MAX9920 Maxim, MAX9920 Datasheet - Page 15

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MAX9920

Manufacturer Part Number
MAX9920
Description
The MAX9918/MAX9919/MAX9920 are single-supply, high-accuracy current-sense amplifiers with a high input common-mode range that extends from -20V to +75V
Manufacturer
Maxim
Datasheet

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Drive SHDN high to enter low-power shutdown mode.
In shutdown mode, the MAX9918/MAX9919/MAX9920
draw 0.5μA (typ) of quiescent current.
The MAX9918/MAX9920 feature externally adjustable
gain set by a resistive-divider network circuit using
resistors R1 and R2 (see the Functional Diagram ). The
gain frequency compensation is set for a minimum gain
of 30V/V for the MAX9918 and 7.5V/V for the MAX9920.
The gain G for the MAX9918/MAX9920 is given by the
following equation:
Ideally, the maximum load current develops the full-
scale sense voltage across the current-sense resistor.
Choose the gain needed to yield the maximum output
voltage required for the application:
where V
the MAX9918/MAX9919, or 200mV for the MAX9920
and G is the gain of the device. G is externally
adjustable for the MAX9918/MAX9920. The MAX9919
has a fixed gain version of 45V/V (MAX9919F) or 90V/V
(MAX9919N).
In unidirectional applications (V
gain of the MAX9918/MAX9920 to utilize the full output
range between GND and V
tions (V
voltage range of ±V
from either rail to achieve the rated gain accuracy.
and
REFIN
Adjustable Gain (MAX9918/MAX9920)
SENSE
Uni-/Bidirectional, Current-Sense Amplifiers
= V
is the full-scale sense voltage, 50mV for
G
Applications Information
G
CC
V
=
______________________________________________________________________________________
=
OUT
/2), select the gain to allow an output
CC
⎝ ⎜
⎝ ⎜
1
1
/2. V
+
+ +
= V
R
4
R
R
R
2
1
2
1
OUT
SENSE
⎠ ⎟
CC
Component Selection
⎠ ⎟
-20V to +75V Input Range, Precision
(
f
. In bidirectional applica-
o o r MAX
(
must be at least 100mV
REFIN
for MAX
x G
Shutdown Mode
= 0V), select the
9918
9920
)
)
Choose R
Accuracy: A high R
to be measured more accurately. This is because off-
sets become less significant when the sense voltage is
larger. In the linear region (100mV < V
- 100mV), there are two components to accuracy: input
offset voltage (V
equation to calculate total error:
For best performance, select R
approximately 50mV (MAX9918/MAX9919) or 200mV
(MAX9920) of sense voltage for the full-scale current in
each application. Sense resistors of 5mΩ to 100mΩ are
available with 1% accuracy or better.
At high current levels, the I
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
the MAX9918/MAX9919/MAX9920 allows the use of
small sense resistors to reduce power dissipation and
reduce hot spots.
Inductance: Keep inductance low if I
high-frequency component by using resistors with low
inductance value.
Bypass the MAX9918/MAX9919/MAX9920’s V
ground with a 0.1μF capacitor. Grounding these
devices requires no special precautions; follow the
same cautionary steps that apply to the rest of the sys-
tem. High-current systems can experience large volt-
age drops across a ground plane, and this drop may
add to or subtract from V
surement between OUT and REFIN prevents this prob-
lem. For highest current-measurement accuracy, use a
single-point star ground. Connect the exposed pad to a
solid ground to ensure optimal thermal performance.
Power-Supply Bypassing and Grounding
SENSE
V
OUT
Efficiency and Power Dissipation
= (G ± GE) x (V
OS
based on the following criteria:
) and gain error (GE). Use the linear
SENSE
OUT
2
value allows lower currents
R losses in R
. Using a differential mea-
Sense Resistor, R
SENSE
SENSE
SENSE
± V
SENSE
OS
OUT
to provide
has a large
)
can be
SENSE
< V
CC
OS
CC
15
to
of

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