Datasheet

LT1366/LT1367
LT1368/LT1369
15
1366fb
Typical applicaTions
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 input voltage, circuit operation is limited by the
LT1366’s absolute maximum ratings and the output power
requirements.
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 impedance
multiplied by the op amp’s open-loop gain. Degradations in
current-source compliance occur when the voltage 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 But-
terworth filter. The filter is a simplified state variable ar-
chitecture consisting of two cascaded 2nd order sections.
Each section uses the 360 degree phase shift around
–
+
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
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
Figure 7. High Side Current Source
Figure 8. 4-Pole 1kHz, 3.3V Single Supply, State Variable Filter Using the LT1367