Home Projects Power LM2585 -12V to 24V @ 1A Step-up switching regulator

 
 

LM2585 12V to 24V @ 1A Step-up switching regulator
author: Michail Papadimitriou - mixosotenet .gr




 


Schematic




click on schematic for better resolution


Description


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.

Specifications

Vin 10-15V DC 
Vout 24V
Iout 1A
frequency 100KHz
   

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.

PCB Design


3D PCB renderings

 

Eagle CAD PCB layout

 

 manufactured PCB

If you would like to receive a PCB, we can ship you one for 6$ (worldwide shipping) click here to contact us

 

Parts List



Part Value Package Part Number Manufacter
 

C1,C2

33uF, 25V, 1

-

EEV-FC1E330P

Panasonic

C3

0.1uF, 50V, 0

1206

C1206C104J5RACTU

Kemet

C4

1 uF, 25V

1206

ECJ-3YB1E105K

Panasonic

C5,C6

220uF, 35V, 0.15

10x10.2mm

EEV-FC1V221P

Panasonic

D1

0.45 V, 3A, 40V Schottky Diode

-

B340LB-13

Diodes Inc.

IC1

LM2585S-ADJ

 TO-263

LM2585S-ADJ

Texas Instruments

L1

120 uH, 0.04

-

PM2120-121K

JW Miller

R1

28 K

1206

ERJ-8ENF2802V

Panasonic

R2

1.5 K

1206

ERJ-8ENF1501V

Panasonic

R3

1.5 K

1206

ERJ-8ENF1501V

Panasonic

R4 8.2 K 1206 - Panasonic
LED white SMD led 1206/1812 - -

 

Simulation


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)

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, 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


Measurements




Output voltage ripple ~100mV

 

 

 



Downloads


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