This is a DC-DC step-up converter based on LM2585-ADJ regulator manufactured by Texas Instruments. This IC was choosen for it’s simplicity of use, requiring minimal external components and for it’s ability to control the output voltage by defining the feedback resistors (R1,R2). NPN switching/power transistor is intergrated inside the regulator and is able to withstand 3A maximum current and 65V maximum voltage. Switching frequency is defined by internal oscillator and it’s fixed at 100KHz.
Additional features include soft-start circuit to eliminate current spikes during start-up and internal current limit. Output voltage regulation is 4% within input voltage and load specifications.
Schematic is a simple boost topology arranement based on datasheet. Input capacitors and diode should be placed close enought to the regulator to minimize inductance effects of pcb traces. IC1, L1, D1, C1,C2 and C5,C6 are the main parts used in voltage convertion. Capacitor C3 is a high frequency bypass capacitor and should be placed as close to IC1 as possible.
All components are selected for their low loss characteristics. So capacitors selected have low ESR and inductor selected has low DC resistance.
At maximum output power there is significant heat produced by IC1 and for that reason we mounted it directly on the ground plane to achieve maximum heat radiation.
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|C1,C2||33uF, 25V, 1Ω||–||EEV-FC1E330P||Panasonic|
|C3||0.1uF, 50V, 0Ω||1206||C1206C104J5RACTU||Kemet|
|C4||1 uF, 25V||1206||ECJ-3YB1E105K||Panasonic|
|C5,C6||220uF, 35V, 0.15Ω||10×10.2mm||EEV-FC1V221P||Panasonic|
|D1||0.45 V, 3A, 40V Schottky Diode||–||B340LB-13||Diodes Inc.|
|L1||120 uH, 0.04Ω||–||PM2120-121K||JW Miller|
|LED||white SMD led||1206/1812||–||–|
We’ve done a simulation of the LM2585 step-up DC-DC converter using the TI’s WEBENCH online softwate tools and some of the results are presented here.
The first graph is the open-loop BODE graph. In this graph we see a plot of GAIN vs FREQUENCY in the range 1Hz – 1M and PHASE vs FREQUENCY in the same range. This plot is usefull as it gives us a detailed view of the stability of the loop and thus the stability and performance of our DC-DC converter.
Bode plot of open control loop
What’s interesting on this plot is the “phase margin” and “gain margin“. Gain margin is the gain for -180deg phase shift and phase margin is the phase difference from 180deg for 0db gain. For the system to be considered stable there should be enough phase margin (>30deg) for 0db gain or when phase is -180deg the gain should be less than 0db.
On the plot above we see that the phase margin is ~90deg and that ensure us that the DC-DC converter will be stable over the measured range.
The next simulation graph is the Input Transient plot over time.
Input Transient simulation
In this plot we see how the output voltage is recovering when input voltage is stepped from 10V to 15V. We see that 4ms after the input voltage is stepped the output has recovered to normal output voltage of 24V.
The next graph is the Load Transient.
Load Transient simulation
Load transient is the response of output voltage to sudden changes of load or Iout. We see that the ouput current suddenly changes from 0,1A to 1A and that the output voltage drops down to 23,2V until it recovers in about 3ms. We also see that when the load is reduced from 1A to 0,1A, output voltage spikes up to ~25,5V, then rings until it recovers to 24V in about 4ms.
The last graph shows Steady State operation of DC-DC converter @ 1A ouput.
This graph show the simulated output voltage ripple and inductor current. We see that output voltage ripple is ~0,6Vpp and the the inductor current has a peak current of 2,4A. The inductor we used is rated at max 5,6A DC current so it can easily withstand such operating current and without much heating of the coil.
Operating point data (Vin=13V, Iout=1A)
|Pulse Width Modulation (PWM) frequency||Frequency||100 kHz|
|Continuous or Discontinuous Conduction mode||Mode||Cont|
|Total Output Power||Pout||24.0 W|
|Vin operating point||Vin Op||13.00 V|
|Iout operating point||Iout Op||1.00 A|
|Operating Point at Vin= 13.00 V,1.00 A|
|Bode Plot Crossover Frequency, indication of bandwidth of supply||Cross Freq||819 Hz|
|Steady State PWM Duty Cycle, range limits from 0 to 100||Duty Cycle||48.3 %|
|Steady State Efficiency||Efficiency||93.2 %|
|IC Junction Temperature||IC Tj||65.2 °C|
|IC Junction to Ambient Thermal Resistance||IC ThetaJA||34.9 °C/W|
|Input Capacitor RMS ripple current||Cin IRMS||0.14 A|
|Output Capacitor RMS ripple current||Cout IRMS||0.48 A|
|Peak Current in IC for Steady State Operating Point||IC Ipk||2.2 A|
|ICs Maximum rated peak current||IC Ipk Max||3.0 A|
|Average input current||Iin Avg||2.0 A|
|Inductor ripple current, peak-to-peak value||L Ipp||0.50 A|
|Power Dissipation Analysis|
|Input Capacitor Power Dissipation||Cin Pd||0.01 W|
|Output Capacitor Power Dissipation||Cout Pd||0.035 W|
|Diode Power Dissipation||Diode Pd||0.45 W|
|IC Power Dissipation||IC Pd||1.0 W|
|Inductor Power Dissipation||L Pd||0.16 W|
Configuring Output Voltage
Output voltage is configured by R1, R2 according to the following expression (Vref=1,23V)
VOUT = VREF (1 + R1/R2)
If R2 has a value between 1k and 5k we can use this expression to calculate R1:
R1 = R2 (VOUT/VREF − 1)
For better thermal response and stability it is suggested to use 1% metal film resistors.