General Description
The MAX9938 high-side current-sense amplifier offers
precision accuracy specifications of VOS less than
500μV (max) and gain error less than 0.5% (max).
Quiescent supply current is an ultra-low 1μA. The
MAX9938 fits in a tiny, 1mm x 1mm UCSP™ package
size or a 5-pin SOT23 package, making the part ideal for
applications in notebook computers, cell phones, PDAs,
and all battery-operated portable devices where accura-
cy, low quiescent current, and small size are critical.
The MAX9938 features an input common-mode voltage
range from 1.6V to 28V. These current-sense amplifiers
have a voltage output and are offered in four gain versions:
25V/V (MAX9938T), 50V/V (MAX9938F), 100V/V
(MAX9938H), and 200V/V (MAX9938W).
The four gain selections offer flexibility in the choice of
the external current-sense resistor. The very low 500μV
(max) input offset voltage allows small 25mV to 50mV
full-scale VSENSE voltage for very low voltage drop at
full-current measurement.
The MAX9938 is offered in tiny 4-bump, UCSP (1mm x
1mm x 0.6mm footprint), 5-pin SOT23, and 6-pin μDFN
(2mm x 2mm x 0.8mm) packages specified for operation
over the -40°C to +85°C extended temperature range.
Applications
Cell Phones
PDAs
Power Management Systems
Portable/Battery-Powered Systems
Notebook Computers
Features
oUltra-Low Supply Current of 1µA (max)
oLow 500µV (max) Input Offset Voltage
oLow < 0.5% (max) Gain Error
oInput Common Mode: +1.6V to +28V
oVoltage Output
oFour Gain Versions Available
25V/V (MAX9938T)
50V/V (MAX9938F)
100V/V (MAX9938H)
200V/V (MAX9938W)
oTiny 1mm x 1mm x 0.6mm, 4-Bump UCSP,
5-Pin SOT23, or 2mm x 2mm x 0.8mm, 6-Pin
µDFN Packages
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
________________________________________________________________
Maxim Integrated Products
1
MAX9938T/F/H/W
MAX9938T/F/H/W
54
132
RS+ RS-
GND OUTGND
SOT23
UCSP
TOP VIEW
(BUMPS ON BOTTOM)
B1 B2
A1
GND
RS+
OUT
RS-A2 1
2
3
6
5
4
RS-
N.C.
RS+
MAX9938FELT
μDFN
TOP VIEW
(PADS ON BOTTOM)
OUT
N.C.
GND
DRAWINGS NOT TO SCALE
Pin Configurations
19-4110; Rev 6; 1/11
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
EVALUATION KIT
AVAILABLE
UCSP is a trademark of Maxim Integrated Products, Inc.
Ordering Information
+
Denotes a lead(Pb)-free/RoHS-compliant package.
G45 indicates protective die coating.
Note: All devices are specified over the -40°C to +85°C
extended temperature range.
PART PIN-
PACKAGE
GAIN
(V/V)
TOP
MARK
MAX9938TEBS+G45 4 UCSP 25 +AGD
MAX9938FEBS+G45 4 UCSP 50 +AGE
MAX9938HEBS+G45
4 UCSP 100 +AGF
MAX9938WEBS+G45
4 UCSP 200 +AGI
MAX9938TEUK+ 5 SOT23 25 +AFFB
MAX9938FEUK+ 5 SOT23 50 +AFFC
MAX9938HEUK+ 5 SOT23 100 +AFFD
MAX9938WEUK+ 5 SOT23 200 +AFGZ
MAX9938FELT+ 6 μDFN 50 +ACM
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VRS+ = VRS- = 3.6V, VSENSE = (VRS+ - VRS-) = 0V, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.) (Note 1)
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.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
VRS+ = 5V, TA = +25°C 0.5 0.85
VRS+ = 5V, -40°C < TA < +85°C 1.1
VRS+ = 28V, TA = +25°C 1.1 1.8
Supply Current (Note 2) ICC
VRS+ = 28V, -40°C < TA < +85°C 2.5
μA
