12V to 24V @ 1A Stepup switching regulator using LM2585
 Michail Papadimitriou
 mixos@otenet.gr

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 moderate
 Tested
Schematic
Description
This is a DCDC stepup converter based on LM2585ADJ regulator manufactured by Texas Instruments. This IC was chosen for its simplicity of use, requiring minimal external components and for its ability to control the output voltage by defining the feedback resistors (R1,R2). NPN switching/power transistor is integrated 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.
The power switch is a 3A NPN device that can standoff 65 V. Protecting the power switch are current and thermal limiting circuits and an undervoltage lockout circuit. This IC contains a 100kHz fixedfrequency internal oscillator that permits the use of small magnetics. Other features include soft start mode to reduce inrush current during startup, current mode control for improved rejection of input voltage, and output load transients and cyclebycycle current limiting. An output voltage tolerance of ±4%, within specified input voltages and output load conditions, is specified for the power supply system.
Specifications
 V_{in}: 1015V DC
 V_{out}: 24V DC
 I_{out}: 1A
 Frequency: 100KHz
Schematic is a simple boost topology arrangement based on datasheet. Input capacitors and diode should be placed close enough to the regulator to minimize the inductance effects of PCB traces. IC1, L1, D1, C1,C2 and C5,C6 are the main parts used in voltage conversion. Capacitor C3 is a highfrequency 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 dissipation.
Photos
Parts List
Part  Value  Package  MPN  Mouser No 

C1 C2  33uF 25V 1Ω  6.3 x 5.4mm  UWX1E330MCL1GB  647UWX1E330MCL1 
C3  0.1uF 50V 0Ω  1206  C1206C104J5RACTU  80C1206C104J5R 
C4  1uF 25V  1206  C1206C105K3RACTU  80C1206C105K3R 
C5 C6  220uF 35V 0.15Ω  10 x 10.2mm  EEEFC1V221P  667EEEFC1V221P 
D1  0.45 V 3A 40V Schottky  SMB  B340LB13F  621B340LBF 
IC1  LM2585SADJ  TO263  LM2585SADJ/NOPB  926LM2585SADJ/NOPB 
L1  120 uH 0.04Ω  30.5 x 25.4 x 22.1 mm  PM2120121KRC  542PM2120121KRC 
R1  28 KΩ  1206  ERJ8ENF2802V  667ERJ8ENF2802V 
R2 R3  1.5 KΩ  1206  ERJ8ENF1501V  667ERJ8ENF1501V 
R4  1 KΩ  1206  RT1206FRE07931KL  603RT1206FRE07931KL 
LED1  RED LED 20mA 2.1V  0805  5990120007F  6455990120007F 
Gerber View
Simulation
We’ve done a simulation of the LM2585 stepup DCDC converter using the TI’s WEBENCH online software tools and some of the results are presented here.
The first graph is the openloop 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 useful as it gives us a detailed view of the stability of the loop and thus the stability and performance of our DCDC converter.
Bode plot of open control loop
What’s interesting on this plot is the “phase margin” and “gain margin“. The gain margin is the gain for 180deg phase shift and phase margin is the phase difference from 180deg for 0db gain as shown in the plot above. 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 ensures that the DCDC 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 the input voltage is stepped from 10V to 15V. We see that 4ms after the input voltage is stepped the output has recovered to the 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 I_{out}. We see that the output 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 the Steady State operation of DCDC converter @ 1A output.
This graph shows the simulated output voltage ripple and inductor current. We see that output voltage ripple is ~0,6Vpp and the inductor current has a peak current of 2,4A. The inductor we used is rated at max 5,6A DC so it can easily withstand such operating current and without much heating of the coil.
Operating point data (Vin=13V, Iout=1A)
Operating Values  
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  
Current Analysis  
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, peaktopeak 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
The output voltage is configured by R1, R2 according to the following expression (Vref=1,23V)
V_{OUT} = V_{REF} (1 + R1/R2)
If R2 has a value between 1k and 5k we can use this expression to calculate R1:
R1 = R2 (V_{OUT}/V_{REF} − 1)
For better thermal response and stability it is suggested to use 1% metal film resistors.
Measurements
LM2585ADJ Datasheet
lm2585
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