Datasheet

Gain (V/V)
Vsense (mV)
R
g
(k:)
500
50
5
100
10
1
0 100 200 300 400 500
V
s
=5V
V
s
=2.7V
V
s
=12V
LMP8645, LMP8645HV
SNOSB29F –NOVEMBER 2009–REVISED MARCH 2013 www.ti.com
The max voltage at the R
G
pin is Vout_max, it means that V
OUT
= V
SENSE
* R
GAIN
/R
IN
≤ Vout_max where
Vout_max is the maximum allowable output voltage according to the Electrical Tables. So all the R
GAIN
resistors
which respect the previous inequality are allowed. The graphical representation in Figure 19 helps in the
selection; all the combinations (V
SENSE
, R
GAIN
) below the curves are allowed.
Figure 19. Allowed Gains for Range 3
Also in this Range once selected the R
GAIN
(Gain) the V
SENSE
Range is fixed too.
From the Ranges showed above a good way to maximize the output voltage swing of the LMP8645 is to select
the max allowable Rgain according to the previous equations. For a fixed supply voltage and Vsense as the
common mode voltage increases, the max allowable Rgain increases too.
DESIGN GUIDELINE
Example 1
The LMP8645 is used to monitor the current supply of an active device (Refer to Figure 20). The LMP8645 is
supplied at 5V the active device is supplied at 12V and the max current sunk is 1A. In this example the LMP8645
will work in all 3 Ranges: in Range 1 at the turning on of the active device, then in Range 3 passing through the
Range 2. Since the purpose of the application is monitor the current of the active device in any operating state
working condition (power on, normal operation, etc.), the gain resistor will be selected according to the Range 1,
the Range which gives more constraints to the output dynamic voltage of the LMP8645.
At the startup of the monitored device the LMP8645 works at 0V common mode, it means that its max output is
1.3V (Range 1). In order to maximize the resolution the Rsense is calculated as max allowed Vsense (Refer to
Figure 17) divided by max current (1A), so Rsesne=0.5Ω. Due to the output limitation the max allowed gain will
be 2.6V/V so R
GAIN
=13kΩ. With this approach the current is monitored at any working condition but without using
the entire output dynamic of the LMP8645. Alternatively if the monitored device doesn’t sink 1A at any supply
voltage, it is possible to design considering the max output voltage of the LMP8645 when operating in Range 3
(V
CM
≥V
S
). Also in this case is possible to maximize the resolution using Rsesne=0.5Ω, and maximize the output
dynamic with Rgain=33kΩ. With this approach the max detectable current when V
CM
is less than 1,8V is about
400mA, while for V
CM
=2.5V the max detectable current is 600mA (Refer to Figure 17) and for V
CM
≥V
S
is 1A.
The second approach maximizes the output dynamic but implies some knowledge on the monitored current.
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