SP682 Datasheet, PDF (4/8 Pages) Sipex Corporation – Ultra-Low Power Inverting Voltage Doubler

The ESR of the charge pump capacitors also
determine the output resistance. Assuming that
switch resistances are approximately equal, the
output resistance can be derived as shown be-
low:
R_OUT = 16*(R_SW1-4) + 4*( ESR_C1+ ESR_C2) + ESR_C3
+ 1 / (f_OSC * C1) + 1 / (f_OSC * C2)
R_OUT is typically 140Ω at +25°C with VCC at
+5V using 3.3µF capacitors. The total internal
switch resistance (16*R_SW) is approximately
90Ω. The table below shows the comparison of
R_OUT versus C1&C2.
C1, C2 (µF)
0.05
0.10
0.47
1.00
3.30
4.70
10.00
22.00
Table 1. R_OUT .vs. C1, C2
ROUT (Ω)
4085
2084
510
285
140
125
105
94
The output voltage ripple is also affected by the
capacitors, specifically C_OUT. Larger values
will reduce the output ripple for a given output
current load of current. The formula represen-
tation is:
V_RIPPLE = {1 / [2 * (f_OSC * C3)] + 2 * (ESR_C3)} * I_OUT
To minimize the output ripple, the C_OUT storage
capacitor can be increased to over 10µF whereas
the pump capacitors can range from 1µF to 5µF.
Table 2 shows the typical V_RIPPLE for given
C_OUT values.
THEORY OF OPERATION
SHUTDOWN FEATURE
The SP682 charge pump includes a shutdown
feature (pin 8) which disables the charge pump
when the V_OUT is not needed. A logic "1" will
activate the shutdown mode. If shutdown is not
needed, it can be left open where an internal
pull-down resistor will always keep the charge
pump active. Typical input current for the
shutdown pin is 3µA. The shutdown feature is
another option for conserving power in portable
applications, reducing current to only 1µA.
PARALLEL DEVICES
Multiple SP682 charge pumps can be connected
in parallel. However, the effective output resis-
tance now is the output resistance of a single
device divided by the number of devices. Con-
necting multiple pumps allows the user to save
on the storage capacitor. The charge pump
capacitors still must be separate for each device.
APPLICATIONS INFORMATION
The SP682 charge pump produces a doubled,
inverted voltage from the V_CC input. As such, it
can serve in many applications where a negative
–5V to –10V output is needed. Typical applica-
tions include powering analog switches, and
biasing LCD displays and panels.
C_OUT (µF)
0.50
1.00
3.30
4.70
10.00
22.00
Table 2. C3 .vs. V_RIPPLE
V_RIPPLE (mV)
1020
520
172
120
70
43
Date: 5/20/04
SP682 Micro Power Inverting Charge Pump
4
© Copyright 2004 Sipex Corporation