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LM25115MTX Arkusz danych(PDF) 10 Page - National Semiconductor (TI) |
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LM25115MTX Arkusz danych(HTML) 10 Page - National Semiconductor (TI) |
10 / 17 page Synchronization (SYNC) and Feed-forward (RAMP) (Continued) The recommended SYNC input current range is 50µA to 150µA. The SYNC pin resistor (R SYNC) should be selected to set the SYNC current (I SYNC) to 150µA with the maximum phase signal amplitude, V PHASE(max). This will guarantee that I SYNC stays within the recommended range over a 3:1 change in phase signal amplitude. The SYNC pin resistor is therefore: R SYNC =(VPHASE(max) / 150µA) - 2.5k Ω Once I SYNC has been established by selecting RSYNC, the RAMP signal amplitude may be programmed by selecting the proper RAMP pin capacitor value. The recommended peak amplitude of the RAMP waveform is 1V to 1.75V. The C RAMP capacitor is chosen to provide the desired RAMP amplitude with the nominal phase signal voltage and pulse width. C RAMP =(3xISYNC xTON )/VRAMP Where C RAMP = RAMP pin capacitance I SYNC = SYNC pin current current T ON = corresponding phase signal pulse width V RAMP = desired RAMP amplitude (1V to 1.75V) For example, Main channel output = 3.3V. Phase signal maximum ampli- tude = 12V. Phase signal frequency = 250kHz • Set I SYNC = 150µA with phase signal at maximum ampli- tude (12V): I SYNC = 150µA = VPHASE(max) /(RSYNC + 2.5 k Ω) = 12V / (R SYNC + 2.5 k Ω) R SYNC = 12V/150µA - 2.5k Ω = 77.5kΩ • T ON = Main channel duty cycle / Phase frequency = (3.3V/12V) / 250kHz = 1.1µs • Assume desired V RAMP = 1.5V • C RAMP =(3xISYNC xTON )/VRAMP = (3 x 150µA x 1.1µs) / 1.5V • C RAMP = 330pF Error Amplifier and Soft-Start (FB, CO, & COMP, SS) An internal wide bandwidth error amplifier is provided within the LM25115 for voltage feedback to the PWM controller. The amplifier’s inverting input is connected to the FB pin. The output of the auxiliary converter is regulated by connect- ing a voltage setting resistor divider between the output and the FB pin. Loop compensation networks are connected between the FB pin and the error amplifier output (COMP). The amplifier’s non-inverting input is internally connected to the SS pin. The SS pin is biased at 0.75V by a resistor divider connected to the internal 1.27V bandgap reference. When the VCC voltage is below the UVLO threshold, the SS pin is discharged to ground. When VCC rises and exceeds the positive going UVLO threshold (4.25V), the SS pin is released and allowed to rise. If an external capacitor is connected to the SS pin, it will be charged by the internal resistor divider to gradually increase the non-inverting input of the error amplifier to 0.75V. The equivalent impedance of the SS resistor divider is nominally 60k Ω which determines the charging time constant of the SS capacitor. During start- up, the output of the LM25115 converter will follow the exponential equation: VOUT(t) = VOUT(final) x (1 - exp(-t/R SS xCSS)) Where Rss = internal resistance of SS pin (60k Ω) Css = external Soft-Start capacitor VOUT(final) = regulator output set point The initial ∆v/ ∆t of the output voltage is VOUT(final) / Rss x Css and VOUT will be within 1% of the final regulation level after 4.6 time constants or when t = 4.6 x Rss x Css. Pull-up current for the error amplifier output is provided by an internal 300µA current source. The PWM threshold signal at the COMP pin can be controlled by either the open drain error amplifier or the open drain current amplifier connected through the CO pin to COMP. Since the internal error ampli- fier is configured as an open drain output it can be disabled by connecting FB to ground. The current sense amplifier and current limiting function will be described in a later section. 20172613 FIGURE 3. Line Feed-forward Waveforms www.national.com 10 |
Podobny numer części - LM25115MTX |
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Podobny opis - LM25115MTX |
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