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 resis-

tors 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:

R2

G

1

(for MAX9918)

R1

and

R2

1

R1

G

(for MAX9920)

4





= +















+













=







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:

VOUT = VSENSE x G

where VSENSE 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 (VREFIN = 0V), select the

gain of the MAX9918/MAX9920 to utilize the full output

range between GND and VCC. In bidirectional applica-

tions (VREFIN = VCC/2), select the gain to allow an output

voltage range of ±VCC/2. VOUT must be at least 100mV

from either rail to achieve the rated gain accuracy.

Sense Resistor, RSENSE

Choose RSENSE based on the following criteria:

Accuracy: A high RSENSE 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 < VOUT < VCC

- 100mV), there are two components to accuracy: input

offset voltage (VOS) and gain error (GE). Use the linear

equation to calculate total error:

VOUT = (G ± GE) x (VSENSE ± VOS)

For best performance, select RSENSE to provide approxi-

mately 50mV (MAX9918/MAX9919) or 200mV (MAX9920)

of sense voltage for the full-scale current in each applica-

tion. Sense resistors of 5mΩ to 100mΩ are available with

1% accuracy or better.

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 MAX9918/

MAX9919/MAX9920 allows the use of small sense resis-

tors to reduce power dissipation and reduce hot spots.

Inductance: Keep inductance low if ISENSE has a large

high-frequency component by using resistors with low

inductance value.

Power-Supply Bypassing and Grounding

Bypass the MAX9918/MAX9919/MAX9920’s VCC to

ground with a 0.1µF capacitor. Grounding these devices

requires no special precautions; follow the same caution-

ary steps that apply to the rest of the system. High-current

systems can experience large voltage drops across a

ground plane, and this drop may add to or subtract from

VOUT. Using a differential measurement between OUT

and REFIN prevents this problem. For highest current-

measurement accuracy, use a single-point star ground.

Connect the exposed pad to a solid ground to ensure

optimal thermal performance.

www.maximintegrated.com

Maxim Integrated │ 16

MAX9918/MAX9919/MAX9920

-20V to +75V Input Range,

Precision Uni-/Bidirectional,

Current-Sense Amplifiers