APPLICATION NOTE 105: Current Sense Circuit Collection Current Control This chapter collects a variety of techniques useful in generating controlled levels of current in circuits. To see other chapters in this Application Note, return to the Introduction. 800 mA/1A White LED Current Regulator D2 LED L1 3µH VIN 3.3V TO 4.2V SINGLE Li-Ion VIN LED CURRENT D1 B130 SHDN VS+ LT6100 GND VS– VCC FB 124k VC MMBT2222 4.7µF 6.3V CER 0.030Ω VSW LT3436 LED ON WARNING! VERY BRIGHT DO NOT OBSERVE DIRECTLY 22µF 16V CER 1210 + – VOUT VEE A4 A2 0.1µF 8.2k OPEN: 1A CLOSED: 800mA 4.99k D1: DIODES INC. D2: LUMILEDS LXML-PW09 WHITE EMITTER L1: SUMIDA CDRH6D28-3R0 6100 TA02 The LT6100 is configured for a gain of either 40V/V or 50V/V depending on whether the switch between A2 and VEE is closed or not. When the switch is open (LT6100 gain of 40V/V), 1A is delivered to the LED. When the switch is closed (LT6100 gain of 50V/V), 800mA is deliv- ered. The LT3436 is a boost switching regulator which governs the voltage/current supplied to the LED. The switch “LED ON” connected to the SHDN pin allows for external control of the ON/OFF state of the LED. Bidirectional Current Source Two Terminal Current Regulator +V VCTL 3 7 + LT1990 2 – 4 1 REF –V 6 RSENSE ILOAD ILOAD = VCTL/RSENSE ≤ 5mA EXAMPLE: FOR RSENSE =100Ω, OUTPUT IS 1mA PER 100mV INPUT 1990 AI03 The LT1990 is a differential amplifier with integrated precision resistors. The circuit shown is the classic Howland current source, implemented by simply adding a sense resistor. The LT1635 combines an op amp with a 200mV reference. Scaling this reference voltage to a potential across resistor R3 forces a controlled amount of current to flow from the +terminal to the –terminal. Power is taken from the loop. Current Control-1 APPLICATION NOTE 105: Current Sense Circuit Collection Precision Voltage Controlled Current Source Variable Current Source A basic high-side current source is implemented at the output, while an input translation amplifier section provides for flexible input scaling. A Rail-to-Rail input capability is required to have both amplifiers in one package, since the input stage has common-mode near ground and the second section operates near VCC. Precision Voltage Controlled Current Source with Ground Referred Input and Output Switchable Precision Current Source 5V 3 INPUT 0V TO 3.7V 5 + 1 LTC2050 4 The ultra-precise LTC2053 instrumentation amplifier is configured to servo the voltage drop on sense resistor R to match the command VC. The LTC2053 output capability limits this basic configuration to low current applications. – 4V TO 44V + 4.7µF 2 LT1004-1.2 2k 0.68µF + 5V 1k 3 LT1637 1/2 LTC6943 7 6 TP0610 – 9 1µF R R* IOUT 1k 1µF 10 12 SHDN 11 15 IOUT = 1.2 R e.g., 10mA = 120Ω IOUT = VIN 1000Ω 14 0.001µF OPERATES FROM A SINGLE 5V SUPPLY 6943 • TA01a The LTC6943 is used to accurately sample the voltage across the 1kΩ sense resistor and translate it to a ground reference by charge balancing in the 1µF capacitors. The LTC2050 integrates the difference between the sense voltage and the input command voltage to drive the proper current into load. Current Control-2 *OPTIONAL FOR LOW OUTPUT CURRENTS, R* = R 1637 TA01 This is a simple current-source configuration where the op amp servos to establish a match between the drop on the sense resistor and that of the 1.2V reference. This particular op amp includes a shutdown feature so the current source function can be switched off with a logic command. The 2kΩ pull-up resistor assures the output MOSFET is off when the op amp is in shutdown mode. APPLICATION NOTE 105: Current Sense Circuit Collection Boosted Bidirectional Controlled Current Source Fast Differential Current Source +V 15V VIN1 1k R* 2 – CZT751 VCTL 3 VIN2 7 + LT1990 2 – 4 6 LT1022 3 + 4 R* IOUT = 6 + VIN2 – VIN1 R R* IOUT –15V RL 10µF RSENSE 1 REF ILOAD 1k R* 10pF 7 CZT651 *MATCH TO 0.01% FULL-SCALE POWER BANDWIDTH = 1MHz FOR IOUTR = 8VP-P = 400kHz FOR IOUTR = 20VP-P MAXIMUM IOUT = 10mAP-P IOUTP-P • RL COMMON-MODE VOLTAGE AT LT1022 INPUT = 2 –V ILOAD = VCTL/RSENSE ≤ 100mA EXAMPLE: FOR RSENSE =10Ω, OUTPUT IS 1mA PER 10mV INPUT LT1022 • TA07 1990 AI04 This is a classical Howland bidirectional current source implemented with an LT1990 integrated difference amplifier. The op amp circuit servos to match the RSENSE voltage drop to the input command VCTL. When the load current exceeds about 0.7mA in either direction, one of the boost transistors will start conducting to provide the additional commanded current. This is a variation on the Howland configuration, where load current actually passes through a feedback resistor as an implicit sense resistance. Since the effective sense resistance is relatively large, this topology is appropriate for producing small controlled currents. 