General Description
The MAX9938 high-side current-sense amplifier offers
precision accuracy specifications of VOSless 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 accuracy, 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 three gain versions: 25V/V (MAX9938T), 50V/V (MAX9938F), and
100V/V (MAX9938H).
The three 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 V
SENSE
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) and 5-pin SOT23 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
♦ Ultra-Low Supply Current of 1µA (max)
♦ Low 500µV (max) Input Offset Voltage
♦ Low < 0.5% (max) Gain Error
♦ Input Common Mode: +1.6V to +28V
♦ Voltage Output
♦ Three Gain Versions Available
25V/V (MAX9938T)
50V/V (MAX9938F)
100V/V (MAX9938H)
♦ Tiny 1mm x 1mm x 0.6mm, 4-Bump UCSP
or 5-Pin SOT23 Package
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
________________________________________________________________
Maxim Integrated Products
1
Pin Configurations
19-4110; Rev 0; 4/08
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.
UCSP is a trademark of Maxim Integrated Products, Inc.
Ordering Information
+
Denotes a lead-free package.
Note: All devices are specified over the -40°C to +85°C
extended temperature range.
PART
PINPACKAGE
TOP MARK
MAX9938TEBS+
4 UCSP 25 +AGD
MAX9938FEBS+
4 UCSP 50 +AGE
MAX9938HEBS+
4 UCSP 100 +AGF
MAX9938TEUK+
5 SOT23 25 +AFFB
MAX9938FEUK+
5 SOT23 50 +AFFC
MAX9938HEUK+
5 SOT23 100 +AFFD
GAIN (V/V)
(BUMPS ON BOTTOM)
A1
RS+
MAX9938T/F/H
B1 B2
GND
TOP VIEW
UCSP
RS-A2
OUT
RS+ RS-
54
MAX9938T/F/H
132
GND OUTGND
SOT23
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(V
RS+
= V
RS-
= 3.6V, V
SENSE
= (V
RS+
- V
RS-
) = 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.
Note 1: All devices are 100% production tested at TA= +25°C. All temperature limits are guaranteed by design.
Note 2: V
OUT
= 0. ICCis the total current into RS+ plus RS- pins.
Note 3: V
OS
is extrapolated from measurements for the gain-error test.
Note 4: Gain error is calculated by applying two values of V
SENSE
and calculating the error of the slope vs. the ideal:
Gain = 25, V
SENSE
is 20mV and 120mV.
Gain = 50, V
SENSE
is 10mV and 60mV.
Gain = 100, V
SENSE
is 5mV and 30mV.
Note 5: The device is stable for any external capacitance value.
Note 6: V
OH
is the voltage from V
RS-
to V
OUT
with V
SENSE
= 3.6V/gain.
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 (T
A
= +70°C)
4-Bump UCSP (derate 3.0mW/°C above +70°C).........238mW
5-Pin SOT23 (derate 3.9mW/°C above +70°C)............312mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range ............................-65°C to +150°C
Bump Temperature (soldering) Reflow............................+235°C
Lead Temperature (soldering, 10s) .................................+300°C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Supply Current (Note 2) I
Common-Mode Input Range V
Common-Mode Rejection Ratio CMRR 1.6V < V
Input Offset Voltage (Note 3) V
Gain G
Gain Error (Note 4) GE
Output Resistance R
OUT High Voltage V
Small-Signal Bandwidth
(Note 5)
Output Settling Time t
Power-Up Time t
V
= 5V, TA = +25°C 0.5 0.85
RS+
V
= 5V, -40°C < TA < +85°C 1.1
CC
CM
OS
RS+
V
= 28V, TA = +25°C 1.1 1.8
RS+
V
= 28V, -40°C < TA < +85°C 2.5
RS+
Guaranteed by CMRR , -40°C < TA < +85°C 1.6 28 V
RS+
TA = +25°C ±100 ±500
-40°C < TA < +85°C ±600
MAX9938T 25
MAX9938F 50
MAX9938H 100
TA = +25°C ±0.1 ±0.5
OUT
-40°C < T
(Note 5) 7.0 10 13.2 kΩ
A
Gain = 25 1.5 15
Gain = 50 3 30OUT Low Voltage V
OL
Gain = 100 6 60
V
OH
BW
S
ON
= V
OH
RS-
V
V
V
= 50mV, gain = 25 125
SENSE
= 50mV, gain = 50 60
SENSE
= 50mV, gain = 100 30
SENSE
1% final value, V
1% final value, V
< 28V, -40°C < TA < +85°C 94 130 dB
< +85°C ±0.6
- V
(Note 6) 0.1 0.2 V
OUT
= 50mV 100 μs
SENSE
= 50mV 200 μs
SENSE
μA
μV
V/V
%
mV
kHz
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
_______________________________________________________________________________________ 3
Typical Operating Characteristics
(V
RS+
= V
RS-
= 3.6V, TA= +25°C, unless otherwise noted.)
