LT1366/LT1367
LT1368/LT1369
U
TYPICAL APPLICATIONS
High-Side Current Source
input voltage, circuit operation is limited by the LT1366’s
absolute maximum ratings and the output power require-
ments.
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 (RSENSE) 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 delivers 1A at 200mV of sense voltage. With a
5V input supply, the power dissipation is 5W. For opera-
tionat70°Cambienttemperature, theMOSFET’sheatsink
must have a thermal resistance of:
θHS = θJA SYSTEM – θJC FET
= (125°C – 70°C)/5W – 1.25°C/W
= 11°C/W –1.25°C/W
= 9.75°C/W
V
R
SENSE
0.2Ω
CC
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.
1k
0.0033µF
100Ω
LT1004-1.2
–
Q1
The circuit’s supply regulation is about 0.03%/V. The
outputimpedanceisequaltotheMOSFET’soutputimped-
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
[VCC – VOUT] < [ILOAD • (RSENSE + RON)].
1/2 LT1366
MTP23P06
R
P
+
10k
I
LOAD
40k
5V < V < 30V
CC
0A < I
< 1A AT V = 5V
LOAD
0mA < I
CC
< 160mA AT V = 30V
LOAD
CC
Q2
2N4340
LT1366 F07
Single Supply, 1kHz, 4th Order Butterworth Filter
Figure 7. High-Side Current Source
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.Eachsectionusesthe360degreephaseshiftaround
The circuit can operate over a wide supply range
(5V < VCC < 30V). At low input voltage, circuit operation is
limited by the MOSFET’s gate drive requirements. At high
C1
10,000pF
C2
10,000pF
R1*
29.5k
10,000pF
R2*
8.6k
V
–
IN
10,000pF
A1
–
1/4 LT1367
11.8k*
A2
+
–
1/4 LT1367
21.5k*
11.8k*
A3
+
–
1/4 LT1367
A4
+
V
OUT
1/4 LT1367
29.5k*
+
10k
3.3V
10k
1µF
*1% RESISTORS
LT1366 F08
Figure 8. 4-Pole 1kHz, 3.3V Single Supply, State Variable Filter Using the LT1367
15
Linear [ Linear ]
