TC1044S Datasheet, PDF (5/8 Pages) TelCom Semiconductor, Inc – CHARGE PUMP DC-TO-DC VOLTAGE CONVERTER

CHARGE PUMP DC-TO-DC
VOLTAGE CONVERTER
1
TC1044S
+
10µF
V+
1
8
2
7
3 ^TC1044S 6
4
"1"
5
* NOTES:
1. V_OUT = –n(V⁺) for 1.5V ≤ V⁺ ≤ 12V
+
10µF
1
8
2
7
3 ^TC1044S 6
4
"n"
5
+ ^10µF
2
VOUT*
+ ^10µF
3
Figure 5. Increased Output Voltage by Cascading Devices
Cascading Devices
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:
V_OUT = –n(V_IN)
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
R_OUT values.
Changing the TC1044S Oscillator Frequency
It may be desirable in some applications (due to noise or
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
Figure 6. In order to prevent possible device latch-up, a 1kΩ
resistor must be used in series with the clock output. In a
+
10µF
1
8
2
7
3 _TC1044S 6
4
5
V+
1kΩ
V+
CMOS
GATE
VOUT
10µF
+
Figure 6. External Clocking
situation where the designer has generated the external
4 ^{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.
It is also possible to increase the conversion efficiency
of the TC1044S at low load levels by lowering the oscillator
frequency. This reduces the switching losses, and is achieved
5 ^{by connecting an additional capacitor, COSC, as shown in}
Figure 7. Lowering the oscillator frequency will cause an
undesirable increase in the impedance of the pump (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
6 ^{frequency of 10kHz (a multiple of 10), and necessitate a}
corresponding increase in the values of C₁ and C₂ (from
10µF to 100µF).
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
7 ^{are used to charge C1 to a voltage level of V+ – VF (where V+}
is the supply voltage and V_F 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⁺) – (2V_F),
or twice the supply voltage minus the combined forward
voltage drops of diodes D₁ and D₂.
The source impedance of the output (V_OUT) will depend
on the output current, but for V⁺ = 5V and an output current
of 10mA, it will be approximately 60Ω.
8
TELCOM SEMICONDUCTOR, INC.
4-47