Datasheet LT6118 (Analog Devices) - 10

LT6118
applicaTions inForMaTion
Connecting an external resistor, R Note that V IN, between SENSEHI SENSE(MAX) can be exceeded without damag- and V ing the amplifier, however, output accuracy will degrade SUPPLY forces a potential, VSENSE, across RIN. A corresponding current, I as V OUTA, equal to VSENSE/RIN, will SENSE exceeds VSENSE(MAX), resulting in increased flow through R output current, I IN. The high impedance inputs of the sense OUTA. amplifier do not load this current, so it will flow through an internal MOSFET to the output pin, OUTA.
Selection of External Current Sense Resistor
The output current can be transformed back into a voltage The external sense resistor, RSENSE, has a significant effect by adding a resistor from OUTA to V–(typically ground). on the function of a current sensing system and must be The output voltage is then: chosen with care. V First, the power dissipation in the resistor should be consid- OUT = V– + IOUTA • ROUT ered. The measured load current will cause power dissipation where ROUT = R1 + R2 as shown in Figure 2. as well as a voltage drop in RSENSE. As a result, the sense
Table 1. Example Gain Configurations
resistor should be as small as possible while still providing
GAIN R
the input dynamic range required by the measurement. Note
IN ROUT VSENSE FOR VOUT = 5V IOUTA AT VOUT = 5V
20 499Ω 10k 250mV 500µA that the input dynamic range is the difference between the maximum input signal and the minimum accurately repro- 50 200Ω 10k 100mV 500µA duced signal, and is limited primarily by input DC offset of the 100 100Ω 10k 50mV 500µA internal sense amplifier of the LT6118. To ensure the specified
Useful Equations
performance, RSENSE should be small enough that VSENSE does not exceed V Input Voltage: V SENSE(MAX) under peak load conditions. As SENSE = ISENSE •RSENSE an example, an application may require the maximum sense VOUT ROUT voltage be 100mV. If this application is expected to draw 2A Voltage Gain: = VSENSE RIN at peak load, RSENSE should be set to 50mΩ. IOUTA RSENSE Once the maximum RSENSE value is determined, the mini- Current Gain: = I RIN mum sense resistor value will be set by the resolution or SENSE dynamic range required. The minimum signal that can be VSUPPLY + RIN RSENSE VSENSE LT6118 – 1 SENSELO SENSEHI 8 LOAD + – VSENSE C1 ISENSE = R V+ V– SENSE 7 V+ 2 LE VLE OUTA 6 V V PULLUP OUT V+ IOUTA R2* CL – INC 5 RC OVERCURRENT 3 OUTC FLAG R1* C + 400mV LC REFERENCE V– 6118 F02 *R 4 OUT = R1 + R2
Figure 2. Typical Connection
6118f 10 For more information www.linear.com/LT6118 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Typical Application Related Parts