MAX1683EUKT Datasheet, PDF (5/9 Pages) Maxim Integrated Products – Switched-Capacitor Voltage Doublers

Switched-Capacitor Voltage Doublers
_____________________Pin Description
PIN NAME
FUNCTION
1
GND Ground
2
OUT
Doubled Output Voltage. Connect C2
between OUT and GND.
3
C1-
Negative Terminal of the Flying
Capacitor
4
IN
Input Supply
5
C1+
Positive Terminal of the Flying
Capacitor
_______________Detailed Description
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_IN. During
the second half cycle, S1 and S2 open, S3 and S4
close, and C1 is level shifted upward by V_IN volts. This
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_IN, since
switches S1–S4 have resistance and the load drains
charge from C2.
Charge-Pump Output
The MAX1682/MAX1683 have a finite output resistance
of about 20Ω (Table 2). As the load current increases,
the devices’ output voltage (V_OUT) droops. The droop
equals the current drawn from V_OUT times the circuit’s
output impedance (R_S), as follows:
V_DROOP = I_OUT x R_S
V_OUT = 2 x V_IN - V_DROOP
S1
S3
V_IN
C1
S2
S4
V_OUT
C2
V_IN
Efficiency Considerations
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:
ΣP_LOSS = P_{INTERNAL LOSSES}
+ P_{PUMP CAPACITOR LOSSES}
+ P_{CONVERSION LOSSES}
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:
P_{PUMP CAPACITOR LOSSES} + P_{SWITCH LOSSES} =
I_OUT2 x R_OUT
( ) ^{ROUT â‰}
1
f_OSC x
C1
+
2R_SWITCHES
+
4ESR_C1
+ ESR_C2
where f_OSC is the oscillator frequency. The first term is
the effective resistance from an ideal switched-
capacitor circuit (Figures 2a and 2b).
f
V+
V_OUT
C2
R_L
C1
Figure 2a. Switched-Capacitor Model
R_EQUIV
V+
R_EQUIV =
1
f × C1
C2
V_OUT
R_L
Figure 1. Simplified Functional Diagram of Ideal Voltage
Doubler
Figure 2b. Equivalent Circuit
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