CAT660

http://onsemi.com

8

Capacitor Selection

Low ESR capacitors are necessary to minimize voltage

losses, especially at high load currents. The exact values of

C1 and C2 are not critical but low ESR capacitors are

necessary.

The ESR of capacitor C1, the pump capacitor, can have a

pronounced effect on the output. C1 currents are

approximately twice the output current and losses occur on

both the charge and discharge cycle. The ESR effects are

thus multiplied by four. A 0.5

Ω ESR for C1 will have the

same effect as a 2

Ω increase in CAT660 output impedance.

Output voltage ripple is determined by the value of C2 and

the load current. C2 is charged and discharged at a current

roughly equal to the load current. The internal switching

frequency is one−half the oscillator frequency.

VRIPPLE + IOUT (FOSC

C2) ) IOUT

ESRC2

For example, with a 10 kHz oscillator frequency (5 kHz

switching frequency), a 150

mF C2 capacitor with an ESR of

0.2

Ω and a 100 mA load peak−to−peak ripple voltage is

87 mV.

Table 5. VRIPPLE vs. FOSC

VRIPPLE (mV)

IOUT (mA)

FOSC (kHz)

C2 (mF)

C2 ESR (W)

87

100

10

150

0.2

28

100

80

150

0.2

Figure 16. Oscillator

Figure 17. External Clocking

+

C2

REQUIRED FOR TTL LOGIC

CAT660

100 k

OSC

NC

+

C1

BOOST/FC

V+

OSC

LV

OUT

BOOST/FC

(1)

OSC

(7)

~18 pF

I

7.0 I

I

7.0 I

LV

(6)

INPUT

CAP+

GND

CAP−

1

2

3

4

8

7

6

5