tc7662b Microchip Technology Inc., tc7662b Datasheet - Page 5

• Manufacturer Part Number: tc7662b
• Description: Charge Pump Dc-to-dc Voltage Converter
• Manufacturer: Microchip Technology Inc.

© 2001 Microchip Technology Inc.
Paralleling Devices
paralleled to reduce output resistance (Figure 5). The reser-
voir capacitor, C
requires its own pump capacitor, C
resistance would be approximately:
C 1
Cascading Devices
larger negative multiplication of the initial supply voltage.
However, due to the finite efficiency of each device, the
practical limit is 10 devices for light loads. The output voltage
is defined by:
where n is an integer representing the number of devices
cascaded. The resulting output resistance would be ap-
proximately the weighted sum of the individual TC7662B
R
CHARGE PUMP DC-TO-DC
VOLTAGE CONVERTER
10 F
OUT
Figure 6. Cascading Devices for Increased Output Voltage
Any number of TC7662B voltage converters may be
The TC7662B may be cascaded as shown to produce
*V OUT = –nV +
values.
+
1
2
3
4
1
2
3
4
TC7662B
R
"1"
TC7662B
OUT
"1"
Figure 5. Paralleling Devices
2
=
, serves all devices, while each device
8
7
6
5
8
7
6
5
n (number of devices)
V
R
V
DS21469A
V +
OUT
+
OUT
10 F
C 1
= – n(V
(of TC7662B)
10 F
+
1
2
3
4
IN
1
1
2
3
4
)
. The resultant output
TC7662B
TC7662B
"n"
"n"
8
7
6
5
8
7
6
5
+
+
10 F
C 2
V OUT
R L
5
Changing the TC7662B Oscillator Frequency
other considerations) to increase the oscillator frequency.
This is achieved by one of several methods described
below:
charge and discharge current is increased and, hence the
oscillator frequency is increased by approximately 3-1/2
times. The result is a decrease in the output impedance and
ripple. This is of major importance for surface mount appli-
cations where capacitor size and cost are critical. Smaller
capacitors, e.g., 0.1 F, can be used in conjunction with the
Boost Pin in order to achieve similar output currents com-
pared to the device free running with C
(Refer to graph of Output Source Resistance as a Function
of Oscillator Frequency).
by overdriving the oscillator from an external clock as shown
in Figure 7. In order to prevent device latchup, a 1k resistor
must be used in series with the clock output. In a situation
where the designer has generated the external clock fre-
quency using TTL logic, the addition of a 10k
resistor to V
quency with external clocking, as with internal clocking, will
be 1/2 of the clock frequency. Output transitions occur on the
positive-going edge of the clock.
of the TC7662B at low load levels by lowering the oscillator
frequency. This reduces the switching losses, and is shown
in Figure 8. However, lowering the oscillator frequency will
cause an undesirable increase in the impedance of the
pump (C
increasing the values of C
the frequency has been reduced. For example, the addition
of a 100pF capacitor between pin 7 (Osc) and V
the oscillator frequency to 1kHz from its nominal frequency
of 10kHz (multiple of 10), and thereby necessitate a corre-
sponding increase in the value of C
100 F).
10 F
It may be desirable in some applications (due to noise or
By connecting the BOOSTPin (Pin 1) to V
Increasing the oscillator frequency can also be achieved
It is also possible to increase the conversion efficiency
+
1
) and reservoir (C
+
1
2
3
4
supply is required. Note that the pump fre-
TC7662B
Figure 7. External Clocking
1
2
) capacitors; this is overcome by
and C
8
7
6
5
V +
2
1 k
by the same factor that
1
and C
1
= C
2
+
2
+
= 1 F or 10 F.
TC7662B
(from 10 F to
, the oscillator
10 F
TC7662B-8 9/11/96
V +
+
will lower
CMOS
GATE
V OUT
pullup