CAT661

8

Doc. No. MD-5003, Rev. K

Figure 5. CAT661 Block Diagram

+

C2

V+

(8)

2

OSC

BOOST/FC

8x

(1)

OSC

(7)

LV

(6)

CLOSED WHEN

V+ > 3.0V

GND

(3)

CAP-

(4)

C1

+

CAP+

(2)

SW2

SW1

Ο

Ο

VOUT

(5)

(N) = Pin Number

OSCILLATOR FREQUENCY CONTROL

The switching frequency can be raised, lowered or driven from an external source. Figure 6 shows a functional diagram

of the oscillator circuit.

The CAT661 oscillator has four control modes:

BOOST/FC Pin Connection

OSC Pin Connection

Nominal Oscillator Frequency

Open

Open

25kHz

BOOST/FC= V+

Open

135kHz

Open or BOOST/FC= V+

External Capacitor

—

Open

External Clock

Frequency of external clock

If BOOST/FC and OSC are left floating (Open), the

nominal oscillator frequency is 25kHz. The pump

frequency is one-half the oscillator frequency.

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 6. The output

current from Pin 7 is small, typically 1

µA to 8µA, so a

CMOS can drive the OSC pin. For 5V applications, a TTL

logic gate can be used if an external 100k

resistor is used as shown in figure 7.