This tutorial is a reproduction
of the MAXIM tutorial http://www.maxim-ic.com/appnotes.cfm/appnote_number/725/
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, and in other cases
the declining voltage of a discharging battery is not 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. Although many conventional charge-pump devices are on
the market, this article focuses on new integrated products that have
become available recently.
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 can't change their voltage level abruptly. A
changing capacitor voltage always follows the exponential function,
which imposes limitations that inductive voltage converters can avoid.
On the other hand, inductive voltage converters are more expensive.
Capacitive voltage conversion is achieved by switching
a capacitor periodically. Passive diodes can perform this switching
function in the simplest cases, provided 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 could be called the prototype of the classic charge pump.
It integrates switches and the oscillator so that the switches S1, S3
and S2, S4 work in alternation (Figure 1). The configuration shown inverts
the input voltage. With a slight change in the external connections,
it can double or divide the input voltage as well.
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,
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 on-resistance.
A detailed calculation of the resulting power dissipation will be shown
later.
Meanwhile, new pin-compatible circuits (MAX660, MAX860,
MAX861, MAX1680, and MAX1681) feature higher switching frequencies and
lower on-resistance in the switches. Because their switching frequencies
are higher, these new charge pumps operate with smaller capacitors and
deliver higher output current. All can be configured as a voltage inverter,
doubler, or divider.
The MAX828, MAX829, MAX870, and MAX871, which were designed
for inverter applications, reduce the required board area with a smaller
package (SOT-23) and smaller external capacitors. New, pin-compatible
versions of these devices (MAX1719-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.