DESIGN IDEAS Battery Charger IC Doubles as Current Sensor 5V C2 0.33µF sible to again mirror this signal to provide a ground referenced output. Circuit operation is as follows: The LT1620 operates by producing a voltage between the V CC pin and the AVG pin that is 10× the voltage across sense resistor R5. C2 filters this voltage. An internal op amp has its noninverting input at the AVG pin (pin 8), its inverting input at the PROG pin (pin 7) and its output at the IOUT pin (pin 2). With the circuit connected as shown in Figure 1, this amplifier will force enough current through R4 to make the voltage drop on R4 equal to the voltage across C2. This current is mirrored through R3 and is filtered by C3, producing a clean, ground-referenced, DC output voltage. Resistor R2 cancels a C1 1000pF LT1620 7 AVG PROG 1 6 VCC SENSE 2 4 IN– IOUT 3 5 IN+ GND R5 0.02Ω INPUT R4 10k 1% R2 1.2M Q1 TP0610T R3 31.6k IL 4.0 3.5 R1 100k 8 LOAD small built-in offset in the LT1620’s amplifiers. The output voltage obeys the following relationship: V O = IL (R5 • R3 • 10)/R4. Changing the value of R3 selects different scale factors. The circuit yields excellent linearity over a wide range of loads and input voltages. The curve shown in Figure 2 was measured with the sense resistor referenced to a 5V input source. The curve looks the same even at inputs over 25V, so only one curve is presented. Maximum input voltage is 36V. There is a small offset at no load, but in a typical microprocessor-based data acquisition system, only a simple 2-point calibration is needed to obtain absolute accuracy. OUTPUT C3 0.33µF OUTPUT VOLTAGE (V) It’s always fun to find applications for an IC that its designer never intended. The circuit shown in Figure 1 is such a design. In many cases, a circuit is required to provide a groundreferenced output voltage that is proportional to a measured current. Frequently, the current must be measured with a shunt in the positive rail that may be well above ground and, worse yet, may vary considerably with time. The LT1620 was originally intended as a contr oller for a synchronous buck regulator in battery-charger applications. The normal operating mode for this IC is to mirror a current signal down to a 5V reference supply. By adding a single small-signal MOSFET and a few resistors, it is pos- by Craig Varga 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0 Figure 1. Current sensor schematic New Device Cameos LT1671: A 60ns, 450µA, Single-Supply Comparator with Complementary Outputs and Output Latch The LT1671 is a low power (450µ A), fast (60ns), single-supply comparator designed to operate on either single 5V or ±5V supplies. It has a maximum offset voltage of 2.5mV, complementary TTL compatible outputs and output-latch capability. The wide input-voltage range extends from the bottom supply rail to within 1.5V of Linear Technology Magazine • August 1998 the top supply rail. The LT1671 is made with Linear Technology’s new 6GHz complementary bipolar technology, which results in a dramatically improved speed/power product compared to industry-standard comparators developed in slower NPN-only technologies. These features combine to make the LT1671 well suited for applications such as high performance crystal oscillators, single-supply voltage-tofrequency converters and high speed, 1 2 3 4 LOAD CURRENT (A) 5 6 Figure 2. Transfer function high accuracy level detectors. The LT1671 is offered in SO-8 and is pin compatible with the industry-standard LT1016 and LT1116 comparators. The LTC1596-1: Ultra-Accurate, Low Power, 16-Bit Multiplying, Current-Output DAC Clears to Midscale The latest addition to LTC’s family of 16-bit current-output DACs is the multiplying LTC1596-1. Based on the LTC1596, this DAC features the same true 16-bit performance (DNL and INL, 1LSB maximum), the low glitch im37