6

ML4818

The ML4818 can also be used in current mode by sensing

load current on the RAMP input (pin 3).

The four output delay timers are programmed via an

value should be no less than 1k

k

Ω the delay, in ns is:

TR

DELAY

DELAY

=×

+

33

45

(1)

The ML4818 contains special logic circuits to provide for

voltage mode feed-forward and lock out long pulses into

the internal logic. This prevents instability from occuring

when the

Φ Comparator trips in voltage mode.

3

RAMP

S

R

Q

Φ MOD

OUTPUT

OSC

Figure 3. Voltage Feed-Forward Circuit.

cycle. When the power cycle terminates, RAMP is pulled

low. In voltage mode operation, a capacitor is connected

provide input voltage feed forward.

OSCILLATOR

reaches the upper threshold (Ramp Peak), the comparator

changes state, turning on the current sink which

To use the CLOCK output for driving external synch-

ronization circuitry, a pull-down resistor is required from

CLOCK to GND.

11

+

–

2

13

5V

CLOCK

OUT

5V

5.5mA

1.7V

250

Ω

Q1

Figure 5. Ocillator Block Diagram

For frequencies of less than 500kHz, oscillator frequency

can be set by using the following formulae:

f

CR

C

OSC

TT

T

=

+

1

0 52

500

.

(2)

ERROR AMPLIFIER

The ML4818 error amplifier is a 2.5MHz bandwidth,

8.5V/

µs slew rate op-amp with provision for limiting the

positive output voltage swing (output inhibit line) to

implement the soft start function. The error amplifier

output source current is limited to 4.5mA.

CLOCK

RAMP PEAK

RAMP VALLEY

Figure 4. Ocillator Timing Diagram