EDNE November 2012 - (Page 68)

designideas with only one dc/dc converter by adding a simple current-mirror scheme to generate the constant current required for each LED channel. The IC used in the figure is the TN1000, a 100-mA current-mode step-down dc/dc converter from Technor Semiconductor (Reference 1). The voltage step-down stage consists of a 12-μH inductor and a 22-μF capacitor. The first LED channel for D1 is driven by a regulated current of 17 mA, which is equal to the IC’s 0.8V reference voltage divided by R1. The voltage across C3 will be regulated to whatever voltage is required to support the voltage across D1 and R4, as well as the 0.8V across R1. Emitter follower Q3 drives the bases of Q1 and Q2, which mirror the 17 mA of D1. The VBE drops of emitter followers Q1 and Q2 are similar to and compensate for the VBE drop of Q3, so the voltage across R5 and R6 is also a constant 0.8V, and D2 and D3 are driven with a constant 17 mA. R4 is set so that the voltage across C3 is high enough such that Q 1 and Q2 do not saturate. R4 should be set so that the voltage across C3 can support the highest LED voltage plus approximately 255 mV to keep Q1 and Q2 from saturating.EDN RefeRence “TN1000 100-mA Monolithic, Synchronous Step-Down DC-DC Converter,” Technor Semiconductor, July 2011, http://bit.ly/MMw1WY. 1 Sense automobile high-side current with discrete components Larry Beaty, Beatys Mills, Comer, Georgia ↘ This Design Idea came about as a range of 25A in for 5V out. This setup result of my not having access to works nicely with an analog input to the those wonderful new ICs that sense cur- microprocessor. rent. I needed a discrete circuit that I The two zener diodes protect the could build easily but that would still be circuit from the automobile electrical as accurate as the new ICs. This circuit system. Such systems have been known seems to do the job. to spike to −90V. Q2 is the first current amplifier; it has SOURCE LOAD a gain of 6.2 (Figure R3A 1). Q1 is the temper0.01 ature-compensation 3W amplifier controlled 24A=5VOUT SMT 4527 13.6V b y I C 1B, w h i c h 13.5178V R3B keeps the Q1 collecR4 R2 R1 0.02 1.78k 1.78k 1.78k tor voltage constant 3W G=6.2 no matter what the 2N3905 temperature does to D1 D2 12.12V 1N4749 the circuit. The refQ1 Q2 1N4749 24V erence voltage for C10 24V 2N3905 0.1 the circuit is the 5V 10V 4.68V R5 system supply. The 5.62k G=4.905 R8 voltages noted on 100k 3 + 8 the schematic are as IC1A 1 VOUT 5.18V R9 built. LM158 2.5V 5 100k 4.31V 2 + IC1A differentially − 4 IC1B 7 amplifies the Q1 and LM158 R6 R7 7.2V R10 Q2 collector voltag6 10k − 10k 499k es. The op amp has a gain of 4.9. R11 VREF 499k 5.18V R3 comprises two surface-mount power resistors, stacked Figure 1 Q1 and Q2 convert the high-side voltage drop across current-sense resistor R3 into a voltage that one atop the other. is within the common-mode range of IC2. The circuit has a If you want to get critical, match R6 and R7; more critical again, also match R1 and R4. I didn’t do this step, and the mismatch did not seem to affect the operation. All resistors except R3 are 1% 0805 SMT. Observe sufficient copper weight and width on your PCB traces for maximum current-carrying capacity, and be sure to use Kelvin connections to R3. This circuit ran slightly warm to the touch at 25A.EDN 68 EDN EuropE | NOVEMBER 2012 www.edn-europe.com http://www.bit.ly/MMw1WY http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE November 2012

Digi-Key
Cover
Agilent Technologies
Contents
International Rectifier
Microchip
Digi-Key
Masthead
EDN comment
Microchip
Pulse
Analog Devices
Maxim Integrated
Maxim Integrated
Texas Instruments
Silicon Laboratories
Test & Measurement World
FTDI
Digi-Key
Strategies for extending product reliability
Managing the 8- to 32-bit processor migration
PCIM
Pickering Interfaces
Mechatronics in Design
Baker's Best
Digi-Key
Emerson
30 years of DSP
Hirose
Vicor
The nuances of variable-frequency drives
Digi-Key
National Instruments
Volt’s battery-stack manager aids Chevy’s drive
Digi-Key
EDNE
Design Ideas
RS Components
RS Components
RS Components
Product Roundup
Tales from the Cube
RS Components

EDNE November 2012

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