7

LT1413

Single Supply Operation

The LT1413 is fully specified for single supply operation,

i.e., when the negative supply is 0V. Input common-mode

range includes ground; the output swings within a few

millivolts of ground.

If the input is more than a few hundred millivolts below

ground, two distinct problems can occur on previous

single supply designs, such as the LM124, LM158, OP-21

and OP-221.

a) When the input is more than a diode drop below

ground, unlimited current will flow from the substrate

the LT1413, the 400

Ω resistors, in series with the input

(see Schematic Diagram), protect the devices even when

the input is 5V below ground.

b) When the input is more than 400mV below ground

(at 25

°C), the input stage saturates (transistors Q3 and

Q4) and phase reversal occurs at the output. This can

cause lock-up in servo systems. Due to a unique phase

reversal protection circuitry (Q21, Q22, Q27, Q28), the

LT1413 outputs do not reverse, as illustrated below,

even when the inputs are at –1.5V. Keep the output of the

other amplifier out of negative saturation for the phase

reversal protection to function properly.

Since the output of the LT1413 cannot go exactly to

ground, but can only approach ground to within a few

millivolts, care should be exercised to ensure that the

output is not saturated. For example, a 1mV input signal

will cause the amplifier to set up in its linear region

in the gain 100 configuration shown below, but is not

enough to make the amplifier function properly in the

voltage-follower mode.

LT1413 • 27

–

+

R

5V

99R

100mV

1mV

–

+

5V

OUTPUT

SATURATED

≈ 15mV

1mV

Gain 100 Amplifier

Voltage Follower

Figure 1.

Comparator Applications

The single supply operation of the LT1413 lends itself to

its use as a precision comparator with TTL compatible

output; the response time is shown below.

0

50

µs/DIV

LT1413 • TA25

0

–100

50

µs/DIV

LT1413 • TA24

4

Voltage Follower with Input Exceeding the Negative Common-Mode Range

Comparator Rise Response Time

10mV, 5mV, 2mV Overdrives

Comparator Fall Response Time

to 10mV, 5mV, 2mV Overdrives

4

2

0

2

0

100

LT1413 • TA21

LT1413 • TA22

4V

2V

0V

LM324, LM358, OP-221

EXHIBIT OUTPUT PHASE REVERSAL

4V

2V

0V

LT1413 • TA23

4V

2V

0V

LT1413

NO PHASE REVERSAL

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.