1A Voltage-Controlled Current Sink 0A to 2A Current Source The LT1995 amplifies the sense resistor drop by 5V/V and subtracts that from VIN, providing an error signal to an LT1880 integrator. The integrated error drives the PMOSFET as required to deliver the commanded current. This is a simple controlled current sink, where the op amp drives the NMOSFET gate to develop a match between the 1Ω sense resistor drop and the VIN current command. Since the common-mode voltage seen by the op amp is near ground potential, a “single-supply” or Rail-to-Rail type is required in this application. Current Control-3 APPLICATION NOTE 105: Current Sense Circuit Collection Voltage Controlled Current Source Adjustable High-Side Current Source VCC V+ 5V RSENSE 0.2Ω 1k 2.5k LT1004-1.2 0.0033µF – 100Ω 1/2 LT1366 – RP 10k RS 1Ω Q1 MTP23P06 + ILOAD + 40k +IN 5V < VCC < 30V 0A < ILOAD < 1A AT VCC = 5V 0mA < ILOAD < 160mA AT VCC = 30V LTC6101 FOR VIN = 0V TO 5V, IOUT = 500mA TO 0mA Q2 2N4340 IOUT = 100mA/V LT1366 F07 10µF + – 0.2V REF LT3021 RLOAD 24k 1k VIN Adding a current sense amplifier in the feedback loop of an adjustable low dropout voltage regulator creates a simple voltage controlled current source. The range of output current sourced by the circuit is set only by the current capability of the voltage regulator. The current sense amplifier senses the output current and feeds back a current to the summing junction of the regulator’s error amplifier. The regulator will then source whatever current is necessary to maintain the internal reference voltage at the summing junction. For the circuit shown a 0V to 5V control input produces 500mA to 0mA of output current. Current Control-4 The wide-compliance current source shown takes advantage of the LT1366’s ability to measure small signals near the positive supply rail. The LT1366 adjusts Q1’s gate voltage to force the voltage across the sense resistor (RSENSE) to equal the voltage between VDC and the potentiometer’s wiper. A rail-to-rail op amp is needed because the voltage across the sense resistor is nearly the same as VDC. Q2 acts as a constant current sink to minimize error in the reference voltage when the supply voltage varies. At low input voltage, circuit operation is limited by the Q1 gate drive requirement. At high input voltage, circuit operation is limited by the LT1366’s absolute maximum ratings. APPLICATION NOTE 105: Current Sense Circuit Collection Programmable Constant Current Source D45VH10 6V TO 28V 0.1Ω IOUT 0A TO 1A 0.1µF 470Ω LT1121CS8-5 8 IN 0.1µF SHDN 5 OUT 1 + GND 3 0.1µF 1µF 18k SHUTDOWN 1 2 AVG SENSE 0.1µF 8 7 PROG LT1620MS8 3 6 VCC GND VN2222LM 2N3904 22Ω 4 10k 1% IOUT +IN –IN IPROG 5 RPROG IOUT = (IPROG)(10,000) RPROG = 40k FOR 1A OUTPUT LT1620/21 • TA01 shutdown command to the LT1121 powers down the LT1620 and eliminates the base-drive to the current regulation pass transistor, thereby turning off IOUT. The current output can be controlled by a variable resistor (RPROG) connected from the PROG pin to ground on the LT1620. The LT1121 is a low-dropout regulator that keeps the voltage constant for the LT1620. Applying a Snap Back Current Limiting 12V R2 39.2k R1 54.9k VCSRC VCSNK VIN +IN –IN EN R3 2.55k 500mA VCC V+ ISRC ISNK TSD OUT LT1970 SENSE+ – SENSE FILTER – V VEE RS 1Ω IOUT RL IMAX 50mA 0 ILOW –500mA IMAX ≈ VCC • R2 (R1 + R2) • 10 • RS ILOW ≈ VCC • (R2||R3) [R1 + (R2||R3)] • 10 • RS COMMON RG 10k –12V RF 10k 1970 F04 The LT1970 provides current detection and limiting features built-in. In this circuit, the logic flags that are produced in a current-limiting event are connected in a feedback arrangement that in turn reduces the current limit command to a lower level. When the load condition permits the current to drop below the limiting level, then the flags clear and full current drive capability is restored automatically. Current Control-5