MAX9928/MAX9929
-0.1V to +28V Input Range, Micropower,
Uni-/Bidirectional, Current-Sense Amplifiers
10 ______________________________________________________________________________________
Voltage Output (MAX9929F)
The output voltage equation for the MAX9929_ is given
below:
V
OUT
= (R
SENSE
x I
LOAD
) x (A
V
)
where V
OUT
= the desired full-scale output voltage,
I
LOAD
= the full-scale current being sensed, R
SENSE
=
the current-sense resistor, A
V
= MAX9929F voltage
gain (50V/V).
SIGN Output
The current/voltage at OUT indicates magnitude. The
SIGN output indicates the current’s direction. The SIGN
comparator compares RS+ to RS-. The sign output is
high when RS+ is greater than RS- indicating positive
current flow. The sign output is low when RS- is greater
than RS+ indicating negative current flow. In battery-
operated systems, this is useful for determining
whether the battery is charging or discharging. The
SIGN output might not correctly indicate the direction of
load current when V
SENSE
is between -1.8mV to -1.2mV
(see Figure 2). Comparator hysteresis of 0.6mV pre-
vents oscillation of SIGN output. If current direction is
not needed, leave SIGN unconnected.
Applications Information
Choosing R
SENSE
The MAX9928F/MAX9929F operate over a wide variety
of current ranges with different sense resistors. Adjust
the R
SENSE
value to monitor higher or lower current lev-
els. Select R
SENSE
using these guidelines:
• Voltage Loss: A high R
SENSE
value causes the
power-source voltage to drop due to IR loss. For
least voltage loss, use the lowest R
SENSE
value.
• Accuracy: A high R
SENSE
value allows lower cur-
rents to be measured more accurately. This is
because offsets become less significant when the
sense voltage is larger.
• Efficiency and Power Dissipation: At high current
levels, the I
2
R losses in R
SENSE
might be significant.
Take this into consideration when choosing the resis-
tor value and power dissipation (wattage) rating.
Also, if the sense resistor is allowed to heat up exces-
sively, its value could drift.
• Inductance: If there is a large high-frequency com-
ponent to I
SENSE
, keep inductance low. Wire-wound
resistors have the highest inductance, while metal
film is somewhat better. Low-inductance metal-film
resistors are available. Instead of being spiral
wrapped around a core, as in metal film or wire-
wound resistors, these are a straight band of metal.
They are made in values under 1Ω.
Use in Systems with Super Capacitors
Since the input common-mode voltage range of the
MAX9928/MAX9929 extends all the way from -0.1V to
28V, they are ideal to use in applications that require
use of super capacitors for temporary or emergency
energy storage systems. Some modern industrial and
automotive systems use multifarad (1F–50F) capacitor
banks to supply enough energy to keep critical sys-
tems alive even if the primary power source is removed
or temporarily disabled. Unlike batteries, these capaci-
tors can discharge all the way down to 0V. The
MAX9928/MAX9929 can continuously help monitor their
health and state of charge/discharge.
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 reliability
testing results, go to Maxim’s website at www.maxim-
ic.com/ucsp to find Application Note 1891:
Understanding the Basics of the Wafer-Level Chip-
Scale Package (WL-CSP).
Chip Information
PROCESS: BiCMOS
