CAT661_12 Datasheet, PDF (8/14 Pages) ON Semiconductor – High Frequency 100 mA CMOS Charge Pump, Inverter/Doubler

CAT661
Oscillator Frequency Control
The switching frequency can be raised, lowered or driven
from an external source. Figure 22 shows a functional
diagram of the oscillator circuit.
The CAT661 oscillator has four control modes:
Table 4.
BOOST/FC
Pin Connection
Open
BOOST/FC = V+
Open or
BOOST/FC = V+
Open
OSC Pin
Connection
Open
Open
External Capacitor
External Clock
Nominal
Oscillator
Frequency
25 kHz
135 kHz
−
Frequency of
external clock
If BOOST/FC and OSC are left floating (Open), the
nominal oscillator frequency is 25 kHz. The pump
frequency is one−half the oscillator frequency.
BOOST/FC
8x
(1)
OSC
(7)
OSC
V⁺
(8)
f
B2
f
By connecting the BOOST/FC pin to V+, the charge and
discharge currents are increased, and the frequency is
increased by approximately 6 times. Increasing the
frequency will decrease the output impedance and ripple
currents. This can be an advantage at high load currents.
Increasing the frequency raises quiescent current but allows
smaller capacitance values for C1 and C2.
If pin 7, OSC, is loaded with an external capacitor the
frequency is lowered. By using the BOOST/FC pin and an
external capacitor at OSC, the operating frequency can be
set.
Note that the frequency appearing at CAP+ or CAP− is
one−half that of the oscillator.
Driving the CAT661 from an external frequency source
can be easily achieved by driving Pin 7 and leaving the
BOOST pin open, as shown in Figure 22. The output current
from Pin 7 is small, typically 1 mA to 8 mA, so a CMOS can
drive the OSC pin. For 5 V applications, a TTL logic gate can
be used if an external 100 kΩ pull−up resistor is used as
shown in Figure 23.
SW1
CAP⁺
(2)
SW2
+
C1
CAP^−
(4)
VOUT
(5)
LV
GND
(6)
CLOSED WHEN
(3)
V⁺ > 3.0 V
Figure 21. CAT661 Block Diagram
C2
+
(N) = Pin Number
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