Common-Mode Input Range VCM Guaranteed by CMRR , -40°C < TA < +85°C 1.6 28 V
Common-Mode Rejection Ratio CMRR 1.6V < VRS+ < 28V, -40°C < TA < +85°C 94 130 dB
TA = +25°C ±100 ±500
Input Offset Voltage (Note 3) VOS -40°C < TA < +85°C ±600 μV
MAX9938T 25
MAX9938F 50
MAX9938H 100
Gain G
MAX9938W 200
V/V
TA = +25°C ±0.1 ±0.5
M AX 9938T/M AX 9938F/
MAX9938H - 40°C < TA < + 85°C ±0.6
TA = +25°C ±0.1 ±0.7
Gain Error (Note 4) GE
MAX9938W - 40°C < TA < + 85°C ±0.8
%
MAX9938T/F/H 7.0 10 13.2
Output Resistance ROUT (Note 5) MAX9938W 14.0 20 26.4 kΩ
Gain = 25 1.5 15
Gain = 50 3 30
Gain = 100 6 60
OUT Low Voltage VOL
Gain = 200 12 120
mV
OUT High Voltage VOH VOH = VRS- - VOUT (Note 6) 0.1 0.2 V
VSENSE = 50mV, gain = 25 125
VSENSE = 50mV, gain = 50 60
VSENSE = 50mV, gain = 100 30
Small-Signal Bandwidth
(Note 5) BW
VSENSE = 50mV, gain = 200 15
kHz
Output Settling Time tS1% final value, VSENSE = 50mV 100 μs
RS+, RS- to GND....................................................-0.3V to +30V
OUT to GND .............................................................-0.3V to +6V
RS+ to RS- ...........................................................................±30V
Short-Circuit Duration: OUT to GND ..........................Continuous
Continuous Input Current (Any Pin)..................................±20mA
Continuous Power Dissipation (TA= +70°C)
4-Bump UCSP (derate 3.0mW/°C above +70°C).........238mW
5-Pin SOT23 (derate 3.9mW/°C above +70°C)............312mW
6-Pin μDFN (derate 4.5mW/°C above +70°C) .............358mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range ............................-65°C to +150°C
Lead Temperature (excluding UCSP, soldering, 10s).....+300°C
Soldering Temperature (reflow) .......................................+260°C
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
_______________________________________________________________________________________ 3
0
10
5
20
15
25
30
-0.4 -0.3 -0.2 -0.1 0 0.2 0.30.1 0.4
INPUT OFFSET VOLTAGE HISTOGRAM
MAX9938 toc01
INPUT OFFSET VOLTAGE (mV)
N (%)
0
10
5
20
15
25
30
-0.4 -0.3 -0.2 -0.1 0 0.2 0.30.1 0.4
GAIN ERROR HISTOGRAM
MAX9938 toc02
GAIN ERROR (%)
N (%)
SUPPLY CURRENT
vs. TEMPERATURE
MAX9938 toc03
TEMPERATURE (°C)
SUPPLY CURRENT (μA)
10-15 35 60
0.4
0.2
0.6
0.8
1.0
1.2
1.4
28V
3.6V
1.8V
0
-40 85
INPUT OFFSET
vs. COMMON-MODE VOLTAGE
MAX9938 toc04
SUPPLY VOLTAGE (V)
INPUT OFFSET (μV)
252010515
-50
-45
-40
-35
-30
-55
030
INPUT OFFSET
vs. TEMPERATURE
MAX9938 toc05
INPUT OFFSET (μV)
20
10
30
40
50
60
0
TEMPERATURE (°C)
10-15 35 60-40 85
SUPPLY CURRENT
vs. COMMON-MODE VOLTAGE
MAX9938 toc06
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (μA)
15 2010 30525
1.0
0.8
0.6
0.4
0.2
1.2
1.4
0
0
Typical Operating Characteristics
(VRS+ = VRS- = 3.6V, TA= +25°C, unless otherwise noted.)
ELECTRICAL CHARACTERISTICS (continued)
(VRS+ = VRS- = 3.6V, VSENSE = (VRS+ - VRS-) = 0V, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.) (Note 1)
Note 1: All devices are 100% production tested at TA= +25°C. All temperature limits are guaranteed by design.
Note 2: VOUT = 0. ICC is the total current into RS+ plus RS- pins.
Note 3: VOS is extrapolated from measurements for the gain-error test.
