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BD8312HFN Datasheet(PDF) 10 Page - Rohm |
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BD8312HFN Datasheet(HTML) 10 Page - Rohm |
10 / 15 page BD8312HFN Technical Note 10/14 www.rohm.com 2009.04- Rev.B © 2009 ROHM Co., Ltd. All rights reserved. (3) Output voltage setting The internal standard voltage of the ERROR AMP is 1.0 V. Output voltage is acquired by Equation (4). (4) DC/DC converter frequency response adjustment system Condition for stable application The condition for feedback system stability under negative feedback is that the phase delay is 135 °or less when gain is 1 (0dB). Since DC/DC converter application is sampled according to the switching frequency, the bandwidth GBW of the whole system (frequency at which gain is 0 dB) must be controlled to be equal to or lower than 1/10 of the switching frequency. In summary, the conditions necessary for the DC/DC converter are: - Phase delay must be 135°or lower when gain is 1 (0 dB). - Bandwidth GBW (frequency when gain is 0 dB) must be equal to or lower than 1/10 of the switching frequency. To satisfy those two points, R1, R2, R3, DS and RS in Fig. 30 should be set as follows. [1] R1, R2, R3 BD8313HFN incorporates phase compensation devices of R4=62kΩ and C2=200pF. These C2 and R1, R2, and R3 valuesdecide the primary pole that determines the bandwidth of DC/DC converter. Primary pole point frequency DC/DC converter DC Gain By Equations (1) and (2), the frequency fsw of point 0 dB under limitation of the bandwidth of the DC gain at the primary pole point is as shown below. fSW = fp×DC Gain = × × It is recommended that fsw should be approx.10 kHz. When load response is difficult, it may be set at approx. 20 kHz. By Equation (3), R1 and R2, which determine the voltage value, will be in the order of several hundred kΩ. If an appropriate resistance value is not available since the resistance is so high and routing may cause noise, the use of R3 enables easy setting. Vo= × 1.0 [V] ・・・ (4) R2 (R1+R2) VREF 1.0V VOUT ERROR AMP R1 R2 INV FB VOUT C2 R1 R2 Rs R4 Cs R3 Inside of IC ・・・・ (1) ・・・・ (2) 2π A×( +R3)×C2 fp= 1 R1×R2 R1+R2 DC Gain =A× × B 1 2πC2×( +R3 ) (R1+R2) (R1・R2) 1 ・・・・ (3) VO VIN B 1 VO VIN Fig.29 Setting of voltage feedback resistance Fig.30 Example of phase compensation setting A: Error AMP Gain About 100dB = 10 5 B: Oscillator amplification = 0.5 VIN: Input voltage VOUT: Output voltage |
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