LTC6101_15 Datasheet, PDF (11/22 Pages) Linear Technology – High Voltage, High-Side Current Sense Amplifi er in SOT-23

LTC6101/LTC6101HV
APPLICATIONS INFORMATION
Selection of External Input Resistor, R_IN
The external input resistor, R_IN, controls the transconduc-
tance of the current sense circuit. Since I_OUT = V_SENSE/R_IN,
transconductance g_m = 1/R_IN. For example, if R_IN = 100,
then I_OUT = V_SENSE/100 or I_OUT = 1mA for V_SENSE = 100mV.
R_IN should be chosen to allow the required resolution
while limiting the output current. At low supply voltage,
I_OUT may be as much as 1mA. By setting R_IN such that
the largest expected sense voltage gives I_OUT = 1mA, then
the maximum output dynamic range is available. Output
dynamic range is limited by both the maximum allowed
output current and the maximum allowed output voltage, as
well as the minimum practical output signal. If less dynamic
range is required, then R_IN can be increased accordingly,
reducing the max output current and power dissipation.
If low sense currents must be resolved accurately in a
system that has very wide dynamic range, a smaller R_IN
than the max current spec allows may be used if the max
current is limited in another way, such as with a Schottky
diode across R_SENSE (Figure 3a). This will reduce the high
current measurement accuracy by limiting the result, while
increasing the low current measurement resolution.
V⁺
R_SENSE
D_SENSE
6101 F03a
LOAD
Figure 3a. Shunt Diode Limits Maximum Input Voltage to Allow
Better Low Input Resolution Without Overranging
This approach can be helpful in cases where occasional
large burst currents may be ignored. It can also be used
in a multirange configuration where a low current circuit
is added to a high current circuit (Figure 3b). Note that
a comparator (LTC1540) is used to select the range, and
transistor M1 limits the voltage across R_{SENSE LO}.
Care should be taken when designing the board layout
for R_IN, especially for small R_IN values. All trace and inter-
connect impedances will increase the effective R_IN value,
causing a gain error. In addition, internal device resistance
will add approximately 0.2Ω to R_IN.
CMPZ4697
10k
M1
Si4465
I_LOAD
V_OUT
R_{SENSE LO}
301
100m 301
301
+IN
–IN
+IN
V^–
V⁺ _VIN
V^–
V_IN
R_{SENSE HI}
10m
301
–IN
V⁺
OUT
LTC6101
LTC6101
OUT
7.5k
R₅
7.5k
( ) V_LOGIC +5V ≤ V_IN ≤ 60V
0 ≤ I_LOAD ≤ 10A
V_LOGIC
(3.3V TO 5V)
7
3
4
8
5
40.2k ₆
1.74M
619k
LTC1540
21
HIGH
RANGE
INDICATOR
(I_LOAD > 1.2A)
HIGH CURRENT RANGE OUT
250mV/A
Q1
CMPT5551
4.7k
V_LOGIC
BAT54C
LOW CURRENT RANGE OUT
2.5V/A
6101 F03b
Figure 3b. Dual LTC6101s Allow High-Low Current Ranging
6101fh
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