MAX4172 Maxim, MAX4172 Datasheet - Page 6

• Manufacturer Part Number: MAX4172
• Description: The MAX4172 is a low-cost, precision, high-side current-sense amplifier for portable PCs, telephones, and other systems where battery/DC power-line monitoring is critical
• Manufacturer: Maxim

flows through R
internal current mirror multiplies I
to give I
(I
The internal current gain of 50 and the factory-trimmed
resistor R
transconductance (G
fined as being equal to I
(V
(I
The output voltage equation for the MAX4172 is given
below:
where V
I
the current-sense resistor, R
resistor, and G
(10mA/V).
The full-scale output voltage range can be set by
changing the R
age must be no greater than V+ - 1.2V. The above
equation can be modified to determine the R
required for a particular full-scale range:
OUT is a high-impedance current source that can be
integrated by connecting it to a capacitive load.
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.
Low-Cost, Precision, High-Side
Current-Sense Amplifier
6
LOAD
OUT
OUT
RS+
_______________________________________________________________________________________
) can be calculated with the following formula:
/ 50) x R
= the full-scale current being sensed, R
- V
R
V
OUT
OUT
OUT
OUT
I
RS-
OUT
G1
I
(10mA/V) x (I
= 50 x I
OUT
) = I
= (G
= (V
= the desired full-scale output voltage,
G1
= 50 x I
combine to result in the MAX4172
G1
OUT
= I
= G
OUT
m
m
LOAD
, I
) x (R
RG1
LOAD
= MAX4172 transconductance
RG1
resistor value, but the output volt-
m
) / (I
LOAD
x (V
. Substituting I
x R
SENSE
x R
m
LOAD
x R
LOAD
) of 10mA/V. G
OUT
RS+
G1
SENSE
x (R
SENSE
OUT
x R
- V
x R
= I
/ (V
x R
SENSE
RG1
= the voltage-setting
SENSE
RS-
LOAD
, the output current
OUT
SENSE
, or:
RS+
Current Output
) =
OUT
by a factor of 50
/ R
x I
)
x R
- V
LOAD
G1
PG Output
x G
/ 50 for I
RS-
SENSE
)
m
m
SENSE
). Since
)
)
is de-
. The
RG1
OUT
=
,
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
rent levels. Select R
mulas in the following section.
Choose R
• Voltage Loss: A high R
Figure 1. Functional Diagram
__________Applications Information
power-source voltage to degrade through IR loss.
For minimal voltage loss, use the lowest R
value.
INPUT
GND
V+
SENSE
SENSE
I
RG1
based on the following criteria:
Suggested Component Values
Q1
CURRENT
value to monitor higher or lower cur-
MIRROR
SENSE
1:50
R
RS+
G1
for Various Applications
V+
Sense Resistor, R
R
V
using the guidelines and for-
SENSE
SENSE
A1
I
OUT
SENSE
V
TH
= 50 I
RS-
R
MAX4172
G2
RG1
value causes the
I
LOAD
TO LOAD/
BATTERY
PG
OUT
SENSE
SENSE