AN725
Maxim > Design Support > Technical Documents > Tutorials > Automotive > APP 725
Maxim > Design Support > Technical Documents > Tutorials > Power-Supply Circuits > APP 725
Keywords: power supply, voltage converter, dc-dc converters, charge pumps, switched capacitor
regulator, regulated charge pump, unregulated charge pump, voltage divider, voltage doubler, regulated
inverter, buck-boost, regulator, charge pump power dissipation
TUTORIAL 725
DC-DC Conversion Without Inductors
Jul 22, 2009
Abstract: Charge pumps are often the best choice for powering an application that requires both low
power and low cost. This application note discusses integrated charge pumps and explains how to
calculate power dissipation in a charge pump.
A familiar problem in system engineering is the subsystem whose power requirements are not met by the
main supply. In such cases, the available supply rails are not directly usable, nor is the direct use of
battery voltage (when available) always an option. Lack of space can prevent inclusion of the optimal
number of cells, or the declining voltage of a discharging battery may not be acceptable for the
application.
Voltage converters can generate the desired voltage levels, and charge pumps are often the best choice
for applications requiring some combination of low power, simplicity, and low cost. Charge pumps are
easy to use, because they require no expensive inductors or additional semiconductors.
Charge Pumps—A General Description
Charge-pump voltage converters use ceramic or electrolytic capacitors to store and transfer energy.
Although capacitors are more common and much cheaper than the coils used in other types of DC-DC
converters, capacitors cannot change their voltage level abruptly. A changing capacitor voltage always
follows the exponential function, which imposes limitations that inductive voltage converters can avoid.
Inductive voltage converters are, however, more expensive.
Capacitive voltage conversion is achieved by switching a capacitor periodically. Passive diodes can
perform this switching function in the simplest cases, if an alternating voltage is available. Otherwise, DC
voltage levels require the use of active switches, which first charge the capacitor by connecting it across
a voltage source and then connect it to the output in a way that produces a different voltage level.
A common integrated circuit using this principle is the ICL7660, which some consider the prototype of
the classic charge pump. The ICL7660 integrates switches and the oscillator so that the switches S1, S3
and S2, S4 work alternately (Figure 1). The configuration shown here inverts the input voltage. With a
slight change in the external connections, it can double or divide the input voltage as well.
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Figure 1. These essential components illustrate the mechanics of charge-pump operation.
Closing S1 and S3 charges the flying capacitor, C1, to V+ in the first half cycle. In the second half, S1
and S3 open and S2 and S4 close. This action connects the positive terminal of C1 to ground and
connects the negative terminal to VOUT . C1 is then in parallel with the reservoir capacitor C2. If the
voltage across C2 is smaller than that across C1, charge flows from C1 to C2 until the voltage across C2
reaches -(V+).
An integrated fixed-frequency oscillator drives the periodic switching. This circuit has no output
regulation, and the switching frequency remains constant for all loads. Thus, the output-voltage variation
depends strongly on the load. With no load, the output voltage corresponds to the negative input voltage:
VOUT = -(V+). As the load increases, VOUT decreases. Output current for the ICL7660 is, therefore,
limited to about 10mA. This is partly due to its low oscillator frequency, and partly due to its integrated
analog switches which are far from ideal. These switches in the "on" state exhibit several ohms of onresistance. A detailed calculation of the resulting power dissipation will be shown later.
Pin-compatible circuits (MAX660, MAX860/MAX861, MAX1680/MAX1681) feature higher switching
frequencies and lower on-resistance in the switches. Because their switching frequencies are higher,
these charge pumps operate with smaller capacitors and deliver higher output current. All the devices
can be configured as a voltage inverter, doubler, or divider.
The MAX828/MAX829 and MAX870/MAX871, which were designed for inverter applications, reduce the
required board area with a smaller package (SOT23) and smaller external capacitors. Pin-compatible
versions of these devices (MAX1719/MAX1720/MAX1721) provide an additional shutdown pin for
switching off the circuit. In that condition, the supply current drops to 1nA, the output disconnects from
the input, and the output voltage drops to zero.
