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MAX4172ESA-MAX4172ESA.-MAX4172EUA
Low-Cost / Precision / High-Side Current-Sense Amplifier
_______________General DescriptionThe MAX4172 is a low-cost, precision, high-side current-
sense amplifier for portable PCs, telephones, and other
systems where battery/DC power-line monitoring is criti-
cal. High-side power-line monitoring is especially useful in
battery-powered systems, since it does not interfere with
the battery charger’s ground path. Wide bandwidth
and ground-sensing capability make the MAX4172
suitable for closed-loop battery-charger and general-
purpose current-source applications. The 0V to 32V input
common-mode range is independent of the supply volt-
age, which ensures that current-sense feedback remains
viable, even when connected to a battery in deep dis-
charge.
To provide a high level of flexibility, the MAX4172 func-
tions with an external sense resistor to set the range of
load current to be monitored. It has a current output that
can be converted to a ground-referred voltage with a sin-
gle resistor, accommodating a wide range of battery volt-
ages and currents.
An open-collector power-good output (PG) indicates
when the supply voltage reaches an adequate level to
guarantee proper operation of the current-sense amplifi-
er. The MAX4172 operates with a 3.0V to 32V supply
voltage, and is available in a space-saving, 8-pin µMAX
or SO package.
________________________ApplicationsPortable PCs: Notebooks/Subnotebooks/Palmtops
Battery-Powered/Portable Equipment
Closed-Loop Battery Chargers/Current Sources
Smart-Battery Packs
Portable/Cellular Phones
Portable Test/Measurement Systems
Energy Management Systems
____________________________FeaturesLow-Cost, High-Side Current-Sense Amplifier±0.5% Typical Full-Scale Accuracy Over
Temperature 3V to 32V Supply Operation0V to 32V Input Range—Independent of
Supply Voltage800kHz Bandwidth [VSENSE= 100mV (1C)]
200kHz Bandwidth [VSENSE= 6.25mV (C/16)]Available in Space-Saving µMAX and SO
Packages
MAX4172
Low-Cost, Precision, High-Side
Current-Sense Amplifier
__________________Pin Configuration
__________Typical Operating Circuit19-1184; Rev 0; 12/96
*Contact factory for availability.
MAX4172
Low-Cost, Precision, High-Side
Current-Sense Amplifier
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(V+ = +3V to +32V; RS+, RS- = 0V to 32V; TA= TMINto TMAX; unless otherwise noted. Typical values are at V+ = +12V, RS+ = 12V, = +25°C.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
V+, RS+, RS-, PG...................................................-0.3V to +36V
OUT..............................................................-0.3V to (V+ + 0.3V)
Differential Input Voltage, VRS+- VRS-............................±700mV
Current into Any Pin..........................................................±50mA
Continuous Power Dissipation (TA= +70°C)
SO (derate 5.88mW/°C above +70°C)..........................471mW
µMAX (derate 4.10mW/°C above +70°C).....................330mW
Operating Temperature Range
MAX4172E_A....................................................-40°C to +85°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10sec).............................+300°C
ELECTRICAL CHARACTERISTICS (continued)(V+ = +3V to +32V; RS+, RS- = 0V to 32V; TA= TMINto TMAX; unless otherwise noted. Typical values are at V+ = +12V, RS+ = 12V, = +25°C.)
MAX4172
Low-Cost, Precision, High-Side
Current-Sense AmplifierSUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX4172-01
V+ (V)
SUPPLY CURRENT (30
OUTPUT ERROR
vs. SUPPLY VOLTAGE
MAX4172-03
V+ (V)
ERROR (%)30
C/16 LOAD OUTPUT ERROR
vs. SUPPLY VOLTAGE
MAX4172-02
V+ (V)
ERROR (%)30
__________________________________________Typical Operating Characteristics
(V+ = +12V, VRS+= 12V, ROUT= 1kΩ, TA= +25°C, unless otherwise noted.)
Note 1:6.25mV = 1/16 of typical full-scale sense voltage (C/16).
Note 2:Valid operation of the MAX4172 is guaranteed by design when PGis low.
OUTPUT ERROR
vs. COMMON-MODE VOLTAGE
MAX4172-06
VRS- (V)
ERROR (%)18
V+ THRESHOLD FOR PG OUTPUT LOW
vs. TEMPERATURE
MAX4172-07
TEMPERATURE (°C)
V+ TRIP THRESHOLD (V)35
0.1m10m100m1m1
ERROR vs. SENSE VOLTAGEMAX4172-04
VSENSE (V)
ERROR (%)
POWER-SUPPLY REJECTION RATIO
vs. FREQUENCY
MAX4172-05
POWER-SUPPLY FREQUENCY (kHz)
ERROR (%)15
MAX4172
Low-Cost, Precision, High-Side
Current-Sense Amplifier
____________________________Typical Operating Characteristics (continued)(V+ = +12V, VRS+= 12V, ROUT= 1kΩ, TA= +25°C, unless otherwise noted.)
