Datasheet

15
LT1366/LT1367
LT1368/LT1369
TYPICAL APPLICATIONS
U
High-Side Current Source
The wide-compliance current source shown in Figure 7
takes advantage of the LT1366’s ability to measure small
signals near the positive supply rail. The LT1366 adjusts
Q1’s gate voltage to force the voltage across the sense
resistor (R
SENSE
) to equal the voltage from the supply to
the potentiometer’s wiper. A rail-to-rail op amp is needed
because the voltage across the sense resistor must drop
to zero when the divided reference voltage is set to zero.
Q2 acts as a constant current sink to minimize error in the
reference voltage when the supply voltage varies.
The circuit can operate over a wide supply range
(5V < V
CC
< 30V). At low input voltage, circuit operation is
limited by the MOSFET’s gate drive requirements. At high
–
+
1/2 LT1366
1k
R
SENSE
0.2Ω
40k
Q1
MTP23P06
I
LOAD
5V < V
CC
< 30V
0A < I
LOAD
< 1A AT V
CC
= 5V
0mA < I
LOAD
< 160mA AT V
CC
= 30V

Q2
2N4340
V
CC
100Ω
0.0033µF
LT1004-1.2
R
P

10k
LT1366 F07
Figure 7. High-Side Current Source
Figure 8. 4-Pole 1kHz, 3.3V Single Supply, State Variable Filter Using the LT1367
input voltage, circuit operation is limited by the LT1366’s
absolute maximum ratings and the output power require-
ments.
The circuit delivers 1A at 200mV of sense voltage. With a
5V input supply, the power dissipation is 5W. For opera-
tion at 70°C ambient temperature, the MOSFET’s heat sink
must have a thermal resistance of:
θ
HS
= θ
JA SYSTEM
– θ
JC FET
= (125°C – 70°C)/5W – 1.25°C/W
= 11°C/W –1.25°C/W
= 9.75°C/W
which is easily achievable with a small heat sink. Input
voltages greater than 5V require the use of a larger heat
sink or a reduction of the output current.
The circuit’s supply regulation is about 0.03%/V. The
output impedance is equal to the MOSFET’s output imped-
ance multiplied by the op amp’s open-loop gain. Degrada-
tions in current-source compliance occur when the volt-
age across the MOSFET’s on-resistance and the sense
resistor drops below the voltage required to maintain the
desired output current. This condition occurs when
[V
CC
– V
OUT
] < [I
LOAD
• (R
SENSE
+ R
ON
)].
Single Supply, 1kHz, 4th Order Butterworth Filter
An LT1367 is used in Figure 8 to form a 4th order
Butterworth filter. The filter is a simplified state variable
architecture consisting of two cascaded 2nd order sec-
tions. Each section uses the 360 degree phase shift around
10,000pF
10,000pF
C2
10,000pF
C1
10,000pF
R1*
29.5k
R2*
8.6k
29.5k*
11.8k*
21.5k*
11.8k*
1µF
10k
10k
V
OUT
V
IN
3.3V
–
+
–
+
–
+
–
+
A1
1/4 LT1367
A2
1/4 LT1367
A3
1/4 LT1367
A4
1/4 LT1367
LT1366 F08
*1% RESISTORS