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LT1671CMS8

60ns, Low Power, Single Supply, Ground-Sensing Comparator

Linear Technology 

LT1671

APPLICATIONS INFORMATION

+V

OUTPUT = 0 → +V (TYPICALLY 3V TO 4V)

1671 F03

Figure 3. Simplified LT1671 Output Stage

15V

1k

+

LT1671

–

RISE TIME = 4ns

FALL TIME = 5ns

2N2369

1k

2N3866

HP5082-2810

OUT

12pF

1k

1671 F04

Figure 4. Level Shift Has Noninverting Voltage Gain

LT1671 is the key to low delay, providing Q2’s base with

nearly ideal drive. This capacitor loads the LT1671’s

output transition, but Q2’s switching is clean with 3ns

delay on the rise and fall of the pulse. Figure 6 is similar to

Figure 4 except that a sink transistor has replaced the

Schottky diode. The two emitter-followers drive a power

MOSFET that switches 1A at 15V. Most of the 7ns to 9ns

delay in this stage occurs in the MOSFET and the 2N2369.

When designing level shifters, remember to use transis-

tors with fast switching times and high fT. To get the kind

of results shown, switching times in the nanosecond

range and an fT approaching 1GHz are required.

15V

+

LT1671

–

2N2369

1k

2N3866

2N5160 1k

1k

12pF

RL

POWER

FET

Figure 5 is a very versatile stage. It features a bipolar swing

that is set by the output transistor’s supplies. This 3ns

delay stage is ideal for driving FET switch gates. Q1, a

gated current source, switches the Baker-clamped output

transistor, Q2. The heavy feedforward capacitor from the

RISE TIME = 7ns

FALL TIME = 9ns

1671 F06

Figure 6. Noninverting Voltage Gain Level Shift

INPUT

+

LT1671

–

1000pF

RISE TIME = 3ns

FALL TIME = 3ns

5V

1N4148

4.7k 430Ω

0.1µF

Q1

2N2907

5V (TYP)

820Ω HP5082-2810 330Ω

820Ω

Q2

2N2369

5V

OUTPUT

–10V

OUTPUT TRANSISTOR SUPPLIES

(SHOWN IN HEAVY LINES)

CAN BE REFERENCED ANYWHERE

BETWEEN 15V AND –15V

1671 F05

–10V (TYP)

Figure 5. Level Shift with Inverting Voltage Gain—Bipolar Swing

9

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