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TC7660EOA 数据表(PDF) 8 Page - Microchip Technology

部件名 TC7660EOA
功能描述  Charge Pump DC-to-DC Voltage Converter
Download  20 Pages
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制造商  MICROCHIP [Microchip Technology]
网页  http://www.microchip.com
标志 MICROCHIP - Microchip Technology

TC7660EOA 数据表(HTML) 8 Page - Microchip Technology

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TC7660
DS21465C-page 8
 2002-2011 Microchip Technology Inc.
5.4
Changing the TC7660 Oscillator
Frequency
The operating frequency of the TC7660 can be
changed in order to optimize the system performance.
The frequency can be increased by over-driving the
OSC input (Figure 5-4). Any CMOS logic gate can be
utilized in conjunction with a 1 k
 series resistor. The
resistor is required to prevent device latch-up. While
TTL level signals can be utilized, an additional 10 k
pull-up resistor to V+ is required. Transitions occur on
the rising edge of the clock input. The resultant output
voltage ripple frequency is one half the clock input.
Higher clock frequencies allow for the use of smaller
pump and reservoir capacitors for a given output volt-
age ripple and droop. Additionally, this allows the
TC7660 to be synchronized to an external clock,
eliminating undesirable beat frequencies.
At light loads, lowering the oscillator frequency can
increase the efficiency of the TC7660 (Figure 5-5). By
lowering the oscillator frequency, the switching losses
are reduced. Refer to Figure 2-3 to determine the typi-
cal operating frequency based on the value of the
external capacitor. At lower operating frequencies, it
may be necessary to increase the values of the pump
and reservoir capacitors in order to maintain the
desired output voltage ripple and output impedance.
FIGURE 5-4:
External Clocking.
FIGURE 5-5:
Lowering Oscillator
Frequency.
5.5
Positive Voltage Multiplication
Positive voltage multiplication can be obtained by
employing two external diodes (Figure 5-6). Refer to
the theory of operation of the TC7660 (Section 4.1
“Theory of Operation”). During the half cycle when
switch S2 is closed, capacitor C1 of Figure 5-6 is
charged up to a voltage of V+ - VF1, where VF1 is the
forward voltage drop of diode D1. During the next half
cycle, switch S1 is closed, shifting the reference of
capacitor C1 from GND to V+. The energy in capacitor
C1 is transferred to capacitor C2 through diode D2, pro-
ducing an output voltage of approximately:
EQUATION
FIGURE 5-6:
Positive Voltage Multiplier.
5.6
Combined Negative Voltage
Conversion and Positive Supply
Multiplication
Simultaneous voltage inversion and positive voltage
multiplication can be obtained (Figure 5-7). Capacitors
C1 and C3 perform the voltage inversion, while capaci-
tors C2 and C4, plus the two diodes, perform the posi-
tive voltage multiplication. Capacitors C1 and C2 are
the pump capacitors, while capacitors C3 and C4 are
the reservoir capacitors for their respective functions.
Both functions utilize the same switches of the TC7660.
As a result, if either output is loaded, both outputs will
drop towards GND.
CMOS
GATE
1k
VOUT
“1”
+
V+
+
1
2
3
4
8
7
6
5
TC7660
10 µF
10 µF
V+
VOUT
+
+
1
2
3
4
8
7
6
5
TC7660
C1
C2
V+
COSC
VOUT
2 V
+
VF1 VF2
+

=
where:
VF1 is the forward voltage drop of diode D1
and
VF2 is the forward voltage drop of diode D2.
+
C2
D1
D2
+
C1
VOUT =
1
2
3
4
8
7
6
5
TC7660
V+
(2 V+) - (2 VF)


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