Positive Doubling Charge Pump with
Shutdown in SOT Package
TC1240
DETAILED DESCRIPTION
The TC1240 charge pump converter doubles the volt-
age applied to the V
IN
pin. Conversion consists of a two-
phase operation (Figure 1). During the first phase, switches
S2 and S4 are open and S1 and S3 are closed. During this
time, C1 charges to the voltage on V
IN
and load current is
supplied from C2. During the second phase, S2 and S4 are
closed, and S1 and S3 are open.
During this second phase, 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 needed. The
actual voltage is slightly lower than 2 x V
IN
since the four
switches (S1 - S4) have an on-resistance and the load drains
charge from reservoir capacitor C2.
S2
(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 exists.
Most of the conversion losses are due to factors (2) and
(3) above. These losses are given by Equation 1(b).
(a) P
LOSS (2, 3)
= I
OUT 2
x R
OUT
(b)
≅
I
OUT2
x
[
(f
1
PUMP
)
+8R
SWITCH
+ 4ESR
C1
+ ESR
C2
C1
Equation 1.
]
S1
V
IN
C1
TC1240
OUT = 2 x V
IN
C2
S3
S4
The pump frequency in Equation 1(b) is defined as one-
half the oscillator frequency (i.e. f
PUMP
= f
OSC
/2). The
1/(f
PUMP
)(C1) term in Equation 1(b) is the effective output
resistance of an ideal switched capacitor circuit (Figures 2a,
2b). The value of R
SWITCH
can be approximated at 1.4Ω for
the TC1240.
The remaining losses in the circuit are due to factor (4)
above, and are shown in Equation 2. The output voltage
ripple is given by Equation 3.
P
LOSS(4)
= [(0.5)(C1) (4V
IN2
– V
OUT2
) + (0.5)(C2)(2V
OUT
V
RIPPLE
– V
RIPPLE2
)] x f
OSC
Equation 2.
V
IN
OSC
Figure 1. Ideal Swiched Capacitor Charge Pump Doubler
APPLICATIONS INFORMATION
Output Voltage Considerations
TheTC1240 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 approximately 17Ω nominal
at +25°C and V
IN
= +2.8V. V
OUT
is approximately +5.6V at
light loads, and droops according to the equation below:
V
DROOP
= I
OUT
x R
OUT
V
OUT
= 2 x V
IN
– V
DROOP
V
+
V
RIPPLE
= I
OUT
+2(I
OUT
)(ESR
C2
)
(f
OSC
)(C2)
Equation 3.
f
V
OUT
R
L
C1
C2
Figure 2a. Ideal Swiched Capacitor Model
R
EQUIV
V
+
R
EQUIV =
1
f x C1
Charge Pump Efficiency
The overall power efficiency of the charge pump is
affected by four factors:
(1) Losses from power consumed by the internal oscil-
lator, switch drive, etc. (which vary with input voltage,
temperature and oscillator frequency).
TC1240-1 7/7/00
V
OUT
C2
R
L
Figure 2b. Equivalent Output Resistance
3
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
