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BD9018KV Datasheet(PDF) 9 Page - Rohm |
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BD9018KV Datasheet(HTML) 9 Page - Rohm |
9 / 17 page ![]() Technical Note 9/16 BD9018KV www.rohm.com 2010.02 - Rev.A ○ c 2010 ROHM Co., Ltd. All rights reserved. A COMP FB C R Feedback -1 8 0 -9 0 0 90 18 0 A 0 0 -90 -180 (a) GBW(b) -180° Phase margin -90° -20dB/decade Gain [dB] Phase [deg] Fig. 25 Fig. 26 Resonance point phase delay-180° fr = [Hz] 1 2π√LC 1 2π√LC fr = [Hz]:Resonance Point fESR = [Hz] :Phase lead 1 2πRESRC Phase delay-90° (8) Method for determining phase compensation In switching regulator applications, the phase needs to be compensated in accordance with the operating conditions as well as the used external parts. In case the margin is not enough, then the output may possibly overshoot or undershoot when the load current, input voltage or switching frequency rapidly changes. The compensation technique is described below. Conditions for application stability Feedback stability conditions are as follows: ・ At the unity (0-dB) gain, the phase delay is 150° or less (i.e., phase margin is at least 30°): Since the DC/DC converter application is sampled according to the switching frequency, GBW (frequency at 0-dB gain) of the overall system should be set to 1/10 or less of the switching frequency. The following section summarizes the targeted characteristics of this application. ・ At the unity (0-dB) gain, the phase delay is 150˚ or less (i.e. the phase margin is 30˚ or more). ・ The GBW for this occasion is 1/10 or less of the switching frequency. Responsiveness is determined with restrictions on the GBW. To improve responsiveness, higher switching frequency should be provided. The key to achieving successful stabilization using phase compensation is to cancel the secondary phase margin/delay (-180°) generated by LC resonance, by employing a dual phase lead. In short, adding two phase leads stabilizes the application. GBW (the frequency at unity gain) is determined by the phase compensation capacitor connected to the error amp. Thus, a larger capacitor will serve to lower GBW if desired. ① General use integrator (low-pass filter) ② Integrator open loop characteristics Fig. 23 Fig. 24 The error amp is provided with phase compensation similar to that depicted in figures ① and ② above and thus serves as the system’s low-pass filter. In DC/DC converter applications, R is established parallel to the feedback resistance. When electrolytic or other high-ESR output capacitors are used: Phase compensation is relatively simple for applications employing high-ESR output capacitors (on the order of several Ω ). In DC/DC converter applications, where LC resonance circuits are always incorporated, the phase margin at these locations is -180°. However, wherever ESR is present, a 90° phase lead is generated, limiting the net phase margin to -90° in the presence of ESR. Since the desired phase margin is in a range less than 150°, this is a highly advantageous approach in terms of the phase margin. However, it also has the drawback of increasing output voltage ripple components. ③ LC resonance circuit ④ ESR connected Vcc Vo L C 1 2πRCA point (a) fa = 1.25[Hz] 1 2πRC point (b) fa = GBW [Hz] Vcc Vo L C RESR |
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