BUCK REGULATOR DESIGN (FIXED OUTPUT) Revision History Version, Date, By Revision 0.1, 29 Sept 2007, Eko Didik Widianto 0.1, 29 Sept 2007, Eko Didik Widianto Fix equations for selecting inductor Add output ripple (current and voltage) estimation Add resume First draft Introduction In this procedure, designing a buck regulator to provide voltage output 2A@3.3V and 2A@5V will be presented. These outputs will be used to power up ip08 board. IP08 has power budgeting as follows: Device 12V 5V 3.3V 1.2V Notes ADSP 50 ma 505 ma SDRAM 255 ma --Curious-- SPI-flash 15 ma M25P40 NAND-flash 60 ma FXS/SLIC (8) 3800 ma 75 ma ETH0&1 200 ma 2ETH, 100BASE-TX USB 400 ma 50 ma RS232 12 ma ADM3202, RL 3kOhm CPLD 80 ma IDE-HD 480 ma seeking, Toshiba 100GB, 5400rpm JTAG 0.8 ma if used, rise from R pull-up LED 45 ma 200 ma 1power+8status led 400MHz, 1.2Vdd-int, Peak [5, p.7], Tj 100'C (highperformance) 0.48Aeach(ring), 0%, 5REN, 75Vrpk, Vunreg=12V [1, p.8], FXO module only needs 3.3V@10mA (max) ISP1362, both for host and device, supports upto 4 usb devices @100mA XC9572XL, High performance, 100MHz(actually it's too high) LCD 0.42 ma HD44780, Icc + Ip [7, p.48] Total 3845 ma 880 ma 998.22 ma 505 ma Rail (V) Current Power Efficiency Requirement 11.5 3845 ma 44217.5 mw 100.00% 44217.5 mw Vf rectifier diode is 0.5V 5 880 ma 4400 mw 80.00% 5500 mw 3.3 998.22 ma 3294.13 mw 81.00% 4066.82 mw 1.2 505 ma 606 mw 36.00% 1683.33 mw Total 55467.66 mw 1.2V is regulated from 3.3V rail, so current requirement from 3.3V is as follow:
Schematic Diodes AP1509 ( a 150KHz, 2A PWM Buck DC/DC Converter) will be used as regulator. Generic schematic is as follow: R ail (V ) C urrent P ow er E ffic ienc y R equirem ent 3.3 998.22 m A 3294.13 m W 100.00% 3294.13 m W 1.2 505 m A 606 m W 36.00% 1683.33 m W T otal 4977.46 m W C urrent requirem ent from 3.3V rail = 1508.32 m A U1 is AP1509-33SG-13 (for 3.3V output) or AP1509-50SG-13 (for 5V output). Target Specification A. 3.3V Regulated Parameter Value Notes Input power Vin (max) = +12V; Vin (min) = +11.5V Regulated Power Vout = +3.3V; ILoad (max) = 2A; ILoad(min) = 0.2A ripple voltage Vripple 0.03V peak-to-peak Efficiency 75% (min) at full load We'll using rectifier diode (Vf=0.5V) prior to Vin ILoad(min) before the circuit becomes discontinuous, typical value is 10% of ILoad(max) Load regulation 1% ½ full load to full load Switching frequency f=150khz ± 15% B. 5V Regulated Drawing 1: Buck regulator schematic Parameter Value Notes Input power Vin (max) = +12V; Vin (min) = +11.5V Regulated Power Vout = +5V; ILoad (max) = 1A; ILoad(min) = 0.15A ripple voltage Vripple 0.03V peak-to-peak We'll using rectifier diode (Vf=0.5V) prior to Vin ILoad(min) before the circuit becomes discontinuous, typical value is 10% of ILoad(max)
Parameter Value Notes Efficiency 75% (min) at full load Load regulation 1% ½ full load to full load Switching frequency f=150khz ± 15% Design objective is to determine the value and rating of inductor L1, output capacitor Co, rectifier D1 and input capacitor Cin. Component Selections Refers to power_calculation_ip08.odt for detailed component calculation, especially in 3_3V_supply_2 and 5V_supply_2 tabs. A. Inductor L(min) The inductor currrent waveform in continuous mode operation (bold line) is as depicted at Drawing 2. r is defined as the ratio of the AC to DC components (current ripple ratio) evaluated at maximum load, Io : A high inductance reduces I and results in lower r (and lower RMS current in the output capacitor), but may result in a very large and impractical inductor. So typically, for most buck regulators, r is chosen to be in the range of 0.25 0.5 (at the maximum rated load). Once the inductance is selected, as we decrease the load on the converter (keeping input voltage constant), I remains fixed but the DC level decreases and so the current ripple ratio increases. Ultimately, at the point of transition to discontinuous mode of operation, the DC level is I/2 as shown in Drawing 2. So The current ripple ratio at the point of transition to discontinuous mode is 2. Therefore, the upper limit for r is also 2. Drawing 2: Inductor Current Waveform