Note 4: Gain error is calculated by applying two values of VSENSE and calculating the error of the slope vs. the ideal:
Gain = 25, VSENSE is 20mV and 120mV.
Gain = 50, VSENSE is 10mV and 60mV.
Gain = 100, VSENSE is 5mV and 30mV.
Gain = 200, VSENSE is 2.5mV and 15mV.
Note 5: The device is stable for any external capacitance value.
Note 6: VOH is the voltage from VRS- to VOUT with VSENSE = 3.6V/gain.
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
4 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(VRS+ = VRS- = 3.6V, TA= +25°C, unless otherwise noted.)
VOUT vs. VSENSE
(SUPPLY = 1.6V)
MAX9938 toc10
VSENSE (mV)
VOUT (V)
80604020
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0
0100
G = 100
G = 50
G = 25
SMALL SIGNAL GAIN
vs. FREQUENCY
MAX9938 toc11
FREQUENCY (kHz)
GAIN (dB)
100kHz10Hz 1MHz100Hz 10kHz1kHz
-5
-10
-15
-20
-25
0
5
-30
1Hz
AV = 25V/V
AV = 100V/V
AV = 50V/V
CMRR
vs. FREQUENCY
MAX9938 toc12
FREQUENCY (kHz)
CMRR (dB)
100kHz10Hz 1MHz100Hz 10kHz1kHz
-40
-60
-80
-100
-120
-140
-20
0
-160
1Hz
G = 25
G = 50
G = 100
SMALL-SIGNAL PULSE RESPONSE
(GAIN = 100)
MAX9938 toc13a
20μs/div
VOUT
VSENSE
1V
1.5V
10mV
15mV
SMALL-SIGNAL PULSE RESPONSE
(GAIN = 50)
MAX9938 toc13b
25μs/div
VOUT
VSENSE
1V
1.5V
20mV
30mV
-0.5
-0.3
-0.4
-0.1
-0.2
0
0.1
010155 202530
GAIN ERROR
vs. COMMON-MODE VOLTAGE
MAX9938 toc07
VOLTAGE (V)
GAIN ERROR (%)
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
_______________________________________________________________________________________ 5
Typical Operating Characteristics (continued)
(VRS+ = VRS- = 3.6V, TA= +25°C, unless otherwise noted.)
SMALL-SIGNAL PULSE RESPONSE
(GAIN = 25)
MAX9938 toc13c
25μs/div
VOUT
VSENSE
1V
1.5V
40mV
60mV
LARGE-SIGNAL PULSE RESPONSE
(GAIN = 100)
MAX9938 toc14a
20μs/div
VOUT
VSENSE
1V
3V
10mV
30mV
LARGE-SIGNAL PULSE RESPONSE
(GAIN = 50)
MAX9938 toc14b
25μs/div
VOUT
VSENSE
0.5V
3V
10mV
60mV
LARGE-SIGNAL PULSE RESPONSE
(GAIN = 25)
MAX9938 toc14c
25μs/div
VOUT
VSENSE
0.5V
3V
20mV
120mV
Pin Description
PIN
UCSP SOT23 µDFN NAME FUNCTION
A1 5 4 RS+ External Sense Resistor Power-Side Connection
A2 4 6 RS- External Sense Resistor Load-Side Connection
B1 1, 2 3 GND Ground
B2 3 1 OUT Output Voltage. VOUT is proportional to VSENSE = VRS+ - VRS-.
— — 2, 5 N.C. No Connection. Not internally connected.
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
6 _______________________________________________________________________________________
Detailed Description
The MAX9938 unidirectional high-side, current-sense
amplifier features a 1.6V to 28V input common-mode
range. This feature allows the monitoring of current out
of a battery with a voltage as low as 1.6V. The
MAX9938 monitors current through a current-sense
resistor and amplifies the voltage across that resistor.
