Voltage Conversion Efficiency TA = +25C RLOAD = 10k RLOAD = 10k TA = +25C RLOAD = 500k, TA =+25C RLOAD = , TA =+25C MAX870 MAX871 MAX870 MAX871 MAX870 MAX871 C1 = C2 = 1F C1 = C2 = 0.47F C1 = C2 = 0.33F MAX871 C1 = C2 = 0.22F C1 = C2 = 0.1F TA = 0C to + 85C 98 96 81 325 125 500 90 75 99.3 99 20 25 20 25 35 65 50 50 MAX870 MAX871 TA = +25C TA = 0C to + 85C 1.4 1.5 5.5 169 675 0.7 2.7 1.0 1.0 3.8 mA V V kHz % %
MAX870 Output Resistance (Note 1) IOUT = TA = +25C 5mA
Note 1: Capacitor contribution is approximately 20% of the output impedance [ESR + 1 / (pump frequency x capacitance)].
ELECTRICAL CHARACTERISTICS
(VIN = +5V, C1 = C2 = 1F (MAX870), C1 = C2 = 0.33F (MAX871), TA = -40C to +85C, unless otherwise noted.) (Note 2) PARAMETER CONDITIONS MIN TYP MAX UNITS Supply Current Minimum Supply-Voltage Range Maximum Supply-Voltage Range Oscillator Frequency Output Resistance Voltage Conversion Efficiency MAX870 MAX871 RLOAD = 10k RLOAD = 10k MAX870 MAX871 IOUT = 5mA RLOAD = MAX870 MAX871 97 95 56 225 1.6 5.5 194 775 65 1.3 4.4 mA V V kHz %
Note 2: All -40C to +85C specifications are guaranteed by design. 2 _______________________________________________________________________________________
Switched-Capacitor Voltage Inverters
__________________________________________Typical Operating Characteristics
(Circuit of Figure 1, VIN = +5V, C1 = C2 = C3, TA = +25C, unless otherwise noted.)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX870/71-TOC01
MAX870/MAX871
OUTPUT RESISTANCE vs. SUPPLY VOLTAGE
MAX828/829-02
MAX870 OUTPUT RESISTANCE vs. TEMPERATURE
45 OUTPUT RESISTANCE () 40 35 30 25 20 15 10 5 VIN = 5.0V VIN = 3.3V VIN = 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 ()
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 (F) VIN = 1.9V, VOUT = -1.5V VIN = 3.15V, VOUT = -2.5V VIN = 4.75V, VOUT = -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 (F) 10 0 VIN = 4.75V, VOUT = -4.0V VIN = 3.15V, VOUT = -2.5V VIN = 1.9V, VOUT = -1.5V
MAX870/871-05
MAX871 OUTPUT RESISTANCE vs. TEMPERATURE
MAX870/71-TOC06
45
450 OUTPUT VOLTAGE RIPPLE (mVp-p)
70 60 OUTPUT RESISTANCE () VIN = 1.5V 50 40 VIN = 3.3V 30 20 VIN = 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 VIN = 5.0V VIN = 3.3V VIN = 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 VIN = 3.15V, VOUT = -2.5V VIN = 1.9V, VOUT = -1.5V VIN = 4.75V, VOUT = -4.0V
0
VIN = 4.75V, VOUT = -4.0V
VIN = 3.15V, VOUT = -2.5V
VIN = 1.9V, VOUT = -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 (F)
CAPACITANCE (F)
OUTPUT CURRENT (mA)
_______________________________________________________________________________________
3
Switched-Capacitor Voltage Inverters MAX870/MAX871
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = +5V, C1 = C2 = C3, TA = +25C, 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 VIN = 2.0V VIN = 3.3V VIN = 5.0V
MAX870/71 TOC11
PUMP FREQUENCY vs. TEMPERATURE
550 500 PUMP FREQUENCY (kHz) 450 400 350 300 250 200 150 100 VIN = 1.5V, MAX870 VIN = 3.3V OR 5.0V, MAX870 VIN = 1.5V, MAX871 VIN = 3.3V OR 5.0V, MAX871
MAX870/71-TOC12
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 5 VIN = 2.0V VIN = 3.3V VIN = 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
2s/div VIN = 3.3V, VOUT = -3.18V, IOUT = 5mA, 20mV/div, AC COUPLED
1s/div VIN = 3.3V, VOUT = -3.14V, IOUT = 5mA, 20mV/div, AC COUPLED
_____________________Pin Description
PIN NAME OUT IN C1GND C1+ FUNCTION Inverting Charge-Pump Output Positive Power-Supply Input Flying Capacitor's Negative Terminal Ground Flying Capacitor's Positive Terminal
*1F (MAX870) 2 3 1 OUT IN MAX870 C3 0.33F* RL
VIN
1
2 3 4 5
VOUT C1+ 5 C2 0.33F* 4 C1 0.33F*
MAX871
C1GND
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 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 VIN volts. 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.