The load at which this happens can be shown by simple geometry to be r/2 times IO. So for example, if the inductance is chosen to be such that r is 0.3 at a load of 2A, the transition to discontinuous mode of operation will occur at 0.15 times 2A, which is 300 ma. The only degree of freedom is in selecting 'r', so all our equation will be written in terms of 'r'. Basic Method to Calculate L From the general rule V = L*dl/dt we get during the ON time of the converter: Solving for I we can write 'r' as: Duty cycle of buck regulator: And L is therefore : Another Method to Calculate L Based on VoltSeconds The ON time is : Voltseconds is: L (min) is: Where: D Vo Vd Vsw f = Duty cycle = voltage = Forward drop across the catch diode ( = 0.5V for Schottky Diode) = across the switch when it is ON (1.25V for AP1509), Internal switch saturation voltage = The switching frequency in Hz The required energy handling capability is next calculated. Every cycle, the peak current is:
The required energy handling capability, e is: Based on LM2596 design procedure [1,p.18], inductor value can be selected from Drawing 3. Inductor selection from Drawing 4 above fulfills inductor minimum requirements both for 3.3V and 5V. Drawing 3: Inductor selection guide 3.3V 5V L(m in) 35.49 uh Irm s Ipk 2.23 A L(m in) 65.66 uh Irm s Ipk 1.14 A Drawing 4: Inductor value selection Inductor Selection SW S-2.24-37 (2.24A, 37uH, 0.10Ohm, $1.36, fprint S4) PE-53832SNL (2.24A, 37uH, 0.10Ohm, $1.95, fprint LCI-44) SW S-1.54-77 (1.54A, 77uH, 0.20Ohm, $1.42, fprint S4) PE-53830SNL (1.54A, 77uH, 0.20Ohm, $2.08, fprint LCI-44) B. Capacitor Co The output capacitor is required to filter the output and provide regulator loop stability. When selecting an output capacitor, the important capacitor parameters are; the 100kHz Equivalent Series Resistance (ESR), the RMS ripples current rating, voltage rating, and capacitance value. For the output capacitor, the ESR value is the most important parameter. Drawing 5: Capacitor parasitics All capacitors contain parasitic elements which make their performance less than ideal as figure below: The ESR (Equivalent Series Resistance) causes internal heating due to power dissipation as the ripple current flows into and out of the capacitor. The capacitor
can fail if ripple current exceeds maximum ratings. Excessive output voltage ripple will result from high ESR, and regulator loop instability is also possible. ESR is highly dependent on temperature, increasing very quickly at temperatures below about 10 C. operate over a wide temperature range. In these cases, an aluminum electrolytic must be paralleled by another type of capacitor with a flatter ESR curve (like Tantalum or Film) so that the effective ESR (which is the parallel value of the two ESR's) stays within the allowable range. ESL: The ESL (Effective Series Inductance) limits the high frequency effectiveness of the capacitor. High ESL is the reason electrolytic capacitors need to be bypassed by film or ceramic capacitors to provide good high-frequency performance. The ESR, ESL and C within the capacitor form a resonant circuit, whose frequency of resonance should be as high as possible. Switching regulators generate ripple voltages on their outputs with very high frequency (>10 Mhz) components, which can cause ringing on the output voltage if the capacitor resonant frequency is low enough to be near these frequencies. 3.3V E S R 0.065 O hm V oltage R ating 4.950 V 5V E S R 0.110 O hm V oltage R ating 7.500 V Drawing 6: capacitor value selection B1. Analysis With L and Co selections above, we can calculate 'r' (ripple current) and then estimate 'Vr' (voltage ripple), as depicted at figure below. Capacitor Selection EEUFC1V391S (390uF/35V, 0.065Ohm, $0.80) UPW 1V331MPD (330uF/35V, 0.068Ohm, $0.40) ELXZ350ELL331MJ16S (330uF/35V, 0.068Ohm, $0.43) EEUFC1V221 (220uF/35V, 0.090 Ohm, $0.44) UPW 1V221MPD (220uF/35V,0.090 Ohm, $0.29) Specification 3.3V r, c urrent ripple 0.221 V r, v oltage ripple 0.030 V r, c urrent ripple 0.221 V r, v oltage ripple 0.029 V r, c urrent ripple 0.221 V r, v oltage ripple 0.023 V 5V r, c urrent ripple 0.233 V r, v oltage ripple 0.021 V Regulator Analysis Drawing 7: Ripple current and voltage estimation Using SWS-2.24-37 and UPW1V331MPD Using SWS-2.24-37 an EEUFC1V391S Using SWS-2.24-37 and UPW1V221MPD + UPW1V101MPD At I = Iload(max)= 1A Using SWS-1.54-77 and UPW1V221MPD or EEUFC1V221 If output capacitor Co small size (or height) requirement is needed, especially in 3.3V design, we can use 220uF/35V (0.09 Ohm ESR) parallelized with 100uF/35V (0.117 Ohm ESR).
