ADM660 Analog Devices, ADM660 Datasheet - Page 7

• Manufacturer Part Number: ADM660
• Description: CMOS Switched-Capacitor Voltage Converter
• Manufacturer: Analog Devices

Specifications of ADM660

• Input Voltage (v): 1.5 - 7
• Vout Preset (v): -Vin or 2*Vin
• Iout: 100mA
• Vout #2 (v): n/a
• Iout #2 (µa): n/a
• Vout #3 (v): n/a
• Iout #3 (µa): n/a
• Power Efficiency (%): 80%
• Supply Current: 4.5mA
• Shutdown Current, Max: 5µA
• Package: PDIP-8,SOIC-8,TSSOP-16

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.
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
negative terminal of C1 connects to V
is voltage inversion at V
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.
REV.
TPC 13. Charge-Pump Frequency vs. Temperature
C
V+
160
140
120
100
80
60
40
20
0
V+
–40
Figure 1. Voltage Inversion Principle
Figure 2. Voltage Doubling Principle
S1
S2
S1
S2
Φ1
–20
Φ1
OSCILLATOR
CAP+
CAP–
LV = GND
FC = V+
C1, C2 = 2.2 F
OSCILLATOR
+ 2
0
OUT
+
CAP+
CAP–
C1
+ 2
+
TEMPERATURE – C
C1
wrt GND. Charge on C1 is trans-
S3
S4
20
Φ2
S3
S4
Φ2
40
V+
OUT
+
C2
via S4. The net result
60
+
C2
OUT = –V+
V
80
OUT
= 2V+
100
–7–
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).
As the switch is toggled between A and B at a frequency f, the
charge transfer per unit time or current is:
Therefore,
where 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.
Figure 4. Switched Capacitor Equivalent Circuit
TPC 14. Output Resistance vs. Temperature
Figure 3. Switched Capacitor Building Block
EQ
60
50
40
30
20
10
0
–40
I = (V1 – V 2)/(1 / fC1) = (V1 – V 2)/(R
= 1/fC1
–20
V1
V1
I = f (∆q) = f (C1)(V1 – V 2)
R
EQ
= 1/fC1
A
0
R
EQ
TEMPERATURE – C
C1
B
20
ADM660/ADM8660
V+ = +1.5V
C2
V+ = +3V
V+ = +5V
C2
40
R
R
L
L
60
V2
V2
80
EQ
)
100