15

LT1366/LT1367

LT1368/LT1369

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

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

limited by the MOSFET’s gate drive requirements. At high

–

+

1/2 LT1366

1k

0.2

Ω

40k

Q1

MTP23P06

Q2

2N4340

100

Ω

0.0033

µF

LT1004-1.2

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:

= (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

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*

1

µF

10k

10k

3.3V

–

+

–

+

–

+

–

+

A1

1/4 LT1367

A2

1/4 LT1367

A3

1/4 LT1367

A4

1/4 LT1367

LT1366 F08

*1% RESISTORS