Datasheet

Gain (V/V)
Vsense (mV)
R
g
(k:)
500
50
5
100
10
1
0 100 200 300 400 500
V
s
=12V @ V
CM
=6V
V
s
=2.7V @ V
CM
=2V
V
s
=5V @ V
CM
=2.5V
GAIN (V/V)
Vsense (mV)
R
g
(kÖ)
500
50
5
100
10
1
0 100 200 300 400 500
V
S
= 5V, V
S
= 12V
V
S
= 2.7V
LMP8645, LMP8645HV
www.ti.com SNOSB29F –NOVEMBER 2009–REVISED MARCH 2013
The following equations explain how to select the gain resistor for various Range of the input common mode
voltage.
Range 1 −2V < V
CM
≤ 1.8V (4)
The max voltage at the RG pin is given by the following inequality V
RG
=V
sense
*R
GAIN
*Gm ≤ min(1.3V; Vout_max)
where Vout_max is the maximum allowable output voltage according to the Electrical Tables. All the gain
resistors (R
GAIN
) which respect the previous inequality are allowed. The graphical representation in Figure 17
helps in the selection; all the combinations (V
SENSE
, R
GAIN
) below the curve are allowed.
Figure 17. Allowed Gains for Range 1
As a consequence once selected the gain (R
GAIN
) the V
SENSE
Range is fixed too. For example if an application
required a Gain of 10, R
G
will be 50 kΩ and V
SENSE
will be in the Range 10 mV to 100 mV.
Range 2 1.8V < V
CM
≤ V
S
(5)
In this Range the max voltage at the R
G
pin is related to the common mode voltage and V
SENSE
. So all the R
GAIN
resistors which respect the following inequalities are allowed:
V
R
G
≤ min (Vout_max; (V
CM
- V
sense
-250mV))
where
• V
RG
= V
SENSE
*R
GAIN
*Gm
• Vout_max is the maximum allowable output voltage according to the Electrical Tables. (6)
The graphical representation in Figure 18 helps in the selection; all the combinations (V
SENSE
, R
GAIN
) below the
curves for given V
CM
and supply voltage are allowed.
Figure 18. Allowed Gains for Range 2
Also in this Range once selected the R
GAIN
(Gain) the V
SENSE
Range is fixed too.
Range 3 V
CM
≥ V
S
(7)
Copyright © 2009–2013, Texas Instruments Incorporated Submit Documentation Feedback 11
Product Folder Links: LMP8645 LMP8645HV