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SG2524D Datasheet(PDF) 4 Page - Microsemi Corporation |
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SG2524D Datasheet(HTML) 4 Page - Microsemi Corporation |
4 / 6 page Rev 1.1a Copyright © 1994 11861 Western Avenue ∞ ∞ ∞ ∞ ∞ Garden Grove, CA 92841 4 (714) 898-8121 ∞ ∞ ∞ ∞ ∞ FAX: (714) 893-2570 FIGURE 1 - OUTPUT STAGE DEADTIME VS. C T FIGURE 2 - OSCILLATOR FREQUENCY VS. R T AND CT APPLICATION NOTES OSCILLATOR The oscillator in the SG1524 uses an external resistor R T to establish a constant charging current into an external capacitor C T. While this uses more current than a series-connected RC, it provides a linear ramp voltage at C T which is used as a time- dependent reference for the PWM comparator. The charging current is equal to 3.6V/R T, and should be restricted to between 30 µA and 2mA. The equivalent range for R T is 1.8K to 100K. The range of values for C T also has limits, as the discharge time of C T 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 C T 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 C T fall between 1000pF and 0.1µF, although successful 120 Hz oscillators have been implemented with val- ues up to 5 µF and a series surge limit resistor of 100 ohms. The oscillator frequency is approximately 1/R T•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 R T 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 R T and CT for the desired frequency. Leave the R T pins on the slaves open, connect the C T pins to the CT of the master, and connect the clock pins to the clock pin of the master. Since C T is a high-impedance node, this sync technique works best when all devices are close together. |
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