High Side Current Monitor

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Oswald Marshall
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1 H78 High Side Current Monitor 8 to 45, oltage Gain of Features Supply voltage 8 to 45 oltage output device Typical gain ±% Max 5 Fast rise and fall times, 7ns to 2µs Maximum quiescent current 5µA 5-Lead SOT-23 Package Applications SMPS current monitor Battery current monitor Motor controls Telecom General Description The H78 high side current monitor IC transfers a highside current measurement voltage to its ground referenced output with an accurate voltage gain of one. The measurement voltage typically originates at a current sense resistor which is located in a high side circuit, such as the positive supply line. This monitor IC features a very wide input voltage range, high accuracy of transfer ratio, small size, low component count, low power consumption, ease of use, and low cost. Offl ine, battery and portable applications can be served equally well due to the wide input voltage range and the low quiescent current of the H78. Typical Application Circuit I SENSE 8 to 45 Input = H78 GND OUT

H78 Ordering Information Device H78 -G indicates package is RoHS compliant ( Green ) Package Option 5-Lead SOT-23 H78K-G Absolute Maximum Ratings Parameter alue, -.5 to +45 -.

2 H78 Ordering Information Device H78 -G indicates package is RoHS compliant ( Green ) Package Option 5-Lead SOT-23 H78K-G Absolute Maximum Ratings Parameter alue, -.5 to to + 2 I Operating ambient temperature Operating junction temperature Storage temperature -.5 to +5. ±ma C to +7 C -4 C to +25 C -65 C to +5 C Absolute maximum ratings are those values beyond which damage to the device may occur. Functional operation under these conditions is not implied. Continuous operation of the device at the absolute rating level may affect device reliability. All voltages are referenced to device ground. Thermal Resistance Package 5-Lead SOT-23 Note: Thermal testboard per JEDEC JESD5-7 Pin Configuration OUT GND NC 5-Lead SOT-23 (top view) θ ja 9 O C/W Electrical Characteristics (T A = 25 C unless otherwise specifi ed) Symbol Parameter Min Typ Max Units Conditions Supply Supply voltage * --- I Q Quiescent supply current µa - = 8 to 45, = Input and Output R OUT OUT pin output resistance kω Output voltage Dynamic Characteristics - - = = = = step 5 to 5 t RISE Output rise time, % to 9% µs step to 5 t FALL Output fall time, 9% to % µs - step 5 to Notes:. Referenced to GND. 2. = - alues of parameters marked with a * apply over the full temperature range. 2

3 H78 Typical Performance Characteristics SEN SE (Maxim um ) I Q µa 5 SEN SE = SEN SE db / (Gain) SEN SE 5 Top Curv e: 4 Bottom Curv e: Hz Frequency db / Top Cur v e: 4 (CMRR) -2 Bottom Curv e: Hz Frequency SEN SE 5 3

4 H78 Typical Performance Characteristics (cont.) Step Response (Pos ) Step Response (Neg ) SEN SE 2 SEN SE µs Time 2 µs Time Pin Description Pin Name Description This pin is an input sense pin that connects to the negative (-) side of sense resistor, R SENSE. The negative side denotes the system load end of the sense resistor. 2 N/C No Connection 3 Positive supply. Input voltage range is 8. to 45DC. This pin is an input sense pin that connects to the positive (+) side of sense resistor, R SENSE. 4 GND This pin is connected to the GND or return of the system. 5 OUT Output oltage referenced to GND terminal. The voltage at OUT will be equal to. The output impedance of this pin is typically 3.6 kω. 4

