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IRF7457 Datasheet(PDF) 9 Page - International Rectifier |
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IRF7457 Datasheet(HTML) 9 Page - International Rectifier |
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9 / 21 page  IRU3037 / IRU3037A 9 Rev. 2.8 03/10/03 www.irf.com For a general solution for unconditionally stability for any type of output capacitors, in a wide range of ESR values we should implement local feedback with a compensa- tion network. The typically used compensation network for voltage-mode controller is shown in Figure 7. Figure 7 - Compensation network with local feedback and its asymptotic gain plot. In such configuration, the transfer function is given by: The error amplifier gain is independent of the transcon- ductance under the following condition: By replacing ZIN and Zf according to Figure 7, the trans- former function can be expressed as: As known, transconductance amplifier has high imped- ance (current source) output, therefore, consider should be taken when loading the E/A output. It may exceed its source/sink output current capability, so that the ampli- fier will not be able to swing its output voltage over the necessary range. The compensation network has three poles and two ze- ros and they are expressed as follows: Cross Over Frequency: The stability requirement will be satisfied by placing the poles and zeros of the compensation network according to following design rules. The consideration has been taken to satisfy condition (14) regarding transconduc- tance error amplifier. 1) Select the crossover frequency: Fo < FESR and Fo [ (1/10 ~ 1/6)3 fS 2) Select R7, so that R7 >> 3) Place first zero before LC’s resonant frequency pole. FZ1 ≅ 75% FLC 4) Place third pole at the half of the switching frequency. C12 > 50pF If not, change R7 selection. 5) Place R7 in (15) and calculate C10: 2 gm 1 - gmZf 1 + gmZIN Ve VOUT = Where: VIN = Maximum Input Voltage VOSC = Oscillator Ramp Voltage Lo = Output Inductor Co = Total Output Capacitors C11 = 1 2 p 3 FZ1 3 R7 C12 = 1 2 p 3 R7 3 FP3 FP3 = fS 2 C10 [ 3 2 p 3 Lo 3 Fo 3 Co R7 VOSC VIN FP1 = 0 1 2 p3C103(R6 + R8) FZ2 = ≅ 1 2 p3C103R6 FZ1 = 1 2 p3R73C11 FP3 = ≅ 1 C12 3C11 C12+C11 2 p3R73 1 2 p3R73C12 FP2 = 1 2 p3R83C10 ( ) VOUT VREF R5 R6 R8 C10 C12 C11 R7 Ve FZ1 FZ2 FP2 FP3 E/A Zf ZIN Frequency Gain(dB) H(s) dB Fb Comp gmZf >> 1 and gmZIN >>1 ---(14) H(s)= 3 (1+sR7C11) 3[1+sC10(R6+R8)] 1 sR6(C12+C11) 1+sR7 3(1+sR8C10) [ ( )] C12 3C11 C12+C11 FO = R7 3C103 3 ---(15) VIN VOSC 1 2 p3Lo3Co |
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