ADM660/ADM8660

REV. A

–6–

TEMPERATURE –

°C

–40

100

–20

0

20406080

160

0

140

80

60

40

20

120

100

LV = GND

FC = V+

C1, C2 = 2.2µF

Figure 13. Charge-Pump Frequency vs. Temperature

TEMPERATURE –

°C

60

0

–40

100

–20

0

20406080

50

40

30

20

10

V+ = +1.5V

V+ = +3V

V+ = +5V

Figure 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 15 and 16.

C2

CAP+

C1

CAP–

S3

S4

S1

S2

OUT = –V+

V+

Φ1

Φ2

÷ 2

OSCILLATOR

Figure 15. Voltage Inversion Principle

C2

CAP+

C1

CAP–

S3

S4

S1

S2

Φ1

Φ2

V+

V+

÷ 2

OSCILLATOR

Figure 16. Voltage Doubling Principle

Figure 15 shows the voltage inverting configuration, while Figure

16 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 17.

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

age 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 be-

tween q1 and q2, so

∆q = q1–q2 = C1 (V1–V2).

C1

AB

C2

V1

V2

Figure 17. 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 fre-

quencies. It should be remembered that as the switching fre-

quency 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.

C2

V1

V2

Figure 18. Switched Capacitor Equivalent Circuit