REV.

ADM660/ADM8660

–7–

TEMPERATURE – C

–40

100

–20

0

20

406080

160

0

140

80

60

40

20

120

100

LV = GND

FC = V+

C1, C2 = 2.2 F

TPC 13. Charge-Pump Frequency vs. Temperature

TEMPERATURE – C

60

0

–40

100

–20

0

20

406080

50

40

30

20

10

V+ = +1.5V

V+ = +3V

V+ = +5V

TPC 14. Output Resistance vs. Temperature

GENERAL INFORMATION

The ADM660/ADM8660 is a switched capacitor voltage con-

verter that can be used to invert the input supply voltage. The

ADM660 can also be used in a voltage doubling mode. The

voltage conversion task is achieved using a switched capacitor

technique using two external charge storage capacitors. An on-

board oscillator and switching network transfers charge between

the charge storage capacitors. The basic principle behind the

voltage conversion scheme is illustrated in Figures 1 and 2.

+

+

V+

S1

S2

S3

S4

CAP+

CAP–

C1

C2

OUT = –V+

Φ1

Φ2

+ 2

OSCILLATOR

Figure 1. Voltage Inversion Principle

+

+

V+

S1

S2

S3

S4

CAP+

CAP–

C1

C2

Φ1

Φ2

+ 2

OSCILLATOR

V+

Figure 2. Voltage Doubling Principle

Figure 1 shows the voltage inverting configuration, while Figure 2

shows the configuration for voltage doubling. An oscillator

generating antiphase signals

φ1 and φ2 controls switches S1, S2,

and S3, S4. During

φ1, switches S1 and S2 are closed charging

C1 up to the voltage at V+. During

φ2, S1 and S2 open and S3

and S4 close. With the voltage inverter configuration during

φ2,

the positive terminal of C1 is connected to GND via S3 and the

ferred to C2 during

φ2. Capacitor C2 maintains this voltage

during

φ1. The charge transfer efficiency depends on the on-

resistance of the switches, the frequency at which they are being

switched, and also on the equivalent series resistance (ESR) of

the external capacitors. The reason for this is explained in the

following section. For maximum efficiency, capacitors with low

ESR are, therefore, recommended.

The voltage doubling configuration reverses some of the con-

nections, but the same principle applies.

Switched Capacitor Theory of Operation

As already described, the charge pump on the ADM660/ADM8660

uses a switched capacitor technique in order to invert or double

the input supply voltage. Basic switched capacitor theory is

discussed below.

A switched capacitor building block is illustrated in Figure 3.

With the switch in position A, capacitor C1 will charge to voltage

V1. The total charge stored on C1 is q1 = C1V1. The switch is

then flipped to position B discharging C1 to voltage V2. The

charge remaining on C1 is q2 = C1V2. The charge transferred

to the output V2 is, therefore, the difference between q1 and

q2, so

∆q = q1–q2 = C1 (V1–V2).

V1

AB

C1

C2

V2

Figure 3. Switched Capacitor Building Block

As the switch is toggled between A and B at a frequency f, the

charge transfer per unit time or current is:

I

= f (∆q) = f (C1)(V1– V 2)

Therefore,

I

= (V1– V 2)/(1 / fC1) = (V1– V 2)/(R

The switched capacitor may, therefore, be replaced by an equivalent

resistance whose value is dependent on both the capacitor size

and the switching frequency. This explains why lower capacitor

values may be used with higher switching frequencies. It should

be remembered that as the switching frequency is increased the

power consumption will increase due to some charge being lost

at each switching cycle. As a result, at high frequencies, the power

efficiency starts decreasing. Other losses include the resistance

of the internal switches and the equivalent series resistance (ESR)

of the charge storage capacitors.

V1

V2

C2

Figure 4. Switched Capacitor Equivalent Circuit

C