5 H78 Block Diagram Principle of Operation R SENSE I SENSE The operational amplifi er and MOSFET forces the voltage across R A to track within the limit of the offset voltage of the opamp, i.e. RA =. R P The current through R A returns to ground through R B. R A and R B are integrated, exhibiting tight matching and excellent tracking. By design, R A and R B have the same resistance. Consequently, RA is equal to RB, resulting in a voltage gain of. R A OUT Pin Loading Effects Bias Circuits Note that the OUT pin has a typical output resistance of 3.6kΩ. Loading the output causes the voltage gain to drop and rise/fall times to increase. For example, assuming an output resistance of 3.6kΩ, the load resistance should exceed 3.6MΩ in order to limit the drop in gain to part in. R B OUT Again assuming an output resistance of 3.6kΩ, capacitive loading of 3pF results in a response pole with a time constant of ns, not yet high enough to materially affect the output rise and fall times (about 7ns). Application Information General The H78 high side current monitor IC features accurate current sensing, small size, low component count, low power consumption, exceptional input voltage range, ease of use and low cost. The part typically performs the measurement of line or load current for overcurrent protection, metering or current regulation. High side current sensing, as opposed to ground referenced or low side current sensing, is desirable or required when: GND H78 The current to be measured does not fl ow in a circuit associated with ground. The measurement at ground level can lead to ambiguity due to changes in the grounding arrangement during fi eld use. The introduction of a sense resistor in the system ground is undesirable due to issues with safety, EMI, or signal degradation due to common impedance coupling. Sense Resistor Considerations Choose a sense resistor that will not exceed 5 during normal operating conditions. Limit the power dissipation in the sense resistor to whatever is practical; a high sense voltage benefi ts accuracy, but increases power dissipation. Consider the use of Kelvin connections for applications where considerable voltage drops may occur in the PCB traces that carry the current to be measured to the sense resistor. A layout pattern that minimizes voltage across the sense lines is shown below. + - R SENSE Choose a low inductance type sense resistor if preservation of bandwidth is important. The use of Kelvin connections helps by excluding the inductive voltage drop across the traces leading to the sense resistor. The inductive voltage drop may be substantial when operating at high frequencies. A trace or component inductance of just nh contributes an impedance of 6.2mΩ at khz, which constitutes a 6% error when using a mω sense resistor. 5

6 H78 Transient Protection Add a protection resistor (R P ) in series with the pin if can exceed 5 in a positive sense or 6 in a negative sense, whether in a steady state or in transient conditions. A large may occur during system startup or shutdown due to the charging and discharging of bulk storage capacitors. may be large due to fault conditions, such as a short circuit condition, or a broken or missing sense resistor. An internal 5 zener diode with a current rating of ma protects the sense amplifi er inputs. The block diagram shows the orientation of this diode. The zener diode provides clamping at 5 for a positive and at 6 for a negative. Under worst case conditions, limit the zener current to ma. A kω resistor limits the zener diode current to 4.5mA when is 45, whether positive or negative. Note that the protection resistor may affect the bandwidth. The resistor forms an RC network with the trace and pin capacitance at the pin. A capacitance of 5pF results in a time constant of 5ns. The protection resistor may cause an offset due to bias current at the input. Under worst case bias current (na), a kω protection resistor could cause an offset of µ or.2% full scale. Note that the bias current is nominally zero as the is a high impedance CMOS input. 5-Lead SOT-23 (K) Package Outline ±.5.6 ± ±.2.9 BSC Top iew.95 BSC Lead Detail.25 BSC (Gauge Plane).5 ±.5.45 MAX -.5 Side iew Front iew Notes:. All dimensions in millimeters; angles in degrees (The package drawing(s) in this data sheet may not refl ect the most current specifi cations. For the latest package outline information go to Doc.# DSFP-H78 A276 6

Supertex inc. HV7801 HV7801. High Side Current Monitor 8.0 to 450V Voltage Gain of 5. Features. General Description. Applications

Supertex inc. HV7801 HV7801. High Side Current Monitor 8.0 to 450V Voltage Gain of 5. Features. General Description. Applications High Side Current Monitor 80 to 450V Voltage Gain of 5 Features Supply voltage 8V to 450V Voltage output device Typical gain 50±1% Max 500mV Fast rise and fall time, 700ns to 20µs Maximum quiescent current