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LMR23630AFDDA Datasheet(PDF) 12 Page - Texas Instruments |
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LMR23630AFDDA Datasheet(HTML) 12 Page - Texas Instruments |
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12 / 42 page  VSW VIN D = tON/ TSW tON tOFF TSW t -VD 0 iL IOUT t 0 ILPK 'iL 12 LMR23630 SNVSAH2C – DECEMBER 2015 – REVISED JUNE 2017 www.ti.com Product Folder Links: LMR23630 Submit Documentation Feedback Copyright © 2015–2017, Texas Instruments Incorporated 8.3 Feature Description 8.3.1 Fixed Frequency Peak Current Mode Control The following operating description of the LMR23630 refers to the Functional Block Diagram and to the waveforms in Figure 13. The LMR23630 is a step-down synchronous buck regulator with integrated high-side (HS) and low-side (LS) switches (synchronous rectifier). The LMR23630 supplies a regulated output voltage by turning on the HS and LS NMOS switches with controlled duty cycle. During high-side switch ON-time, the SW pin voltage swings up to approximately VIN, and the inductor current iL increase with linear slope (VIN – VOUT) / L. When the HS switch is turned off by the control logic, the LS switch is turned on after an anti-shoot-through dead time. Inductor current discharges through the LS switch with a slope of –VOUT / L. The control parameter of a buck converter is defined as duty cycle D = tON / TSW, where tON is the high-side switch ON-time and TSW is the switching period. The regulator control loop maintains a constant output voltage by adjusting the duty cycle D. In an ideal buck converter, where losses are ignored, D is proportional to the output voltage and inversely proportional to the input voltage: D = VOUT / VIN. Figure 13. SW Node and Inductor Current Waveforms in Continuous Conduction Mode (CCM) The LMR23630 employs fixed-frequency peak-current-mode control. A voltage feedback loop is used to get accurate DC voltage regulation by adjusting the peak current command based on voltage offset. The peak inductor current is sensed from the high-side switch and compared to the peak current threshold to control the ON-time of the high-side switch. The voltage feedback loop is internally compensated, which allows for fewer external components, makes it easy to design, and provides stable operation with almost any combination of output capacitors. The regulator operates with fixed switching frequency at normal load condition. At light load condition, the LMR23630 operates in PFM mode to maintain high efficiency (PFM option) or in FPWM mode for low output-voltage ripple, tight output-voltage regulation, and constant switching frequency (FPWM option). 8.3.2 Adjustable Frequency The switching frequency can be programmed for the adjustable-switching-frequency version of LMR23630 by the impedance RT from the RT pin to ground. The frequency is inversely proportional to the RT resistance. The RT pin can be left floating and the LMR23630 operates at 400-kHz default switching frequency. The RT pin is not designed to be shorted to ground. For a desired frequency, typical RT resistance can be found by Equation 1. Table 1 gives typical RT values for a given fSW. RT(kΩ) = 40200 / fSW(kHz) – 0.6 (1) |
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