TC7660EJA Просмотр технического описания (PDF) - Microchip Technology
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TC7660EJA Datasheet PDF : 20 Pages
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TC7660
4.0 DETAILED DESCRIPTION
4.1 Theory of Operation
The TC7660 charge pump converter inverts the voltage
applied to the V + pin. The conversion consists of a two-
phase operation (Figure 4-1). During the first phase,
switches S2 and S4 are open and switches S1 and S3
are closed. C1 charges to the voltage applied to the V +
pin, with the load current being supplied from C2. Dur-
ing the second phase, switches S2 and S4 are closed
and switches S1 and S3 are open. Charge is trans-
ferred from C1 to C2, with the load current being
supplied from C1.
V+
S1
S2
+
C1
GND S3
+
C2
S4
VOUT = -VIN
FIGURE 4-1:
Inverter.
Ideal Switched Capacitor
In this manner, the TC7660 performs a voltage inver-
sion, but does not provide regulation. The average out-
put voltage will drop in a linear manner with respect to
load current. The equivalent circuit of the charge pump
inverter can be modeled as an ideal voltage source in
series with a resistor, as shown in Figure 4-2.
ROUT
-
V+
+
VOUT
FIGURE 4-2:
Switched Capacitor Inverter
Equivalent Circuit Model.
The value of the series resistor (ROUT) is a function of
the switching frequency, capacitance and equivalent
series resistance (ESR) of C1 and C2 and the on-resis-
tance of switches S1, S2, S3 and S4. A close
approximation for ROUT is given in the following
equation:
EQUATION
ROUT =
--------------1---------------
fPUMP C1
+
8RSW
+
4ESRC1
+
ESRC2
Where:
fPUMP
=
-f-O----S---C-
2
RSW = on-resistance of the switches
ESRC1 = equivalent series resistance of C1
ESRC2 = equivalent series resistance of C2
4.2 Switched Capacitor Inverter
Power Losses
The overall power loss of a switched capacitor inverter
is affected by four factors:
1. Losses from power consumed by the internal
oscillator, switch drive, etc. These losses will
vary with input voltage, temperature and
oscillator frequency.
2. Conduction losses in the non-ideal switches.
3. Losses due to the non-ideal nature of the
external capacitors.
4. Losses that occur during charge transfer from
C1 to C2 when a voltage difference between the
capacitors exists.
Figure 4-3 depicts the non-ideal elements associated
with the switched capacitor inverter power loss.
RSW S1
RSW S2
V+
+
IDD
+
C1
-
ESRC1
+
C2
ESRC2
IOUT LOAD
RSW S3
RSW S4
FIGURE 4-3:
Non-Ideal Switched
Capacitor Inverter.
The power loss is calculated using the following
equation:
EQUATION
PLOSS = IO2 UT ROUT + IDD V+
DS21465C-page 6
2002-2011 Microchip Technology Inc.
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