MAX4172 Datasheet, PDF (6/8 Pages) Maxim Integrated Products – Low-Cost, Precision, High-Side Current-Sense Amplifier

Low-Cost, Precision, High-Side
Current-Sense Amplifier
flows through R_G1, I_RG1 x R_G1 = I_LOAD x R_SENSE. The
internal current mirror multiplies I_RG1 by a factor of 50
to give I_OUT = 50 x I_RG1. Substituting I_OUT / 50 for I_RG1,
(I_OUT / 50) x R_G1 = I_LOAD x R_SENSE, or:
I_OUT = 50 x I_LOAD x (R_SENSE / R_G1)
The internal current gain of 50 and the factory-trimmed
resistor R_G1 combine to result in the MAX4172
transconductance (G_m) of 10mA/V. G_m is de-
fined as being equal to I_OUT / (V_RS+ - V_RS-). Since
(V_RS+ - V_RS-) = I_LOAD x R_SENSE, the output current
(I_OUT) can be calculated with the following formula:
I_OUT = G_m x (V_RS+ - V_RS-) =
(10mA/V) x (I_LOAD x R_SENSE)
Current Output
The output voltage equation for the MAX4172 is given
below:
V_OUT = (G_m) x (R_SENSE x R_OUT x I_LOAD)
where V_OUT = the desired full-scale output voltage,
I_LOAD = the full-scale current being sensed, R_SENSE =
the current-sense resistor, R_OUT = the voltage-setting
resistor, and G_m = MAX4172 transconductance
(10mA/V).
The full-scale output voltage range can be set by
changing the R_OUT resistor value, but the output volt-
age must be no greater than V+ - 1.2V. The above
equation can be modified to determine the R_OUT
required for a particular full-scale range:
R_OUT = (V_OUT ) / (I_LOAD x R_SENSE x G_m)
OUT is a high-impedance current source that can be
integrated by connecting it to a capacitive load.
PG Output
The PG output is an open-collector logic output that
indicates the status of the MAX4172’s V+ power sup-
ply. A logic low on the PG output indicates that V+ is
sufficient to power the MAX4172. This level is tempera-
ture dependent (see Typical Operating Characteristics
graphs), and is typically 2.7V at room temperature. The
internal PG comparator has a 100mV (typical) hystere-
sis to prevent possible oscillations caused by repeated
toggling of the PG output, making the device ideal for
power-management systems lacking soft-start capabili-
ty. An internal delay (15µs typical) in the PG compara-
tor allows adequate time for power-on transients to
settle out. The PG status indicator greatly simplifies the
design of closed-loop systems by ensuring that the
components in the control loop have sufficient voltage
to operate correctly.
INPUT
R_SENSE
I_LOAD
RS+ V_SENSE RS-
TO LOAD/
BATTERY
R_G1
R_G2
I_RG1
A1
Q1
V+
MAX4172
1:50
CURRENT
I_OUT = 50 I_RG1
OUT
MIRROR
V+
PG
V_TH
GND
Figure 1. Functional Diagram
__________Applications Information
Suggested Component Values
for Various Applications
The Typical Operating Circuit is useful in a wide variety
of applications. Table 1 shows suggested component
values and indicates the resulting scale factors for vari-
ous applications required to sense currents from
100mA to 10A.
Adjust the R_SENSE value to monitor higher or lower cur-
rent levels. Select R_SENSE using the guidelines and for-
mulas in the following section.
Sense Resistor, R_SENSE
Choose R_SENSE based on the following criteria:
• Voltage Loss: A high R_SENSE value causes the
power-source voltage to degrade through IR loss.
For minimal voltage loss, use the lowest R_SENSE
value.
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