Capacitive Voltage-Divider
Consider a circuit designed to divide the input voltage by two and double the output current. It offers
advantages over linear regulators (which usually convert power into heat), and benefits applications that
require a limited output current. A 4mA to 20mA interface, for example, often provides a relatively high
output voltage but a limited preset output current. Other applications include the many op amps and
microcontrollers that now operate with very low supply voltages. In those circuits, dividing the supply
voltage by two theoretically divides the power consumption by four.
The configuration of Figure 2 generates a regulated VOUT (= VIN/2) using the capacitive voltage divider
C3, C4 and C5, C6. By switching the flying capacitor, C2, alternately between upper and lower halves of
this divider, the IC counterbalances any load-dependant voltage differences. The circuit's switching
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frequency is 35kHz and its quiescent current is only 36µA. When load currents exceed 1mA, the circuit's
efficiency exceeds 90%. Given very small load currents (i.e., below 100µA), however, even this low
36mA quiescent current reduces conversion efficiency. This switched-capacitor configuration provides
both better regulation than a simple resistive voltage-divider, and higher efficiency than that obtained
from a simple combination of a voltage-divider and an op-amp buffer. The IC specification limits VIN to
5.5V maximum.
Figure 2. With the connections shown, this inverting charge-pump IC divides the input voltage by two.
Calculating Charge-Pump Power Dissipation
Consider now a simple model in which a capacitor, C1, switches between the output voltage and V+ at
frequency f (Figure 3). This model enables a discussion of charge-pump power dissipation.
Figure 3. This model of a switched capacitor shows that it behaves like a resistor.
A reservoir capacitor, C2, and load, R L , are connected to VOUT . The charge transmitted per cycle is:
ΔQ = C1(V+ - VOUT )
Which produces a current, I, that depends on the frequency f:
I = fΔQ = fC1(V+ - VOUT ).
After changing the equation according to Ohm's Law, an equivalent resistance, R ERS, for the switched
capacitor can be calculated as:
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R ERS = 1/fC1
This equation shows that the resistance and, consequently, the resistive losses decrease with increasing
frequency and higher capacitance. Higher capacitance lowers the output resistance only until the
switches' resistance and the capacitors' equivalent series resistance (ESR) exceed R ERS. This internal
loss (switching loss) can be reduced only by choosing low-ESR capacitors. Switch on-resistance can be
lowered through the use of sophisticated new charge pumps.
Switching loss is caused by the voltage difference between the flying capacitor and the output capacitor,
as well as by on-resistance in the switches. This voltage difference appears across the switches, causing
dissipation in the application. As shown before, a switched capacitor behaves like a resistance. Thus,
you can reduce output resistance and increase the output power by connecting several switchedcapacitor devices in parallel.
Regulated Charge Pumps
Integrated charge pumps that regulate the output voltage operate without inductors. They offer regulated
output voltages (e.g., 5V) and several power-saving modes. Devices such as the MAX682 regulated
upconverters operate either in the efficient skip mode or in a fixed-frequency mode with reduced output
ripple.
When a drop in output voltage is sensed by the internal comparator, the power-saving skip mode avoids
unnecessary switching by activating only the internal oscillator. The result is lower quiescent current and
lower switching dissipation, especially for light loads. Skip mode is preferable for low-power applications,
because higher levels of quiescent current reduce overall efficiency.
To minimize output ripple, the circuit can oscillate in a fixed-frequency mode that is regulated between
50kHz and 2MHz. Regulation ensures that the flying capacitor is charged through an internal MOSFET
with a charging current that depends on the load. A decreasing output voltage, caused by increasing
power consumption, charges the capacitor with more energy. As benefits of the fixed-frequency mode,
the output ripple is lower and the external components are smaller. If you have the impression that
charge pumps provide only low output currents of a few milliamps, you will be surprised to learn that the
MAX682 delivers as much as 250mA from a 5V output.
Regulated-Charge-Pump Design Idea
An improved design for maintaining a switching frequency that is constant and independent of the input
voltage is shown in Figure 4.