S1 IN C1 S2
MAX870/MAX871
C2 S3 S4 VOUT = -(VIN)
Charge-Pump Output
The MAX870/MAX871 are not voltage regulators: the charge pump's output source resistance is approximately 20 at room temperature (with VIN = +5V), and VOUT approaches -5V when lightly loaded. VOUT will droop toward GND as load current increases. The droop of the negative supply (VDROOP-) equals the current draw from OUT (IOUT) times the negative converter's source resistance (RS-): VDROOP- = IOUT x RSThe negative output voltage will be: VOUT = -(VIN - VDROOP-)
Figure 2. Ideal Voltage Inverter
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: PPUMP CAPACITOR LOSSES +PCONVERSION LOSSES = IOUT x ROUT ROUT
2
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:
PLOSS = PINTERNAL LOSSES + PSWITCH LOSSES + PPUMP CAPACITOR LOSSES + PCONVERSION LOSSES
f
V+ VOUT
(fOSC )
1 x C1
+ 2RSWITCHES + 4ESRC1 + ESRC2
where fOSC is the oscillator frequency. The first term is the effective resistance from an ideal switchedcapacitor circuit. See Figures 3a and 3b.
REQUIV V+ 1 REQUIV = f x C1 C2 VOUT RL
C1
C2
RL
Figure 3a. Switched-Capacitor Model
Figure 3b. Equivalent Circuit
5
_______________________________________________________________________________________
Switched-Capacitor Voltage Inverters MAX870/MAX871
Conversion losses occur during the charge transfer between C1 and C2 when there is a voltage difference between them. The power loss is:
PCONV.LOSS = [1 / 2 C1 VIN2 - VOUT2 + 2 1 / C2 V RIPPLE - 2VOUT VRIPPLE ] x fOSC 2
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 IOUT 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 IOUT constantly, except for short switching spikes. A 0.1F bypass capacitor is sufficient.
__________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.
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.
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: IOUT VRIPPLE = + 2 x IOUT x ESRC2 f x C2 OSC Input Bypass Capacitor Bypass the incoming supply to reduce its AC impedance and the impact of the MAX870/MAX871's switching
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
Table 1. Low-ESR Capacitor Manufacturers
PRODUCTION METHOD MANUFACTURER AVX Surface-Mount Tantalum Matsuo Sprague Surface-Mount Ceramic 6 AVX Matsuo SERIES TPS series 267 series 593D, 595D series X7R X7R PHONE (803) 946-0690 (714) 969-2491 (603) 224-1961 (803) 946-0690 (714) 969-2491 FAX (803) 626-3123 (714) 960-6492 (603) 224-1430 (803) 626-3123 (714) 960-6492
_______________________________________________________________________________________
Switched-Capacitor Voltage Inverters MAX870/MAX871
...
2 3 C1 4 5 +VIN 3 2 2 +VIN
...
2 3
MAX870 MAX871 "1"
C1 1
4 5
MAX870 MAX871 "n"
3 1 VOUT C2 C1 4 5
...
C2 VOUT = -nVIN
MAX870 MAX871 "1"
C1 1
4 5
MAX870 MAX871 "n"
1
VOUT
...
ROUT OF SINGLE DEVICE ROUT = NUMBER OF DEVICES VOUT = -VIN C2
Figure 4. Cascading MAX870s or MAX871s to Increase Output Voltage
Figure 5. Paralleling MAX870s or MAX871s to Reduce Output Resistance
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.
+VIN 3 C1 4 2 D1, D2 = 1N4148
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).
MAX870 MAX871
5 1
D1 VOUT = -VIN C2 D2 VOUT = (2VIN) (VFD1) - (VFD2)
C3
C4
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.
Figure 6. Combined Doubler and Inverter
GND
4
MAX870 MAX871
1
OUT
Figure 7. High V- Load Current
_______________________________________________________________________________________
7
Switched-Capacitor Voltage Inverters MAX870/MAX871
Shutdown Control
If shutdown control is necessary, use the circuit in Figure 8. The output resistance of the MAX870/MAX871 will typically be 20 plus two times the output resistance of the buffer driving IN. The 0.1F 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.
INPUT 3 C1 5 4 C1IN 2 CIN 0.1F SHUTDOWN LOGIC SIGNAL OFF ON OUT 1 C2 OUTPUT
MAX870 C1+ MAX871
GND
___________________Chip Information
TRANSISTOR COUNT: 58 SUBSTRATE CONNECTED TO IN
Figure 8. Shutdown Control
________________________________________________________Package Information
SOT5L.EPS
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 (c) 1997 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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