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
(V
–
terminal) to the input. This can destroy the unit. On
the LT1413, the 400Ω 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
V
S
= 5V, 0V 50µs/DIV
LT1413 • TA25
0
–100
V
S
= 5V, 0V 50µs/DIV
LT1413 • TA24
INPUT (mV) OUTPUT (V)
INPUT (mV) OUTPUT (V)
4
APPLICATIONS INFORMATION
UUWU
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
6V
P-P
INPUT, –1.5V TO 4.5V
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.
