LT6105
1
DESCRIPTION
Demonstration Circuit 1192 is a Precision, Gain Select-
able High Side Current Sense Amplifier featuring the
LT6105.
This demo board amplifies a current-sense resistor IR-
drop, providing a precision uni-polar output voltage pro-
portional to load current. The LT6105 is pow ered from a
supply voltage (from 2.85V up to 36V) w hile it measures
current of a voltage pow er bus (0V to 44V). This device is
suitable for measuring battery system current since the
monitoring-input current draw s become negligible w hen
V
CC
is pow ered dow n. Its 0V and up input range makes it
suitable for measuring supply bus loading as they are
brought up. W hen provided w ith a negative supply, it is
capable of monitoring negative supplies.
The key performance characteristics of the LT6105 and
DC1192 are show n in the Performance Summary below .
Design files for this circuit board are available. Call the
LTC factory.
L
, LT, Over-The-Top are registered trademarks of Linear Technology Corporation. Other
product names may be trademarks of the companies that manufacture the products.
PERF ORM ANCE SU M M ARY
Specifications are at T
A
= 25°C
SYM BOL PARAM ETER CONDITIONS M IN TYP M AX UNITS
V
IN
M onitored Supply Input Range
-0.3 0… 44 44
V
V
CC
Amplifier Supply Range 2.85 3… 28 36
V
A
V
Voltage Gain R
IN1
= R
IN2
= 100, R
OUT
= 4.99k
50
V/V
A
Z
Total Current to Voltage Gain R
SENSE
= .020 Ohm
1 V
/A
V
OUT
Output Voltage Range V+ = 3V 2
V
I
M AX
M aximum Sense Current Thermal limit of R
SENSE
(2W atts) 10
A
I
CC
V
+
(=Vcc) Supply Current V+= 36V, V
S+
=3V, V
SENSE
=0V 150 250
µA
I
BIAS
V
S+
Bias Current V
S+
= 3V, V
SENSE
=0V
15 25
µA
I
INPD
V
S+
Current during Pow er Dow n V
+
= 0V, V
S+
=44V, V
SENSE
=0V
.001 1 µ
A
t
R
V
M ON
step-response time I
OUT
step from 0A to 5A 50 µs
OPERATING PRINCIPL ES
The LT6105 operates by amplifying the voltage drop on a
sense resistor placed in series w ith the pow er source of a
load to be monitored. The sense inputs of the amplifier
differentially measure the sense-resistor drop to control
an internal variable current source that allow s translation
of the input information to a level referenced to V
EE
(ground in this demo circuit). An internal post-amplifier
provides additional gain and buffers the output signal.
The post-amplifier gain is established by the pin strap
configuration of the part. In DC1192 as shipped, the
sense resistor installed is 20m
Ω
and the resistors set the
gain to 50V/V, so the nominal output scaling is 1V per
Ampere of load current. Other scalings can be produced
by changing the resistors.
The DC1192 Schematic diagram is show n in Figure 2.
DEMO CIRCUIT
Q UICK S TA RT G UIDE
L T6 1 0 5
P re c isio n , G
a in S e le c ta b le H ig h
S id e Cu rre n t S e n se A m p lifie r
LT6105
2
Q U ICK START PROCEDU RE
Demonstration circuit 1192 is easy to set up to evaluate
the performance of the LT6105. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below :
1.
W ith pow er off, connect the load driving pow er supply
positive to SOURCE and the common to GN D. This
supply should be in the range of 0V to 44V. If this
supply is not equipped w ith an accurate current read-
out, a DM M (set to measure Amperes) may be con-
nected in series w ith the supply as show n in Figure 1.
NOTE.
Be sure that the current capacity of the DM M is adequate to
handle the intended load current.
2.
W ith pow er off, connect the amplifier supply positive
to VCC and the common to GND. This supply should
be in the range of 2.85V to 36V.
3.
Connect a voltmeter or oscilloscope probe to the Vout
terminal, w ith the common connection or ground clip
tied to GND.
4.
Connect a load to the TO LOAD terminal (positive) and
return (negative) to the common of the load driving
pow er supply. The load may be a pow er resistor, ac-
tive load instrument, or other circuit of interest.
5.
Turn on the pow er supplies.
6.
Check for an output voltage that corresponds to the
load current: V
M ON
= 1.0
×
I
OUT
.
LOAD
(10Ω)
+ COM
POWER
SUPPLY
10.
0
1.00
0
DMM
A COM V
1.00
0
DMM
A COM V
+ COM
POWER
SUPPLY
3.0
Figure 1.
Proper M easurem ent Equipm ent Setup
LT6105
3