Circuit Note CN-0241 Devices Connected/Referenced 30 V, Micropower, Overvoltage Protection, Circuits from the Lab reference circuits are engineered and ADA4096-2 Rail-to-Rail Input/Output Amplifier tested for quick and easy system integration to help solve todays analog, mixed-signal, and RF design challenges. For more AD7920 250 kSPS, 12-bit, 250 kSPS ADC in 6-Lead SC70 information and/or support, visit www.analog.com/CN0241. High Accuracy Ultralow IQ 500 mA anyCAP ADP3336 Adjustable Low Dropout Linear Regulator High-Side Current Sensing with Input Overvoltage Protection in Figure 1, uses the overvoltage protected ADA4096-2 op amp EVALUATION AND DESIGN SUPPORT connected as a difference amplifier to monitor the high-side Circuit Evaluation Boards current. The ADA4096-2 has input overvoltage protection, CN-0241 Circuit Evaluation Board (EVAL-CN0241-SDPZ) without phase reversal or latch-up, for voltages of 32 V higher System Demonstration Platform (EVAL-SDP-CB1Z) than and lower than the supply rails. Design and Integration Files Schematics, Layout Files, Bill of Materials The circuit is powered by the ADP3336 adjustable low dropout 500 mA linear regulator, which can also be used to supply power to CIRCUIT FUNCTION AND BENEFITS other parts of the system, if desired. Its input voltage can range High-side current monitors are likely to encounter overvoltage from 5.2 V to 12 V when set for a 5 V output. To save power, conditions from transients or when the monitoring circuits are the current sensing circuit can be powered down by removing connected, disconnected, or powered down. This circuit, shown power to the ADP3336 however, the power source, such as a solar panel, can still operate. +5V 20k *SEE TEXT V +5V* DD V A 1k SCLK +5V/2.5A V R OUT +V 0.1 V AD7920 SDATA INPUT 1k IN V SUPPLY C CS V B GND 20k ADA4096-2 2 OPTIONAL FILTER LOAD SDP BOARD ADP3336 +5V +6V IN OUT IN 210k 1F 470pF 1F FB SD 64.9k GND Figure 1. High-Side Current Sensing with Input Overvoltage Protection (Simplified Schematic: All Connections and Decoupling Not Shown) Rev. A Circuits from the Lab circuits from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices Tel: 781.329.4700 www.analog.com be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 2012 Analog Devices, Inc. All rights reserved. 10155-001CN-0241 Circuit Note V CC R7 R1 R2 R5 I1 I3 D6 D10 Q20 Q5 Q6 Q11 Q12 C2 D9 Q18 D3 Q3 Q4 OUT Q13 Q14 D4 Q7 Q8 C1 Q9 D8 Q17 Q10 1 Q19 +IN OVP Q1 Q2 R6 D2 Q16 I2 Q15 D7 OVP IN R3 R4 D11 D1 V EE Figure 2. ADA4096-2 Simplfied Schemactic This applies voltage to the inputs of the unpowered ADA4096-2 Figure 3 shows the input current limiting capability of the however, no latch-up or damage occurs for input voltages up to ADA4096-2 provided by low RDS internal series FETs (green ON 32 V. If slower throughput rates are required, the AD7920 can curves) compared to using a 5 k external series resistor with also be powered down between samples. The AD7920 draws a an unprotected op amp (red curves). maximum of 5 W when powered down and 15 mW when Figure 3 was generated with the ADA4096-2 in a unity-gain powered up. The ADA4096-2 requires only 120 A under buffer configuration with the supplies connected to GND (or operational conditions. When operating at 5 V, this is only 15 V) and the positive input swept until it exceeds the supplies 0.6 mW. The ADP3336 draws only 1 A in the shutdown mode. by 32 V. In general, input current is limited to 1 mA during positive overvoltage conditions and 200 A during negative undervoltage CIRCUIT DESCRIPTION conditions. For example, at an overvoltage of 20 V, the ADA4096-2 The circuit is a classic high-side current sensing circuit topology input current is limited to 1 mA, providing a current limit with a single sense resistor. The other four resistors (dual 1 k/ equivalent to a series 20 k resistor. 20 k divider) are in a thin film network (for ratio matching) 7 and are used to set the difference amplifier gain. This amplifies 6 the difference between the two voltages seen across the sense 5 resistor and rejects the common-mode voltage. 4 3 VOUT = (VA VB) (20 k/1 k ) 2 Figure 2 shows a simplified schematic of the ADA4096-2. The 1 0 input stage comprises two differential pairs (Q1 to Q4 and Q5 to 1 Q8) operating in parallel. When the input common-mode voltage 2 approaches V 1.5 V, Q1 to Q4 shut down as I1 reaches its CC 3 minimum voltage compliance. Conversely, when the input 4 common-mode voltage approaches VEE + 1.5 V, Q5 to Q8 shut 5 LOW RDS SERIES FET ON 6 down as I2 reaches its minimum voltage compliance. This topology 5k SERIES RESISTOR 7 allows for maximum input dynamic range because the amplifier 48 40 32 24 16 8 0 8 16 24 32 40 48 can function with its inputs at 200 mV outside the rail (at room V (V) IN temperature). Figure 3. Input Current Limiting Capability As with any rail-to-rail input amplifier, V mismatch between OS Figure 3 also shows that the current limiting circuitry is active the two input pairs determines the CMRR of the amplifier. If the whether the amplifier is powered or not. input common-mode voltage range is within 1.5 V of each rail, Figure 3 represents input protection under abnormal conditions transitions between the input pairs are avoided, thus improving only. The correct amplifier operation input voltage range (IVR) the CMRR by approximately 10 dB. is specified in Table 2 to Table 4 of the ADA4096-2 data sheet. The ADA4096-2 inputs are protected from input voltage excursions The AD7920 is a 12-bit, high speed, low power, successive up to 32 V outside each rail. This feature is of particular importance approximation ADC. The part operates from a single 2.35 V to in applications with power supply sequencing issues that can cause 5.25 V power supply and features throughput rates up to 250 kSPS. the signal source to be active before the supplies to the amplifier The part contains a low noise, wide bandwidth track-and-hold are applied. amplifier that can handle input frequencies in excess of 13 MHz. Rev. A Page 2 of 6 INPUT BIAS CURRENT (mA) V = 15V EE V = 0V EE V = +15V CC 10155-002 10155-003