N (%)
INPUT OFFSET (μV)
30
25
20
15
10
-30
-35
-40
-45
-50
INPUT OFFSET VOLTAGE HISTOGRAM
5
0
-0.4 -0.3 -0.2 -0.1 0 0.2 0.30.1 0.4
INPUT OFFSET VOLTAGE (mV)
INPUT OFFSET
vs. COMMON-MODE VOLTAGE
MAX9938 toc01
MAX9938 toc04
GAIN ERROR HISTOGRAM
30
25
20
15
N (%)
10
5
0
-0.4 -0.3 -0.2 -0.1 0 0.2 0.30.1 0.4
GAIN ERROR (%)
INPUT OFFSET
vs. TEMPERATURE
60
50
40
30
INPUT OFFSET (μV)
20
10
MAX9938 toc02
MAX9938 toc05
SUPPLY CURRENT
vs. TEMPERATURE
1.4
1.2
1.0
0.8
0.6
SUPPLY CURRENT (μA)
0.4
0.2
0
-40 85
10-15 35 60
TEMPERATURE (°C)
28V
3.6V
1.8V
SUPPLY CURRENT
vs. COMMON-MODE VOLTAGE
1.4
1.2
1.0
0.8
0.6
SUPPLY CURRENT (μA)
0.4
0.2
MAX9938 toc03
MAX9938 toc06
-55
030
SUPPLY VOLTAGE (V)
252010515
0
GAIN ERROR
vs. COMMON-MODE VOLTAGE
MAX9938 toc07
0.08
0.07
0.06
0.05
0.04
0.03
GAIN ERROR (%)
0.02
0.01
0
-40
0.1
0
-0.1
-0.2
GAIN ERROR (%)
-0.3
-0.4
-0.5
010155 202530
VOLTAGE (V)
10-15 35 60-40 85
TEMPERATURE (°C)
GAIN ERROR
vs. TEMPERATURE
10 60 8535-15
TEMPERATURE (°C)
MAX9938 toc08
0
0
15 2010 30525
SUPPLY VOLTAGE (V)
V
vs. V
OUT
SENSE
(SUPPLY = 3.6V)
4.0
3.5
3.0
2.5
(V)
2.0
OUT
V
1.5
1.0
0.5
0
0 150
G = 50
V
SENSE
G = 100
10050
(mV)
MAX9938 toc09
G = 25
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
4 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(V
RS+
= V
RS-
= 3.6V, TA= +25°C, unless otherwise noted.)
V
OUT
vs. V
SENSE
(SUPPLY = 1.6V)
MAX9938 toc10
V
SENSE
(mV)
V
OUT
(V)
80604020
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0
0 100
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)
GAIN (dB)
100kHz10Hz 1MHz100Hz 10kHz1kHz
-40
-60
-80
-100
-120
-140
-20
0
-160
1Hz
G = 25
G = 50
G = 100
SMALL-SIGNAL PULSE RESPONSE
(G = 100)
MAX9938 toc13a
20μs/div
V
OUT
V
SENSE
1V
1.5V
10mV
15mV
SMALL-SIGNAL PULSE RESPONSE
(G = 50)
MAX9938 toc13b
25μs/div
V
OUT
V
SENSE
1V
1.5V
20mV
30mV
SMALL-SIGNAL PULSE RESPONSE
(G = 25)
MAX9938 toc13c
25μs/div
V
OUT
V
SENSE
1V
1.5V
40mV
60mV
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
_______________________________________________________________________________________ 5
Typical Operating Characteristics (continued)
(V
RS+
= V
RS-
= 3.6V, TA= +25°C, unless otherwise noted.)
LARGE-SIGNAL PULSE RESPONSE
(G = 100)
MAX9938 toc14a
20μs/div
V
OUT
V
SENSE
1V
3V
10mV
30mV
LARGE-SIGNAL PULSE RESPONSE
(G = 50)
MAX9938 toc14b
25μs/div
V
OUT
V
SENSE
0.5V
3V
10mV
60mV
LARGE-SIGNAL PULSE RESPONSE
(G = 25)
MAX9938 toc14c
25μs/div
V
OUT
V
SENSE
0.5V
3V
20mV
120mV
Pin Description
UCSP SOT23
A1 5 RS+ External Sense Resistor Power-Side Connection
A2 4 RS- External Sense Resistor Load-Side Connection
B1 1, 2 GND Ground
B2 3 OUT Output Voltage. V
PIN
NAME FUNCTION
is proportional to V
OUT
SENSE
= V
RS+
- V
RS-
.