C. Rectifier D1 The Rectifier D1 current rating must be greater than the peak switch current IPK. The reverse voltage rating of the output rectifier D1 should be at least 1.25 times the maximum input voltage. The output rectifier D1 must be fast (short reverse recovery time) and must be located close to the AP1509 using short leads and short printed circuit traces. Because of their fast switching speed and low forward voltage drop, Schottky diodes provide the best performance and efficiency, and should be the first choice, especially in low output voltage applications. Diode Rectifier Selection 3.3V V R R M 15 V MBRS340T3G (Vf=0.5V, If = 3A, VRRM=40V, $3.8/10) IP K 2.23 A 5V V R R M 15 V IP K 1.14 A Drawing 8: Diode rectifier component selection D. Input Capacitor The RMS current rating of the input capacitor can be calculated from the following formula table. The important parameters for the Input capacitor are the input voltage rating and the RMS current rating. We need to assure that this ratings is not exceeded.
3.3V Iin, rm s 1.192 A V oltage R ating 18 V 5V Iin, rm s 0.717 A V oltage R ating 18 V Input Capacitor Selection Drawing 9: Input capacitor component selection EEUFC1V391S (390uF/35, 1205mA) UPW 1V331MPD (330uF/35V, 1050mA) UPW 1V221MPD + UPW 1V101MPD (100uF/35V, 555mA) UPW 1V221MPD (220uF/35V, 755mA) EEUFC1V221 (220uF/35V, 755mA) Same as output capacitor, if we need short capacitor, we can use 2 capacitors in parallel. References 1. LM2596 Datasheet: SIMPLE SW ITCHER Power Converter 150 khz 3A Step-Down Regulator. National Semiconductor. May 2002 2. Application Note AP1509 150KHz, 2A PWM Buck DC/DC Converter. Diodes Incorporates. August 2007 3. LM2592HV Datasheet :SIMPLE SWITCHER Power Converter 150 khz 2A, Step-Down Regulator Drawing 10: RMS current rating for low ESR electrolytic capacitor (typical)
Design Resume Power specification V Out Specification Notes 3.3V V in 12 V V in (m in) 11.5 V Due to rectifier diode Vf V r (m ax ) 0.03 V IL (m ax ) 2 A fs 150 k H z Switching frequency 5V V in 12 V V in (m in) 11.5 V Due to rectifier diode Vf V r (m ax ) 0.03 V IL (m ax ) 1 A fs 150 k H z Switching frequency 3.3V 5V L(m in) 35.49 uh Irm s Ipk 2.23 A L(m in) 65.66 uh Irm s Ipk 1.14 A Inductor Selection SW S-2.24-37 (2.24A, 37uH, 0.10Ohm, $1.36, fprint S4) PE-53832SNL (2.24A, 37uH, 0.10Ohm, $1.95, fprint LCI-44) SW S-1.54-77 (1.54A, 77uH, 0.20Ohm, $1.42, fprint S4) PE-53830SNL (1.54A, 77uH, 0.20Ohm, $2.08, fprint LCI-44) 3.3V E S R 0.065 O hm V oltage R ating 4.950 V 5V E S R 0.110 O hm V oltage R ating 7.500 V 3.3V V R R M 15 V IP K 2.23 A 5V V R R M 15 V IP K 1.14 A 3.3V Iin, rm s 1.192 A V oltage R ating 18 V 5V Iin, rm s 0.717 A V oltage R ating 18 V Specification 3.3V r, c urrent ripple 0.221 V r, v oltage ripple 0.030 V r, c urrent ripple 0.221 V r, v oltage ripple 0.029 V r, c urrent ripple 0.221 V r, v oltage ripple 0.023 V 5V r, c urrent ripple 0.233 V r, v oltage ripple 0.021 V Capacitor Selection EEUFC1V391S (390uF/35V, 0.065Ohm, $0.80) UPW 1V331MPD (330uF/35V, 0.068Ohm, $0.40) ELXZ350ELL331MJ16S (330uF/35V, 0.068Ohm, $0.43) EEUFC1V221 (220uF/35V, 0.090 Ohm, $0.44) UPW 1V221MPD (220uF/35V,0.090 Ohm, $0.29) Diode Rectifier Selection MBRS340T3G (Vf=0.5V, If = 3A, VRRM=40V, $3.8/10) Input Capacitor Selection EEUFC1V391S (390uF/35, 1205mA) UPW 1V331MPD (330uF/35V, 1050mA) UPW 1V221MPD + UPW 1V101MPD (100uF/35V, 555mA) UPW 1V221MPD (220uF/35V, 755mA) EEUFC1V221 (220uF/35V, 755mA) Regulator Analysis Using SWS-2.24-37 and UPW1V331MPD Using SWS-2.24-37 an EEUFC1V391S Using SWS-2.24-37 and UPW1V221MPD + UPW1V101MPD At I = Iload(max)= 1A Using SWS-1.54-77 and UPW1V221MPD or EEUFC1V221