CHARGE PUMP DC-TO-DC
VOLTAGE CONVERTER
1
2
3
4
5
6
7
8
TC1044S
+
V
1
2
8
7
6
5
1
2
3
4
8
7
6
5
+
TC1044S
3
4
10µF
+
TC1044S
"n"
"1"
10µF
V
*
OUT
10µF
+
*NOTES:
1. VOUT = –n(V+) for 1.5V ≤ V+ ≤ 12V
10µF
+
Figure 5. Increased Output Voltage by Cascading Devices
Cascading Devices
situation where the designer has generated the external
clock frequency using TTL logic, the addition of a 10kΩ pull-
up resistor to V+ supply is required. Note that the pump
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.
The TC1044S may be cascaded as shown (Figure 5) to
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:
It is also possible to increase the conversion efficiency
of the TC1044S at low load levels by lowering the oscillator
frequency.Thisreducestheswitchinglosses,andisachieved
by connecting an additional capacitor, COSC, as shown in
Figure 7. Lowering the oscillator frequency will cause an
undesirableincreaseintheimpedanceofthepump(C1)and
the reservoir (C2) capacitors. To overcome this, increase
the values of C1 and C2 by the same factor that the
frequency has been reduced. For example, the addition of
a 100pF capacitor between pin 7 (OSC) and pin 8 (V+) 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 C1 and C2 (from
10µF to 100µF).
VOUT = –n(VIN)
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 TC1044S
ROUT values.
Changing the TC1044S Oscillator Frequency
Itmaybedesirableinsomeapplications(duetonoiseor
other considerations) to increase the oscillator frequency.
Pin 1, frequency boost pin may be connected to V+ to
increase oscillator frequency to 45kHz from a nominal of
10kHz for an input supply voltage of 5.0 volts. The oscillator
may also be synchronized to an external clock as shown in
Figure6. Inordertopreventpossibledevicelatch-up, a1kΩ
resistor must be used in series with the clock output. In a
Positive Voltage Multiplication
The TC1044S may be employed to achieve positive
voltage multiplication using the circuit shown in Figure 8. In
this application, the pump inverter switches of the TC1044S
are used to charge C1 to a voltage level of V+ – VF (where V+
is the supply voltage and VF is the forward voltage drop of
diode D1). On the transfer cycle, the voltage on C1 plus the
supply voltage (V+) is applied through diode D2 to capacitor
C2. The voltage thus created on C2 becomes (2V+) – (2VF),
or twice the supply voltage minus the combined forward
voltage drops of diodes D1 and D2.
+
+
V
V
1
2
3
4
8
7
6
5
1kΩ
CMOS
GATE
+
TC1044S
10µF
V
OUT
The source impedance of the output (VOUT) will depend
on the output current, but for V+ = 5V and an output current
of 10mA, it will be approximately 60Ω.
10µF
+
Figure 6. External Clocking
TELCOM SEMICONDUCTOR, INC.
4-47
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
