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TC1240ECH Datenblatt(PDF) 3 Page - Microchip Technology

Teilenummer TC1240ECH
Bauteilbeschribung  Positive Doubling Charge Pump with Shutdown in SOT Package
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Hersteller  MICROCHIP [Microchip Technology]
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TC1240
Positive Doubling Charge Pump with
Shutdown in SOT Package
TC1240-1 7/7/00 DS21333A
© 2001 Microchip Technology Inc.
DETAILED DESCRIPTION
The TC1240 charge pump converter doubles the volt-
age applied to the VIN 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 VIN 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
VIN 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 VIN since the four
switches (S1 - S4) have an on-resistance and the load drains
charge from reservoir capacitor C2.
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 17
Ω nominal
at +25
°C and VIN = +2.8V. VOUT is approximately +5.6V at
light loads, and droops according to the equation below:
VDROOP = IOUT x ROUT
VOUT = 2 x VIN – VDROOP
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).
Figure 1. Ideal Swiched Capacitor Charge Pump Doubler
(2) I
2R 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) PLOSS (2, 3) = IOUT
2 x ROUT
(b) ≅ I
OUT
2 x
[
1
+8RSWITCH + 4ESRC1 + ESRC2
]
(fPUMP) C1
Equation 1.
The pump frequency in Equation 1(b) is defined as one-
half the oscillator frequency (i.e. fPUMP = fOSC/2). The
1/(fPUMP)(C1) term in Equation 1(b) is the effective output
resistance of an ideal switched capacitor circuit (Figures 2a,
2b). The value of RSWITCH can be approximated at 1.4
Ω 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.
PLOSS(4) = [(0.5)(C1) (4VIN
2– VOUT2 ) + (0.5)(C2)(2VOUT
VRIPPLE – VRIPPLE
2 )] x fOSC
Equation 2.
VRIPPLE =
IOUT +2(IOUT)(ESRC2)
(fOSC)(C2)
Equation 3.
Figure 2a. Ideal Swiched Capacitor Model
V+
V
OUT
R
L
C1
C2
f
Figure 2b. Equivalent Output Resistance
C1
C2
TC1240
S1
S3
S4
S2
OSC
VIN
OUT = 2 x VIN
VIN
V+
V
OUT
R
EQUIV
R
EQUIV =
R
L
C2
f x C1
1


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