ML4818 데이터 시트보기 (PDF) - Micro Linear Corporation

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ML4818

Phase modulation /soft switching controller

Micro Linear Corporation 

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

external RDELAY resistor as shown below. This resistor

value should be no less than 1kΩ. Expressing RDELAY in

kΩ the delay, in ns is:

TDELAY = 33 × RDELAY + 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.

Φ MOD

OUTPUT S

Q

OSC

R

RAMP

3

QR

Figure 3. Voltage Feed-Forward Circuit.

The collector of QR in figure 3 is high only during a power

cycle. When the power cycle terminates, RAMP is pulled

low. In voltage mode operation, a capacitor is connected

from RAMP to GND with a resistor from RAMP to VIN to

provide input voltage feed forward.

OSCILLATOR

The ML4818 oscillator charges the external capacitor, CT,

with a current (ISET) equal to 5/RT. When the CT voltage

reaches the upper threshold (Ramp Peak), the comparator

changes state, turning on the current sink which

discharges CT to the lower threshold (Ramp Valley). The

CT pin is clamped to Ramp Valley by Q1 (Figure 5) to

prevent inaccuracy due to undershoot on CT.

To use the CLOCK output for driving external synch-

ronization circuitry, a pull-down resistor is required from

CLOCK to GND.

ISET

11

RT

1.7V

Q1

2

CT

5.5mA

5V

ISET

+

–

5V

CLOCK

OUT

13

250Ω

Figure 5. Ocillator Block Diagram

For frequencies of less than 500kHz, oscillator frequency

can be set by using the following formulae:

fOSC

=

1

0.52CT RT

+

500 CT

(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

CT

RAMP VALLEY

TC

TD

Figure 4. Ocillator Timing Diagram

6

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