MAX1682 Maxim, MAX1682 Datasheet - Page 5

• Manufacturer Part Number: MAX1682
• Description: Switched-Capacitor Voltage Doublers
• Manufacturer: Maxim

The MAX1682/MAX1683 capacitive charge pumps
double the voltage applied to their input. Figure 1
shows a simplified functional diagram of an ideal volt-
age doubler. During the first half-cycle, switches S1
and S2 close, and capacitor C1 charges to V
the second half cycle, S1 and S2 open, S3 and S4
close, and C1 is level shifted upward by V
connects C1 to the reservoir capacitor C2, allowing
energy to be delivered to the output as necessary. The
actual voltage is slightly lower than 2 x V
switches S1–S4 have resistance and the load drains
charge from C2.
The MAX1682/MAX1683 have a finite output resistance
of about 20Ω (Table 2). As the load current increases,
the devices’ output voltage (V
equals the current drawn from V
output impedance (R
_____________________Pin Description
Figure 1. Simplified Functional Diagram of Ideal Voltage
Doubler
_______________Detailed Description
PIN
1
2
3
4
5
V
IN
V
V
NAME
GND
OUT
C1+
DROOP
OUT
C1-
IN
= 2 x V
S2
S1
_______________________________________________________________________________________
= I
Ground
Doubled Output Voltage. Connect C2
between OUT and GND.
Negative Terminal of the Flying
Capacitor
Input Supply
Positive Terminal of the Flying
Capacitor
S
OUT
), as follows:
C1
IN
Switched-Capacitor Voltage Doublers
- V
x R
DROOP
Charge-Pump Output
S
S4
S3
OUT
FUNCTION
OUT
) droops. The droop
times the circuit’s
C2
V
IN
IN
IN
volts. This
V
IN
OUT
. During
, since
The power efficiency of a switched-capacitor voltage
converter is affected by three factors: the internal losses
in the converter IC, the resistive losses of the capacitors,
and the conversion losses during charge transfer
between the capacitors. The total power loss is:
The internal losses are associated with the IC’s internal
functions, such as driving the switches, oscillator, etc.
These losses are affected by operating conditions such
as input voltage, temperature, and frequency.
The next two losses are associated with the voltage
converter circuit’s output resistance. Switch losses
occur because of the on-resistance of the MOSFET
switches in the IC. Charge-pump capacitor losses
occur because of their ESR. The relationship between
these losses and the output resistance is as follows:
where f
the effective resistance from an ideal switched-
capacitor circuit (Figures 2a and 2b).
Figure 2a. Switched-Capacitor Model
Figure 2b. Equivalent Circuit
P
R
PUMP CAPACITOR LOSSES
OUT
OSC
P
V+
LOSS
I
OUT
V+
f
OSC
ESR
is the oscillator frequency. The first term is
C1
R
2
EQUIV
1
C
R
f
x R
EQUIV
2
=
x C
P
INTERNAL LOSSES
f
P
P
1
OUT
PUMP CAPACITOR LOSSES
CONVERSION LOSSES
C1
1
Efficiency Considerations
2
R
C2
SWITCHES
C2
P
SWITCH LOSSES
R
L
R
L
V
V
OUT
OUT
4
ESR
C
1
5