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IRU3048 Datasheet(PDF) 10 Page - International Rectifier |
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IRU3048 Datasheet(HTML) 10 Page - International Rectifier |
10 / 18 page 10 Rev. 1.7 09/12/02 IRU3048 www.irf.com 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: 2) Select R7, so that R7 >> 3) Place first zero before LC’s resonant frequency pole. 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: 6) Place second pole at ESR zero. FP2 = FESR Check if R8 > If R8 is too small, increase R7 and start from step 2. 7) Place second zero around the resonant frequency. FZ2 = FLC FP3 = fS 2 8) Use equation (1) to calculate R5: These design rules will give a crossover frequency ap- proximately one-tenth of the switching frequency. The higher the band width, the potentially faster the load tran- sient speed. The gain margin will be large enough to provide high DC-regulation accuracy (typically -5dB to - 12dB). The phase margin should be greater than 45 8 for overall stability. LDO Section Output Voltage Programming Output voltage for LDO is programmed by reference volt- age and external voltage divider. The Fb3 pin is the in- verting input of the error amplifier, which is internally ref- erenced to 1.25V. The divider is ratioed to provide 1.25V at the Fb3 pin when the output is at its desired value. The output voltage is defined by using the following equa- tion: Results to: RHIGH=1K LDO Power MOSFET Selection The first step in selecting the power MOSFET for the linear regulator is to select the maximum RDS(ON) based on the input to the dropout voltage and the maximum load current. Results to: RDS(ON)(MAX) = 0.4 V Note that since the MOSFET RDS(ON) increases with tem- perature, this number must be divided by ~1.5 in order to find the RDS(ON)(MAX) at room temperature. The IRLR2703 has a maximum of 0.065 V RDS(ON) at room temperature, which meets our requirements. 1 gm C12 = 1 2 p 3 R7 3 FP3 C10 [ 3 2 p 3 Lo 3 FO 3 Co R7 VOSC VIN R8 = 1 2 p 3 C10 3 FP2 R6 = - R8 1 2 p 3 C10 3 FZ2 R5 = 3 R6 VREF VOUT - VREF RDS(ON) = VIN3 - VOUT2 IOUT2 For: VIN3 = 3.3V VOUT2 = 2.5V IOUT2 = 2A Where: VIN = Maximum Input Voltage VOSC = Oscillator Ramp Voltage Lo = Output Inductor Co = Total Output Capacitors FO1 = R7 3C103 3 VIN VOSC 1 2 p3Lo3Co ---(15) Fo < FESR and Fo [ (1/10 ~ 1/6)3 fS 2 gm FZ1 ≅ 75% FLC C11 = 1 2 p 3 FZ1 3 R7 For: VOUT2 = 2.5V VREF = 1.25V RLOW = 1K ( ) RHIGH RLOW VOUT2 = VREF 3 1+ |
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