Datasheet

 2001-2012 Microchip Technology Inc. DS21516D-page 7
TC1240/TC1240A
4.0 DETAILED DESCRIPTION
The TC1240/TC1240A charge pump converter dou-
bles the voltage applied to the V
IN
pin. Conversion con-
sists of a two-phase operation (Figure 4-1). During the
first phase, switches S
2
and S
4
are open and S
1
and S
3
are closed. During this time, C
1
charges to the voltage
on V
IN
and load current is supplied from C
2
. During the
second phase, S
2
and S
4
are closed, while S
1
and S
3
are open.
During this second phase, C
1
is level-shifted upward by
V
IN
volts. This connects C
1
to the reservoir capacitor
C
2
, allowing energy to be delivered to the output as
needed. The actual voltage is slightly lower than 2 x V
IN
since the four switches (S
1
-S
4
) have an on-resistance
and the load drains charge from reservoir capacitor C
2
.
FIGURE 4-1: Ideal Switched Capacitor
Charge Pump Doubler.
5.0 TYPICAL APPLICATIONS
5.1 Output Voltage Considerations
The TC1240/TC1240A performs voltage doubling but
does not provide regulation. The output voltage will
droop in a linear manner with respect to load current.
The value of this equivalent output resistance is approx-
imately 12 nominal at +25°C and V
IN
= +5.0V for the
TC1240A and 17 nominal at +25°C and V
IN
= +2.8V
for the TC1240. V
OUT
is approximately +10.0V at light
loads for the TC1240A and +5.6V for the TC1240, and
droops according to the equation below:
EQUATION
5.2 Charge Pump Efficiency
The overall power efficiency of the charge pump is
affected by four factors:
1. Losses from power consumed by the internal
oscillator, switch drive, etc. (which vary with
input voltage, temperature and oscillator
frequency).
2. I
2
R losses due to the on-resistance of the
MOSFET switches on-board the charge pump.
3. Charge pump capacitor losses due to effective
series resistance (ESR).
4. Losses that occur during charge transfer (from
commutation capacitor to the output capacitor)
when a voltage difference between the two
capacitors exist.
Most of the conversion losses are due to factors (2) and
(3) above. These losses are given by Equation 5-1.
EQUATION 5-1:
V
OUT
= 2 x V
IN
C1
C
2
TC1240/TC1240A
V
IN
S
1
S
3
S
4
S
2
OSC
V
IN
V
OUT
2V
IN
V
DROOP
–=
V
DROOP
I
OUT
R
OUT
=
a)
b)
P
LOSS(2,3)
I
OUT
2
R
OUT
=
R
OUT
1
F
SW
C
1

---------------------- 8R
SWITCH
4ESR
C1
ESR
C2
+++=