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 resistor, R
SENSE
. There is an internal resistor at RS- with the
same value as R
1
to minimize offset voltage. The current 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, R
OUT
. This
produces an output voltage, V
OUT
, whose magnitude is
I
LOAD
x R
SENSE
x R
OUT/R1
. The gain accuracy is
based on the matching of the two gain resistors R1and
R
OUT
(see Table 1). Total gain = 25V/V for the
MAX9938T, 50V/V for the MAX9938F, and 100V/V for
the MAX9938H. The output is protected from input
overdrive by use of an output current limiting circuit of
7mA (typical) and a 6V clamp protection circuit.
Applications Information
Choosing the Sense Resistor
Choose R
SENSE
based on the following criteria:
Voltage Loss
A high R
SENSE
value causes the power-source voltage
to drop due to IR loss. For minimal voltage loss, use the
lowest R
SENSE
value.
Table 1. Internal Gain Setting Resistors
(Typical Values)
Typical Operating Circuit
I
V
BATT
LOAD
= 1.6V TO 28V
R
SENSE
RS+ RS-
R
R
OUT
1
P
10kΩ
GND
GAIN
(V/V)
100 100 10
50 200 10
25 400 10
R
(Ω)
1
R
(kΩ)
OUT
R
1
MAX9938
OUT
VDD = 3.3V
μC
ADC
LOAD
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
_______________________________________________________________________________________ 7
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 VCCsupply voltage
pin. Therefore, the OUT voltage swing is limited by the
minimum voltage at RS+.
V
OUT
(max) = V
RS+
(min) - V
SENSE
(max) - V
OH
and
V
SENSE
full scale should be less than V
OUT
/gain at the
minimum RS+ voltage. For best performance with a
3.6V supply voltage, select R
SENSE
to provide approximately 120mV (gain of 25V/V), 60mV (gain of 50V/V), or
30mV (gain of 100V/V) of sense voltage for the fullscale current in each application. These can be
increased by use of a higher minimum input voltage.
Accuracy
In the linear region (V
OUT
< V
OUT(max)
), there are two
components to accuracy: input offset voltage (V
OS
) and
gain error (GE). For the MAX9938, VOS= 500μV (max)
and gain error is 0.5% (max). Use the linear equation:
V
OUT
= (gain ± GE) x V
SENSE
± (gain x VOS)
to calculate total error. A high R
SENSE
value allows lower
currents to be measured more accurately because offsets are less significant when the sense voltage is larger.
Efficiency and Power Dissipation
At high current levels, the I2R losses in R
SENSE
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 V
OS
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 R
SENSE
,
take care to eliminate parasitic trace resistance from
causing errors in the sense voltage. Either use a fourterminal 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 V
OUT
constant during sampling. This also
decreases the small-signal bandwidth of the currentsense amplifier and reduces noise at OUT.
R
SENSE
V
OUT
=
GI
×
(max)
LOAD
(max)
MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
8 _______________________________________________________________________________________
Bidirectional Application
Battery-powered systems may require a precise bidirectional 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 1).
UCSP Applications Information
For the latest application details on UCSP construction,
dimensions, tape carrier information, PCB techniques,
bump-pad layout, and recommended reflow temperature profile, as well as the latest information on reliability
testing results, refer to the Application Note 1891:
Understanding the Basics of the Wafer-Level ChipScale Package (WL-CSP)
available on Maxim’s website
at www.maxim-ic.com/ucsp.
Chip Information
PROCESS: BiCMOS
Figure 1. Bidirectional Application
I
V
= 1.6V TO 28V
BATT
LOAD
R
SENSE
RS+ RS-RS- RS+
R
1
R
1
R
1
R
1
LOAD
TO WALL-CUBE/
CHARGER
P
R
OUT
MAX9938
GND
OUT OUT
P
MAX9938
R
10kΩ10kΩ
OUT
GND
VDD = 3.3V
μC
ADC
ADC
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 _____________________
9
© 2008 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.
Package Information
For the latest package outline information, go to www.maxim-ic.com/packages.
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
2 x 2 UCSP B4-1
21-0117
5 SOT23 U5-2
21-0057
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