1524J データシートの表示（PDF） - Microsemi Corporation

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1524J

REGULATING PULSE WIDTH MODULATOR

Microsemi Corporation 

SG1524/SG2524/SG3524

APPLICATION NOTES

OSCILLATOR

The oscillator in the SG1524 uses an external resistor RT to

establish a constant charging current into an external capacitor

CT. While this uses more current than a series-connected RC, it

provides a linear ramp voltage at CT which is used as a time-

dependent reference for the PWM comparator. The charging

current is equal to 3.6V/RT, and should be restricted to between

30µA and 2mA. The equivalent range for RT is 1.8K to 100K.

The range of values for CT also has limits, as the discharge time

of CT determines the pulse width of the oscillator output pulse.

The pulse is used (among other things) as a blanking pulse to

both outputs to insure that there is no possibility of having both

outputs on simultaneously during transitions. This output

deadtime relationship is shown in Figure 1. A pulse width below

0.35 microseconds may cause failure of the internal flip-flop to

toggle. This restricts the minimum value of CT to 1000pF. (Note:

Although the oscillator output is a convenient oscilloscope sync

input, the probe capacitance will increase the pulse width and

decrease the oscillator frequency slightly.) Obviously, the upper

limit to the pulse width is determined by the modulation range

required in the power supply at the chosen switching frequency.

Practical values of CT fall between 1000pF and 0.1µF, although

successful 120 Hz oscillators have been implemented with val-

ues up to 5µF and a series surge limit resistor of 100 ohms.

The oscillator frequency is approximately 1/RT•CT; where R is in

ohms, C is in microfarads, and the frequency is in Megahertz. For

greater accuracy, the chart in Figure 2 may be used for a wide

range of operating frequencies.

Note that for buck regulator topologies, the two outputs can be

wire-ORed for an effective 0-90% duty cycle range. With this

connection, the output frequency is the same as the oscillator

frequency. For push-pull applications, the outputs are used

separately; the flip-flop limits the duty cycle range at each output

to 0-45%, and the effective switching frequency at the trans-

former is 1/2 the oscillator frequency.

If it is desired to synchronize the SG1524 to an external clock, a

positive pulse may be applied to the clock pin. The oscillator

should be programmed with RT and CT values that cause it to free-

run at 90% of the external sync frequency. A sync pulse with a

maximum logic 0 of +0.3 volts and a minimum logic 1 of +2.4 volts

applied to Pin 3 will lock the oscillator to the external source. The

minimum sync pulsewidth should be 200 nanoseconds, and the

maximum is determined by the required deadtime. The clock pin

should never be driven more negative than -0.3 volts, nor more

positive than +5.0 volts. The nominal resistance to ground is

3.2K at the clock pin, ±25% over temperature.

If two or more SG1524s must be synchronized together, program

one master unit with RT and CT for the desired frequency. Leave

the RT pins on the slaves open, connect the CT pins to the CT of

the master, and connect the clock pins to the clock pin of the

master. Since CT is a high-impedance node, this sync technique

works best when all devices are close together.

FIGURE 1 - OUTPUT STAGE DEADTIME VS. CT

FIGURE 2 - OSCILLATOR FREQUENCY VS. RT AND CT

4/90 Rev 1.1 2/94

Copyright © 1994

LINFINITY Microelectronics Inc.

4

11861 Western Avenue ∞ Garden Grove, CA 92841

(714) 898-8121 ∞ FAX: (714) 893-2570

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