MAX4172
Low-Cost, Precision, High-Side
Current-Sense Amplifier
______________________________________________________________Pin Description
____________________________Typical Operating Characteristics (continued)(V+ = +12V, VRS+= 12V, ROUT= 1kΩ, TA= +25°C, unless otherwise noted.)
_______________Detailed DescriptionThe MAX4172 is a unidirectional, high-side current-sense
amplifier with an input common-mode range that is inde-
pendent of supply voltage. This feature not only allows
the monitoring of current flow into a battery in deep dis-
charge, but also enables high-side current sensing at
voltages far in excess of the supply voltage (V+).
The MAX4172 current-sense amplifier’s unique topolo-
gy simplifies current monitoring and control. The
MAX4172’s amplifier operates as shown in Figure 1.
The battery/load current flows through the external
sense resistor (RSENSE), from the RS+ node to the RS-
node. Current flows through RG1and Q1, and into the
current mirror, where it is multiplied by a factor of 50
before appearing at OUT.
To analyze the circuit of Figure 1, assume that current
flows from RS+ to RS-, and that OUT is connected to
GND through a resistor. Since A1’s inverting input is
high impedance, no current flows though RG2(neglect-
ing the input bias current), so A1’s negative input is
equal to VSOURCE- (ILOADx RSENSE). A1’s open-loop
gain forces its positive input to essentially the same
voltage level as the negative input. Therefore, the drop
across RG1equals ILOADx RSENSE. Then, since IRG1
MAX4172flows through RG1, IRG1x RG1= ILOADx RSENSE. The
internal current mirror multiplies IRG1by a factor of 50
to give IOUT= 50 x IRG1. Substituting IOUT/ 50 for IRG1,
(IOUT/ 50) x RG1 = ILOADx RSENSE, or:
IOUT= 50 x ILOADx (RSENSE/ RG1)
The internal current gain of 50 and the factory-trimmed
resistor RG1combine to result in the MAX4172
transconductance (Gm) of 10mA/V. Gmis de-
fined as being equal to IOUT/ (VRS+- VRS-). Since
(VRS+- VRS-) = ILOADx RSENSE, the output current
(IOUT) can be calculated with the following formula:
IOUT= Gmx (VRS+- VRS-) =
(10mA/V) x (ILOADx RSENSE)
Current OutputThe output voltage equation for the MAX4172 is given
below:
VOUT= (Gm) x (RSENSEx ROUTx ILOAD)
where VOUT= the desired full-scale output voltage,
ILOAD= the full-scale current being sensed, RSENSE=
the current-sense resistor, ROUT= the voltage-setting
resistor, and Gm= MAX4172 transconductance
(10mA/V).
The full-scale output voltage range can be set by
changing the ROUTresistor value, but the output volt-
age must be no greater than V+ - 1.2V. The above
equation can be modified to determine the ROUT
required for a particular full-scale range:
ROUT= (VOUT) / (ILOADx RSENSEx Gm)
OUT is a high-impedance current source that can be
integrated by connecting it to a capacitive load.
OutputThe PGoutput is an open-collector logic output that
indicates the status of the MAX4172’s V+ power sup-
ply. A logic low on the PGoutput indicates that V+ is
sufficient to power the MAX4172. This level is tempera-
ture dependent (see Typical Operating Characteristics
graphs), and is typically 2.7V at room temperature. The
internal PG comparator has a 100mV (typical) hystere-
sis to prevent possible oscillations caused by repeated
toggling of the PGoutput, making the device ideal for
power-management systems lacking soft-start capabili-
ty. An internal delay (15µs typical) in the PGcompara-
tor allows adequate time for power-on transients to
settle out. The PGstatus indicator greatly simplifies the
design of closed-loop systems by ensuring that the
components in the control loop have sufficient voltage
to operate correctly.
__________Applications Information
Suggested Component Values
for Various ApplicationsThe Typical Operating Circuitis useful in a wide variety
of applications. Table 1 shows suggested component
values and indicates the resulting scale factors for vari-
ous applications required to sense currents from
100mA to 10A.
Adjust the RSENSEvalue to monitor higher or lower cur-
rent levels. Select RSENSEusing the guidelines and for-
mulas in the following section.
Sense Resistor, RSENSEChoose RSENSEbased on the following criteria:
Voltage Loss:A high RSENSEvalue causes the
power-source voltage to degrade through IR loss.
For minimal voltage loss, use the lowest RSENSE
value.
Low-Cost, Precision, High-Side
Current-Sense Amplifier
