Rainbow Electronics MAX9920 - Manual (Page 15)

Shutdown Mode
Drive SHDN high to enter low-power shutdown mode.
In shutdown mode, the MAX9918/MAX9919/MAX9920
draw 0.5μA (typ) of quiescent current.
Adjustable Gain (MAX9918/MAX9920)
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:
Applications Information
Component Selection
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:
V OUT = V SENSE x G
where V SENSE is the full-scale sense voltage, 50mV for
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 REFIN = 0V), select the
gain of the MAX9918/MAX9920 to utilize the full output
range between GND and V
CC
. In bidirectional applica-
tions (V REFIN = V CC /2), select the gain to allow an output
voltage range of ±V CC /2. V OUT must be at least 100mV
from either rail to achieve the rated gain accuracy.
Sense Resistor, R SENSE
Choose R SENSE based on the following criteria:
Accuracy: A high R SENSE
value allows lower currents
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
OUT < V CC
- 100mV), there are two components to accuracy: input
offset voltage (V
OS ) and gain error (GE). Use the linear
equation to calculate total error:
V OUT = (G ± GE) x (V SENSE ± V OS )
For best performance, select R
SENSE
to provide
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.
Efficiency and Power Dissipation
At high current levels, the I 2 R 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 MAX9918/MAX9919/MAX9920 allows the use of
small sense resistors to reduce power dissipation and
reduce hot spots.
Inductance: Keep inductance low if I SENSE has a large
high-frequency component by using resistors with low
inductance value.
Power-Supply Bypassing and Grounding
Bypass the MAX9918/MAX9919/MAX9920’s V CC to
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
OUT . Using a differential mea-
surement between V 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.
( G
R
R
f
= +
⎛
⎝⎜
⎞
⎠⎟
1
2
1
o or MAX
and
G
9918
1
)
=
++
⎛
⎝⎜
⎞
⎠⎟
⎛
⎝
⎜
⎜
⎜⎜
⎞
⎠
⎟
⎟
⎟⎟
R
R
for MAX
2
1
4
9920 ( )
MAX9918/MAX9919/MAX9920
-20V to +75V Input Range, Precision
Uni-/Bidirectional, Current-Sense Amplifiers
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