APPLICATION NOTE 105: Current Sense Circuit Collection Negative Voltage This chapter discusses solutions for negative voltage current sensing. To see other chapters in this Application Note, return to the Introduction. Telecom Supply Current Monitor + LOAD IL 5V 48V – 3 7 + RS G2 LT1990 2 – 4 G1 5 6 VOUT 8 1 –77V ≤ VCM ≤ 8V VOUT = VREF – (10 • IL • RS) REF VREF = 4V 4 5 IN OUT LT6650 1 GND FB 2 1nF 174k 20k 1990 AI01 1µF mately 4V by the LT6650. The output signal moves downward from the reference potential in this connection so that a large output swing can be accommodated. The LT1990 is a wide common-mode range difference amplifier used here to amplify the sense resistor drop by 10. To provide the desired input range when using a single 5V supply, the reference potential is set to approxi–48V Hot Swap Controller GND RIN 3× 1.8k IN SERIES 1/4W EACH GND (SHORT PIN) CIN 1µF 1 VIN R1 402k 1% + 9 R2 32.4k 1% EN OV PWRGD UV DRAIN 10 TIMER 3 CT SS 0.33µF C1 10nF CSS 68nF GATE VEE 5 LOAD R3 5.1k LTC4252-1 8 CL 100µF SENSE * 2 7 VOUT RD 1M 6 Q1 IRF530S 4 RC 10Ω CC 18nF RS 0.02Ω –48V * M0C207 This load protecting circuit employs low-side current sensing. The N-MOSFET is controlled to soft-start the load (current ramping) or to disconnect the load in the event of supply or load faults. An internal shunt regulator establishes a local operating voltage. Negative Voltage-1 APPLICATION NOTE 105: Current Sense Circuit Collection –48V Low Side Precision Current Sense The first stage amplifier is basically a complementary form of the “classic” high-side current sense, designed to operate with telecom negative supply voltage. The Zener forms an inexpensive “floating” shunt-regulated supply for the first op amp. The N-MOSFET drain delivers a metered current into the virtual ground of the second stage, configured as a trans-impedance amplifier (TIA). The second op amp is powered from a positive supply and furnishes a positive output voltage for increasing load current. . A dual op amp cannot be used for this implementation due to the different supply voltages for each stage. This circuit is exceptionally precise due to the use of Zero Drift op amps. The scaling accuracy is established by the quality of the user-selected resistors. Smallsignal range is limited by VOL in single-supply operation of the second stage. Fast Compact –48V Current Sense VOUT = 3V – 0.1Ω • ISENSE ISENSE = 0A TO 30A ACCURACY ≈ 3% VOUT Q1 FMMT493 30.1Ω 1% – 3.3k 0805 ×3 + R1 4.7k 1k 1% VS = 3V R1 REDUCES Q1 DISSIPATION LT1797 0.1µF SETTLES TO 1% IN 2µs, 1V OUTPUT STEP BZX84C6V8 VZ = 6.8V –48V SUPPLY (–42V TO –56V) 0.003Ω 1% 3W – ISENSE + This amplifier configuration is essentially the complementary implementation to the classic high-side configuration. The op amp used must support common-mode operation at its lower rail. A “floating” shunt-regulated local supply is provided by the Zener diode, and the transistor provides metered current to an output load resis- Negative Voltage-2 1797 TA01 –48V LOAD tance (1kΩ in this circuit). In this circuit, the output voltage is referenced to a positive potential and moves downward when representing increasing –48V loading. Scaling accuracy is set by the quality of resistors used and the performance of the NPN transistor. APPLICATION NOTE 105: Current Sense Circuit Collection –48V Current Monitor In this circuit an economical ADC is used to acquire the sense resistor voltage drop directly. The converter is powered from a “floating” high-accuracy shunt-regulated supply and is configured to perform continuous conversions. The ADC digital output drives an opto-isolator, level-shifting the serial data stream to ground. For wider supply voltage applications, the 13k biasing resistor may be replaced with an active 4mA current source such as shown to the right. For complete dielectric isolation and/or higher efficiency operation, the ADC may be powered from a small transformer circuit as shown below. Simple Telecom Power Supply Fuse Monitor 47k 5V FUSE STATUS –48V RETURN R1 100k R2 100k MOC207 3 RTN 1 8 OUT F VA 5V SUPPLY A STATUS VB LTC1921 2 47k 4 FUSE B OUT A OUT B SUPPLY A –48V SUPPLY B –48V F1 D1 F2 D2 5V SUPPLY B STATUS 5 6 MOC207 R3 47k 1/4W The LTC1921 provides an all-in-one telecom fuse and supply-voltage monitoring function. Three opto-isolated SUPPLY A STATUS 0 0 1 1 SUPPLY B STATUS 0 1 0 1 OK: WITHIN SPECIFICATION OV: OVERVOLTAGE UV: UNDERVOLTAGE MOC207 FUSE A 47k 7 VA VB OK OK OK UV OR OV UV OR OV OK UV OR OV UV OR OV –48V OUT VFUSE A = VA = VA ≠ VA ≠ VA VFUSE B = VB ≠ VB = VB ≠ VB FUSE STATUS 0 1 1 1* 0: LED/PHOTODIODE ON 1: LED/PHOTODIODE OFF *IF BOTH FUSES (F1 AND F2) ARE OPEN, ALL STATUS OUTPUTS WILL BE HIGH SINCE R3 WILL NOT BE POWERED = LOGIC COMMON status flags are generated that indicate the condition of the supplies and the fuses. Negative Voltage-3