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MAX9928FAUA+ Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX9928FAUA+ Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 14 page -0.1V to +28V Input Range, Micropower, Uni-/Bidirectional, Current-Sense Amplifiers 8 _______________________________________________________________________________________ Detailed Description The MAX9928F/MAX9929F micropower uni-/bidirectional, current-sense amplifiers feature -0.1V to +28V input common-mode range that is independent of the supply voltage. This wide input voltage range feature allows the monitoring of the current flow out of a power supply dur- ing short-circuit/fault conditions, and also enables high- side current sensing at voltages far in excess of the supply voltage (VCC). The MAX9928F/MAX9929F oper- ate from a 2.5V to 5.5V single supply and draw a low 20µA quiescent supply current. Current flows through the sense resistor, generating a sense voltage VSENSE (Figure 1). The comparator sens- es the direction of the sense voltage and configures the amplifier for either positive or negative sense voltages by controlling the S1 and S2 switches. For positive VSENSE voltage, the amplifier’s inverting input is high impedance and equals VIN - VSENSE. The amplifier’s output drives the base of Q1, forcing its non- inverting input terminal to (VIN - VSENSE); this causes a current to flow through RG1 equal to |VSENSE|/RG1. Transistor Q2 and the current mirror amplify the current by a factor of M. For negative VSENSE voltage, the amplifier’s noninvert- ing input is high impedance and the voltage on RS- ter- minal equals VIN + VSENSE. The amplifier’s output drives the base of Q1 forcing its inverting input terminal to match the voltage at the noninverting input terminal; this causes a current to flow through RG2 equal to |VSENSE|/RG2. Again, transistor Q2 and the current mir- ror amplify the current by a factor of M. +VSENSE vs. -VSENSE The amplifier is configured for either positive VSENSE or negative VSENSE by the SIGN comparator. The com- parator has a built-in offset skew of -1.2mV so that ran- dom offsets in the comparator do not affect the precision of IOUT (VOUT) with positive VSENSE. The comparator has a small amount of hysteresis (typically 0.6mV) to prevent its output from oscillating at the crossover sense voltage. The ideal transfer characteris- tic of IOUT (VOUT) and the output of the comparator (SIGN) is shown in Figure 2. The amplifier VOS is only trimmed for the positive VSENSE voltages (VRS+ > VRS-). The SIGN comparator reconfig- ures the internal structure of the amplifier to work with negative VSENSE voltages (VRS- > VRS+) and the preci- sion VOS trim is no longer effective and the resulting VOS is slightly impacted. See details in the Electrical Characteristics Note 2. The user can choose the direc- tion that needs the best precision to be the direction where VRS+ > VRS-. For example, when monitoring Li+ battery currents, the discharge current should be VRS+ > VRS- to give the best accuracy over the largest dynamic range. When the battery charger is plugged in, the charge current flows in the opposite direction and is usually much larger, and a higher VOS error can be tolerated. See the Typical Operating Circuit. For applications with unidirectional currents (e.g., bat- tery discharge only), the SIGN output can be ignored. Note that as VSENSE increases, the output current (IOUT for the MAX9928 or VOUT/10kΩ for the MAX9929) also increases. This additional current is supplied from VCC. Pin/Bump Description PIN BUMP µMAX UCSP NAME FUNCTION 1 B3 RS- Negative Current-Sense Input. Load-side connection for the external sense resistor. 2 B2 SIGN SIGN Output. Indicates polarity of VSENSE. SIGN = H indicates VRS+ > VRS- SIGN = L indicates VRS+ < VRS- 3 B1 RS+ Positive Current-Sense Input. Power-side connection to the external sense resistor. 4, 5 — N.C. No Connection. Not internally connected. 6A1 VCC Supply Voltage Input. Bypass to GND with a 0.1µF capacitor. 7 A2 GND Circuit Ground 8 A3 OUT Current-Sense Output. MAX9928: Current output (IOUT is proportional to |VSENSE|). MAX9929: Voltage output (VOUT is proportional to |VSENSE|). |
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