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Figure 4. This regulated charge pump maintains a constant switching frequency.
The IC's internal switching frequency is controlled with current into its shutdown pin. The governing
equation is taken from the device's data sheet:
R EXT = 45000(V IN - 0.69V)/f OSC, with R EXT in kΩ and fOSC in kHz
Normally, you calculate the value of the external shutdown resistor with the given input voltage and the
desired switching frequency. In this case, however, the equation shows that switching frequency and
current in the shutdown pin depend on the input voltage, VIN. If the input voltage varies, the switching
frequency varies too.
Two diodes direct current into the shutdown pin. D1 ensures a reliable startup by directing current from
the input to the shutdown pin when the supply voltage is first turned on. When the output voltage
achieves 5V or rises higher than VIN, the switching frequency becomes constant because D2 conducts
current from the stable output voltage. A tiny diode array in a 3-pin SOT23 package (BAV70) is
recommended for the D1-D2 combination. Note that a shutdown function is still available. Driving the
shutdown pin to ground with an open-drain MOSFET simply short circuits the preset-frequency current to
ground.
Regulated Inverter
Many applications need an additional negative voltage such as -5V. Such a voltage can be generated
with a regulating charge-pump inverter (MAX868) and a few external components (Figure 5). When
charging, the left-side switches close and the right-side switches open. Both flying capacitors are
charged in parallel, and the load is serviced entirely by charge stored in the output capacitor. During
discharge, the switches reconfigure to connect the flying capacitors in series. When connected to the
output capacitor, they then transfer charge as required to maintain output-voltage regulation.
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Figure 5. Internal components illustrate the operation of this regulated charge-pump inverter (MAX868).
The internal oscillator frequency (450kHz) is sufficiently high to ensure small external capacitors and high
output current. Controlled by a comparator, the oscillator becomes active only when the output voltage is
lower than its threshold. This regulation enables the circuit to provide constant output voltages as high as
-2VIN. At the same time, the circuit draws minimum quiescent currents at light loads.
Buck/Boost Combination
Another problem common in battery-powered applications is a battery voltage that ranges above and
below the regulated output voltage. The output voltage of a Li+ cell varies from 3.6V to 1.5V during its
lifetime, before it is recharged. To derive a constant 3.3V from this changing input, a combined
buck/boost converter is required. Initially this device downconverts the full battery voltage (3.6V) to 3.3V.
When the battery voltage drops below 3.3V, the step-up converter function guarantees the regulated
3.3V output voltage.
Though usually complicated, this approach can now be implemented with a simple charge-pump IC like
the MAX1759. Operating from input voltages ranging from 1.6V to 5.5V, the MAX1759 generates an
output either fixed (3.3V) or adjustable (2.5V to 5.5V) and delivers output currents up to 100mA. This IC
comes in a 10-pin µMAX® package and operates with three external capacitors. An additional shutdown
mode disconnects output from input while lowering the quiescent current to 1µA.
Charge-Pump Overview
Tables 1 and 2 list some of the regulated and unregulated charge pumps available from Maxim,
including those with special functions and all those mentioned in the text. These tables enable designers
to choose a suitable charge pump according to the application's required package, functions, and outputcurrent specifications.
Maxim continually introduces new products. We encourage you to browse Maxim's product lines for the
most updated list of charge-pump devices. Capacitor charge pumps are shown under the Power and
Battery Management category.
