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LM1771SMM Scheda tecnica(PDF) 10 Page - Texas Instruments |
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LM1771SMM Scheda tecnica(HTML) 10 Page - Texas Instruments |
10 / 31 page VOUT fSW = D VOUT F = VIN x TON 1 TP = fSW VOUT D = VIN LM1771 SNVS446D – JUNE 2006 – REVISED JANUARY 2016 www.ti.com 7.3 Feature Description 7.3.1 Timing Opinion Three versions of the LM1771 are available each with a predetermined TON set internally by EEPROM. This TON setting determines the switching frequency for the application. Derivation and calculation of the dependence of the switching frequency on VIN and TON is shown in Equation 1 through Equation 6. In a PWM buck switcher, Equation 1, Equation 2, and Equation 3 can be manipulated to obtain the switching frequency. Equation 1 shows the standard duty-cycle equation given by the volts-seconds balance on the inductor with Equation 2 and Equation 3 defining standard relationships: (1) TON = D x TP (2) (3) Using these equations and solving for duty-cycle for Equation 4: D = fSW x TON (4) Frequency can now be expressed in Equation 5: (5) Or simply written as Equation 6: where • α = VIN x TON (6) To maintain a set frequency in an application, α is always held constant by varying TON inversely with VIN. The three versions of the LM1771 are identified by the ON-times at a VIN of 3.3 V for consistency. For clarification, see Table 1. Table 1. Timing for All Variants Product ID TON at 3.3 V α (V µs) LM1771S 0.5 µs 1.65 LM1771T 1.0 µs 3.3 LM1771U 2.0 µs 6.6 The variation of TON versus VIN can also be expressed graphically. These graphs can be found in the Typical Characteristics of the data sheet. With α being a constant regardless of the version of the LM1771 used, Equation 6 shows that the only dependent variable remaining is VOUT. Because VOUT is a constant in any application, the frequency also remains constant. The switching frequency at which the application runs depends upon the VOUT desired and the LM1771 version chosen. For any VOUT, three frequency options (LM1771 versions) can be selected. This can be seen Table 2. The recommended frequency range of operation is 100 kHz to 1000 kHz. 10 Submit Documentation Feedback Copyright © 2006–2016, Texas Instruments Incorporated Product Folder Links: LM1771 |
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