The MAX9938 is a unidirectional current-sense amplifier
that has a well-established history. An op amp is used
to force the current through an internal gain resistor at
RS+, which has a value of R1, such that its voltage drop
equals the voltage drop across an external sense resis-
tor, RSENSE. There is an internal resistor at RS- with the
same value as R1to minimize offset voltage. The cur-
rent through R1is sourced by a high-voltage p-channel
FET. Its source current is the same as its drain current,
which flows through a second gain resistor, ROUT. This
produces an output voltage, VOUT, whose magnitude is
ILOAD x RSENSE x ROUT/R1. The gain accuracy is
based on the matching of the two gain resistors R1and
ROUT (see Table 1). Total gain = 25V/V for the
MAX9938T, 50V/V for the MAX9938F, 100V/V for the
MAX9938H, and 200V/V for the MAX9938W. The output
is protected from input overdrive by use of an output
current limiting circuit of 7mA (typical) and a 6V clamp
protection circuit.
GAIN (V/V) R1 (Ω)R
OUT (kΩ)
200 100 20
100 100 10
50 200 10
25 400 10
Table 1. Internal Gain Setting Resistors (Typical Values)
Typical Operating Circuit
VBATT = 1.6V TO 28V
RSENSE
R1
ILOAD
ROUT
R1
GND
OUT
P
MAX9938
RS+ RS-
ADC
LOAD
μC
VDD = 3.3V
10kΩ
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
_______________________________________________________________________________________ 7
Applications Information
Choosing the Sense Resistor
Choose RSENSE based on the following criteria:
Voltage Loss
A high RSENSE value causes the power-source voltage
to drop due to IR loss. For minimal voltage loss, use the
lowest RSENSE value.
OUT Swing vs. V
RS+
and V
SENSE
The MAX9938 is unique since the supply voltage is the
input common-mode voltage (the average voltage at
RS+ and RS-). There is no separate VCC supply voltage
pin. Therefore, the OUT voltage swing is limited by the
minimum voltage at RS+.
VOUT(max) = VRS+ (min) - VSENSE (max) - VOH
and
VSENSE full scale should be less than VOUT/gain at the
minimum RS+ voltage. For best performance with a
3.6V supply voltage, select RSENSE to provide approxi-
mately 120mV (gain of 25V/V), 60mV (gain of 50V/V),
30mV (gain of 100V/V), or 15mV (gain of 200V/V) of
sense voltage for the full-scale current in each applica-
tion. These can be increased by use of a higher mini-
mum input voltage.
Accuracy
In the linear region (VOUT < VOUT(max)), there are two
components to accuracy: input offset voltage (VOS) and
gain error (GE). For the MAX9938, VOS = 500μV (max)
and gain error is 0.5% (max). Use the linear equation:
VOUT = (gain ± GE) x VSENSE ± (gain x VOS)
to calculate total error. A high RSENSE value allows lower
currents to be measured more accurately because off-
sets are less significant when the sense voltage is larger.
Efficiency and Power Dissipation
At high current levels, the I2R losses in RSENSE can be
significant. Take this into consideration when choosing
the resistor value and its power dissipation (wattage)
rating. Also, the sense resistor’s value might drift if it is
allowed to heat up excessively. The precision VOS of
the MAX9938 allows the use of small sense resistors to
reduce power dissipation and reduce hot spots.
Kelvin Connections
Because of the high currents that flow through RSENSE,
take care to eliminate parasitic trace resistance from
causing errors in the sense voltage. Either use a four-
terminal current-sense resistor or use Kelvin (force and
sense) PCB layout techniques.
Optional Output Filter Capacitor
When designing a system that uses a sample-and-hold
stage in the ADC, the sampling capacitor momentarily
loads OUT and causes a drop in the output voltage. If
sampling time is very short (less than a microsecond),
consider using a ceramic capacitor across OUT and
GND to hold VOUT constant during sampling. This also
decreases the small-signal bandwidth of the current-
sense amplifier and reduces noise at OUT.
Input Filters
Some applications of current-sense amplifiers need to
measure currents accurately even in the presence of both
differential and common-mode ripple, as well as a wide
variety of input transient conditions. For example, high-fre-
quency ripple at the output of a switching buck or boost
regulator results in a common-mode voltage at the inputs
of the MAX9938. Alternatively, fast load-current transients,
when measuring at the input of a switching buck or boost
regulator, can cause high-frequency differential sense
voltages to occur at the inputs of the MAX9938, although
the signal of interest is the average DC value. Such high-
frequency differential sense voltages may result in a volt-
age offset at the MAX9938 output.
