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TC1219 Datasheet(PDF) 5 Page - Microchip Technology

Part No. TC1219
Description  Switched Capacitor Voltage Converters with Shutdown in SOT Packages
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Maker  MICROCHIP [Microchip Technology]
Homepage  http://www.microchip.com
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TC1219 Datasheet(HTML) 5 Page - Microchip Technology

 
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 2002 Microchip Technology Inc.
DS21366B-page 5
TC1219/TC1220
4.0
APPLICATIONS INFORMATION
4.1
Output Voltage Considerations
The TC1219/TC1220 perform voltage conversion but
do 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 approxi-
mately 25
Ω nominal at +25°C and VIN = +5V. VOUT is
approximately -5V at light loads, and droops according
to the equation below:
VDROP = IOUT x ROUT
VOUT = – (VIN – VDROP)
4.2
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.
I2R 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
the
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 4-1(b).
EQUATION 4-1:
The 1/(fOSC)(C1) term in Equation 4-1(b) is the
effective output resistance of an ideal switched
capacitor circuit (Figure 4-1 and Figure 4-2). The value
of RSWITCH can be approximated at 0.5Ω for the
TC1219/TC1220.
The remaining losses in the circuit are due to factor (4)
above, and are shown in Equation 4-2. The output
voltage ripple is given by Equation 4-3.
EQUATION 4-2:
EQUATION 4-3:
FIGURE 4-1:
IDEAL SWITCHED
CAPACITOR MODEL
FIGURE 4-2:
EQUIVALENT OUTPUT
RESISTANCE
a) PLOSS (2, 3) = IOUT2 x ROUT
b) where ROUT =
[ 1 / [fOSC(C1) ] + 8RSWITCH +
4ESRC1 + ESRC2
]
PLOSS(4) =
[(0.5)(C1)(VIN2 – VOUT2)+(0.5)
(C2)(VRIPPLE2 – 2VOUT VRIPPLE)
] xfOSC
VRIPPLE =[ IOUT /2 x ( fOSC)(C2)] + 2 ( IOUT) (ESRC2)
V+
VOUT
RL
C2
C1
f
V+
VOUT
REQUIV
REQUIV =
RL
C2
f x C1
1


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