TC660 TELCOM [TelCom Semiconductor, Inc], TC660 Datasheet - Page 7

• Manufacturer Part Number: TC660
• Description: 100mA CHARGE PUMP DC-TO-DC VOLTAGE CONVERTER
• Manufacturer: TELCOM [TelCom Semiconductor, Inc]

Cascading Devices
produce 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 TC660 R
values.
Changing the TC660 Oscillator Frequency
other considerations) to increase the oscillator frequency.
Pin 1, the FC pin, may be connected to V
oscillator frequency to 90kHz from a nominal of 10 kHz for
an input supply voltage of 5.0 volts. The oscillator may also
be synchronized to an external clock as shown in Figure 6
and LV must be grounded when overdriving OSC. In a
situation where the designer has generated the external
100mA CHARGE PUMP DC-TO-DC
VOLTAGE CONVERTER
150 F
The TC660 may be cascaded as shown (Figure 5) to
It may be desirable in some applications (due to noise or
TELCOM SEMICONDUCTOR, INC.
+
*
150 F
NOTE:
1
2
3
4
. V OUT = –n(V + ) for 1.5V
Figure 6. External Clocking
+
TC660
V
OUT
1
2
3
4
= –n (V
8
7
6
5
TC660
"1"
IN
V +
Figure 5. Increased Output Voltage by Cascading Devices
)
OSC
8
7
6
5
V +
V
+
+
+
5.5V
to increase
150 F
V +
CMOS
GATE
V OUT
150 F
OUT
+
150 F
+
clock frequency using TTL logic, the addition of a 10k pull-
up resistor to V
frequency 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 TC660 at low load levels by lowering the oscillator
frequency. This reduces the switching losses, and is achieved
by connecting an additional capacitor, C
Figure 7. Lowering the oscillator frequency will cause an
undesirable increase in the impedance of the pump (C
the reservoir (C
values of C
has been reduced. For example, the addition of a 100pF
capacitor between pin 7 (OSC) and GND will lower the
oscillator frequency to 1kHz from its nominal frequency of
10kHz (a multiple of 10), and necessitate a corresponding
increase in the values of C
Positive Voltage Doubler
C 1
It is also possible to increase the conversion efficiency
+
1
2
3
4
1
Figure 7. Lowering Oscillator Frequency
and C
1
2
3
4
TC660
2
"n"
) capacitors. To overcome this, increase the
+
supply is required. Note that the pump
TC660
2
by the same factor that the frequency
8
7
6
5
1
and C
8
7
6
5
2
.
V +
+
C OSC
OSC
150 F
+
, as shown in
V OUT
C 2
TC660
V OUT
*
1
) and
4-11
1
2
3
4
5
6
7
8