7

LTC660

Figure 5. Oscillator

OSC

(7)

LTC660 • F05

LV

(6)

BOOST

(1)

∼18pF

I

I

7.0I

7.0I

SCHMITT

TRIGGER

Figure 6. External Clocking

Capacitor Selection

While the exact values of C1 and C2 are noncritical, good

quality, low ESR capacitors are necessary to minimize

voltage losses at high currents. For C1 the effect of the ESR

of the capacitor will be multiplied by four, due to the fact

the switch currents are approximately two times higher

than the output current and losses will occur on both the

charge and discharge cycle. This means using a capacitor

with 1

Ω of ESR for C1 will have the same effect as

increasing the output impedance of the LTC660 by 4

Ω.

This represents a significant increase in the voltage losses.

For C2 the effect of ESR is less dramatic. A C2 with 1

Ω of

ESR will increase the output impedance by 1

Ω. The size

of C2 and the load current will determine the output

voltage ripple. It is alternately charged and discharged at

a current approximately equal to the output current. This

will cause a step function to occur in the output voltage at

the switch transitions. For example, for a switching fre-

quency of 5kHz (one-half the nominal 10kHz oscillator

frequency) and C2 = 150

µF with an ESR of 0.2Ω, ripple is

approximately 90mV with a 100mA load current.

discharge current is increased and, hence, the frequency

is increased by approximately four and a half times.

Increasing the frequency will decrease output impedance

and ripple for high load currents.

Loading Pin 7 with more capacitance will lower the fre-

quency. Using the BOOST (Pin 1) in conjunction with

external capacitance on Pin 7 allows user selection of the

frequency over a wide range.

Driving the LTC660 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 1.1

µA to 8µA, so a logic gate is

capable of driving this current. (A CMOS logic gate can be

used to drive the OSC pin.) For 5V applications, a TTL logic

gate can be used by simply adding an external pull-up

resistor (see Figure 6).

8

7

6

5

4

3

2

1

C1

C2

100k

REQUIRED FOR TTL LOGIC

LTC660 • F06

NC

OSC INPUT

LTC660

+