Datasheet

I
LOAD
x R
SENSE
. Then, since I
RG1
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 device’s transcon-
ductance (G
m
) of 10mA/V. G
m
is defined 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
equals the full-scale current being sensed, R
SENSE
equals the current-sense resistor, R
OUT
equals the
voltage-setting resistor, and G
m
equals the device’s
transconductance (10mA/V).
The full-scale output voltage range can be set by changing
the R
OUT
resistor value, but the output voltage 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 supply. A
logic low on the PG output indicates that V+ is sufficient to
power the MAX4172. This level is temperature dependent
(see Typical Operating Characteristics graphs), and is
typically 2.7V at room temperature. The internal PG
comparator has a 100mV (typ) hysteresis to prevent
possible oscillations caused by repeated toggling of the
PG output, making the device ideal for power-management
systems lacking soft-start capability. An internal delay
(15μs, typ) in the PG comparator 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 various applications
required to sense currents from 100mA to 10A.
Adjust the R
SENSE
value to monitor higher/lower current
levels. Select R
SENSE
using the guidelines and formulas
in the following section.
Figure 1. Functional Diagram
R
G1
RS+
R
SENSE
INPUT
R
G2
TO LOAD/
BATTERY
RS-V
SENSE
A1
1:50
CURRENT
MIRROR
MAX4172
OUT
V
TH
I
OUT
= 50 I
RG1
I
RG1
Q1
V+
PG
V+
GND
I
LOAD
MAX4172 Low-Cost, Precision, High-Side
Current-Sense Amplier
www.maximintegrated.com
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