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
)
MAX4172
R
SENSE
INPUT
RS+
V
SENSE
RS-
R
G2
I
LOAD
TO LOAD/
BATTERY
R
G1
I
RG1
A1
Q1
V+
1:50
CURRENT
MIRROR
V+
MAX4172
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.
I
OUT
= 50 I
RG1
OUT
PG
V
TH
GND
Figure 1. Functional Diagram
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.
__________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.
6
_______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