RV
GI
SENSE OUT
LOAD
=×
(max)
(max)
The MAX9938 allows two methods of filtering to help
improve performance in the presence of input common-
mode voltage and input differential voltage transients.
Figure 1 shows a differential input filter.
The capacitor CIN between RS+ and RS- along with the
resistor RIN between the sense resistor and RS- helps
filter against input differential voltages and prevents
them from reaching the MAX9938.
The corner frequency of this filter is determined by the
choice of RIN, CIN, and the value of the input resis-
tance at RS- (R1). See Table 1 for R1values at the dif-
ferent gain options.
The value of RIN should be chosen to minimize its
effect on the input offset voltage due to the bias current
at RS-. RIN x IBIAS contributes to the input voltage off-
set. IBIAS is typically 0.2μA.
Placing RIN at the RS- input does not affect the gain
error of the device because the gain is given by the
ratio between ROUT and R1at RS+.
Figure 2 shows the input common-mode filter.
Again, the corner frequency of the filter is determined
by the choice of RIN, CIN and is affected by R1.
In this case RIN affects both gain error and input offset
voltage. RIN should be smaller than R1so that it has
negligible effect on the device gain. If, for example, a fil-
ter with RIN = 10Ωand CIN = 1μF is built, then depend-
ing upon the gain selection, the gain error is affected by
either 2.5% (G = 25V/V, R1= 400Ω) or 5% (G = 50V/V,
R1= 200Ω) or 10% (G = 100V/V, R1= 100Ω) or 10%
(G = 200V/V, R1= 100Ω).
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
8 _______________________________________________________________________________________
Figure 2. Input Common-Mode Filter
RSENSE
CIN CIN
RIN
RIN
RS-
OUT
GND
RS+
MAX9938
LOAD
RSENSE
CIN
RIN
RS-
OUT
GND
RS+
MAX9938
LOAD
Figure 1. Differential Input Filter
MAX9938
VBATT = 1.6V TO 28V
RSENSE
R1
ILOAD
ROUT
R1
GND
P
MAX9938
R1
ROUT
R1
GND
P
OUT OUT
TO WALL-CUBE
/
CHARGER
MAX9938
RS+ RS-RS- RS+
ADC
ADC
LOAD
μC
VDD = 3.3V
10kΩ10kΩ
Figure 3. Bidirectional Application
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
_______________________________________________________________________________________ 9
Chip Information
PROCESS: BiCMOS
Bidirectional Application
Battery-powered systems may require a precise bidi-
rectional current-sense amplifier to accurately monitor
the battery’s charge and discharge currents.
Measurements of the two separate outputs with respect
to GND yields an accurate measure of the charge and
discharge currents respectively (Figure 3).
UCSP Applications Information
For the latest application details on UCSP construction,
dimensions, tape carrier information, PCB techniques,
bump-pad layout, and recommended reflow tempera-
ture profile, as well as the latest information on reliabili-
ty testing results, refer to the Application Note 1891:
Wafer-Level Packaging (WLP) and Its Applications
available on Maxim’s website at www.maxim-
ic.com/ucsp.
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
10 ______________________________________________________________________________________
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the
package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the
package regardless of RoHS status.
PACKAGE TYPE PACKAGE CODE OUTLINE NO. LAND
PATTERN NO.
2 x 2 UCSP B4+1 21-0117 —
5 SOT23 U5-2 21-0057 90-0174
6 μDFN L622+1 21-0164 90-0004
MAX9938
SOT-23 5L .EPS
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
______________________________________________________________________________________ 11
Package Information (continued)
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the
package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the
package regardless of RoHS status.
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
12 ______________________________________________________________________________________
Package Information (continued)
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the
package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the
package regardless of RoHS status.
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
______________________________________________________________________________________ 13
Package Information (continued)
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the
package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the
package regardless of RoHS status.
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
14
____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2011 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0 4/08 Initial release —
1 9/08 Added μDFN package information 1, 2, 4, 5, 9
2 2/09 Added G45 designation to part number 1
3 10/09 Added Input Filters section and MAX9938W to the data sheet 1, 2, 6–9
4 2/10 Updated EC table and Input Filters section 2, 8
5 8/10 Removed Power-Up Time parameter 2
6 1/11 Corrected error on Figure 2 8