Datasheet MAX471 (Maxim) - 9

Precision, High-Side Current-Sense Amplifiers
R RS+ RS- SENSE TO LOAD/CHARGER RS+ RS- POWER MAX471 3V SIGN LOGIC SOURCE TO RG2 RG1 SUPPLY OR 36V BATTERY LOGIC OUT GND SUPPLY TO LOAD/ 100k CHARGER 3V 100k TO MAX472 OUT RS+ RS- 1 SHDN 8 36V RS+ RS- 2 N.C. VCC 7 ROUT 3 RG1 RG2 6
MAX471/MAX472
MAX471 SIGN 4 GND SIGN 5 OUT GND VOUT 1k Figure 3. Paralleling MAX471s to Sense Higher Load Current Figure 4. MAX472 Standard Application Circuit for current summing. A single scaling resistor is
Shutdown
required when summing OUT currents from multiple When SHDN is high, the MAX471/MAX472 are shut devices (Figure 3). Current can be integrated by con- down and consume less than 18µA. In shutdown mode, necting OUT to a capacitive load. SIGN is high impedance and OUT turns off.
SIGN Output __________Applications Information
The current at OUT indicates magnitude. The SIGN out- put indicates the current’s direction. Operation of the
MAX471
SIGN comparator is straightforward. When Q1 (Figures The MAX471 obtains its power from the RS- pin. This 1 and 2) conducts, the output of A1 is high while A2’s includes MAX471 current consumption in the total sys- output is zero. Under this condition, a high SIGN output tem current measured by the MAX471. The small drop indicates positive current flow (from RS+ to RS-). In bat- across RSENSE does not affect the MAX471’s perfor- tery-operated systems, this is useful for determining mance. whether the battery is charging or discharging. The
Resistor Selection
SIGN output may not correctly indicate if the load cur- Since OUT delivers a current, an external voltage gain- rent is such that IOUT is less than 3.5µA. The MAX471’s setting resistor (ROUT to ground) is required at the OUT SIGN output accurately indicates the direction of cur- pin in order to get a voltage. RSENSE is internal to the rent flow for load currents greater than 7mA. MAX471. RG1 and RG2 are factory trimmed for an out- SIGN is an open-collector output (sinks current only), put current ratio (output current to load current) of allowing easy interface with logic circuits powered from 500µA/A. Since they are manufactured of the same any voltage. Connect a 100kΩ pull-up resistor from material and in very close proximity on the chip, they SIGN to the logic supply. The convention chosen for provide a high degree of temperature stability. Choose the polarity of the SIGN output ensures that it draws no ROUT for the desired full-scale output voltage up to RS- current when the battery is being discharged. If current - 1.5V (see the Current Output section). direction is not needed, float the SIGN pin.
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