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Table 1. Unregulated Charge Pumps
Input
Output Output Switching
Part Number
Voltage Voltage Current Frequency
Features
MAX828/MAX829
1.5V to
-VIN
5.5V
25mA
12kHz/35kHz
Inverter; 5-pin SOT23 package
MAX1720
1.5V to
-VIN
5.5V
25mA
12kHz
Inverter; low quiescent current,
shutdown, 6-pin SOT23 package
MAX1719/1721
1.5V to
-VIN
5.5V
25mA
125kHz
Inverter; 6-pin SOT23 package,
shutdown
MAX870/871
1.4V to
-VIN
5.5V
25mA
125kHz/500kHz Inverter; 5-pin SOT23 package
MAX1682/1683
2V to
5.5V
2 × VIN
30mA
12kHz/35kHz
Doubler; 5-pin SOT23 package
ICL7660
MAX1044
1.5V to -VIN,
2 × VIN
10V
10mA
10kHz
Doubler or inverter; DIP, 8-pin SO
MAX860
1.5V to -VIN,
2 × VIN
5.5V
50mA
6kHz; 50kHz;
130kHz
Doubler or inverter; 8-pin SO/
µMAX
MAX861
1.5V to -VIN,
2 × VIN
5.5V
50mA
13kHz;
100kHz;
250kHz
Doubler or inverter; 8-pin SO/
µMAX
MAX1680
2V to
5.5V
-VIN,
2 × VIN
125mA
125kHz;
250kHz
Doubler or inverter; 8-pin SO/
µMAX
MAX1681
3V to
5.5V
-VIN,
2 × VIN
125mA
500kHz; 1MHz
Doubler or inverter; 8-pin SO/
µMAX
Table 2. Regulated Charge Pumps
Part
Input
Output
Output
Number Voltage
Voltage
Current
Switching
Frequency
Features
MAX619
2V to 3.6V +5V
50mA
500kHz
Regulated 5V, 8-pin SO
package
MAX682
2.7V to
5.5V
+5V
250mA
50kHz to 2MHz
Regulated 5V, 8-pin SO/µMAX
packages
MAX868
1.8V to
5.5V
Up to
-2 × VIN
30mA
up to 450kHz
Variable inverted voltage; µMAX
package
MAX1673
2.0V to
5.5V
Up to -VIN
125mA
350kHz
Variable inverted voltage; fixed
frequency
MAX1759
1.6V to
5.5V
2.5V to
5.5V
100mA
1.5MHz
Buck/boost converter
µMAX is a registered trademark of Maxim Integrated Products, Inc.
Related Parts
Page 7 of 8
ICL7660
Switched-Capacitor Voltage Converters
Free Samples MAX1044
Switched-Capacitor Voltage Converters
Free Samples MAX1673
Regulated, 125mA Output, Charge Pump DC-DC Inverter
Free Samples MAX1680
125mA, Frequency-Selectable, Switched-Capacitor
Voltage Converters
Free Samples MAX1681
125mA, Frequency-Selectable, Switched-Capacitor
Voltage Converters
Free Samples MAX1682
Switched-Capacitor Voltage Doublers
Free Samples MAX1719
SOT23, Switched-Capacitor Voltage Inverters with
Shutdown
Free Samples MAX1720
SOT23, Switched-Capacitor Voltage Inverters with
Shutdown
Free Samples MAX1721
SOT23, Switched-Capacitor Voltage Inverters with
Shutdown
Free Samples MAX1759
Buck/Boost Regulating Charge Pump in µMAX
Free Samples MAX619
Regulated 5V Charge Pump DC-DC Converter
Free Samples MAX682
3.3V Input to Regulated 5V Output Charge Pumps
Free Samples MAX828
Switched-Capacitor Voltage Inverters
Free Samples MAX829
Switched-Capacitor Voltage Inverters
Free Samples MAX860
50mA, Frequency-Selectable, Switched-Capacitor
Voltage Converters
Free Samples MAX861
50mA, Frequency-Selectable, Switched-Capacitor
Voltage Converters
Free Samples MAX868
Regulated, Adjustable -2x Inverting Charge Pump
Free Samples MAX870
Switched-Capacitor Voltage Inverters
Free Samples MAX871
Switched-Capacitor Voltage Inverters
Free Samples More Information
For Technical Support: http://www.maximintegrated.com/support
For Samples: http://www.maximintegrated.com/samples
Other Questions and Comments: http://www.maximintegrated.com/contact
Application Note 725: http://www.maximintegrated.com/an725
TUTORIAL 725, AN725, AN 725, APP725, Appnote725, Appnote 725
Copyright © by Maxim Integrated Products
Additional Legal Notices: http://www.maximintegrated.com/legal
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