LT1498/LT1499
14989fg
18
APPLICATIONS INFORMATION
Overdrive Protection
To prevent the output from reversing polarity when the
input voltage exceeds the power supplies, two pair of
crossing diodes D1 to D4 are employed. When the input
voltage exceeds either power supply by approximately
700mV, D1/D2 or D3/D4 will turn on, forcing the output
to the proper polarity. For the phase reversal protection to
work properly, the input current must be less than 5mA.
If the amplifi er is to be severely overdriven, an external
resistor should be used to limit the overdrive current.
Furthermore, the LT1498/LT1499’s input stages are pro-
tected by a pair of back-to-back diodes, D5/D6. When a
differential voltage of more than 0.7V is applied to the
inputs, these diodes will turn on, preventing the Zener
breakdown of the input transistors. The current in D5/D6
should be limited to less than 10mA. Internal resistors R6
and R7 (700Ω total) limit the input current for differential
input signals of 7V or less. For larger input levels, a re-
sistor in series with either or both inputs should be used
to limit the current. Worst-case differential input voltage
usually occurs when the output is shorted to ground. In
addition, the amplifi er is protected against ESD strikes up
to 3kV on all pins.
Figure 2b. LT1498 Large-Signal Response
Figure 2a. LT1498 Small-Signal Response
C
L
= 10nF
C
L
= 500pF
C
L
= 0pF
14989 F02a
V
S
= 5V
A
V
= 1
C
L
= 10nF
C
L
= 500pF
C
L
= 0pF
14989 F02b
V
S
= 5V
A
V
= 1
Capacitive Load
The LT1498/LT1499 are designed for ease of use. The
amplifi er can drive a capacitive load of more than 10nF
without oscillation at unity gain. When driving a heavy
capacitive load, the bandwidth is reduced to maintain
stability. Figures 2a and 2b illustrate the stability of the
device for small-signal and large-signal conditions with
capacitive loads. Both the small-signal and large-signal
transient response with a 10nF capacitive load are well
behaved.
Feedback Components
To minimize the loading effect of feedback, it is possible
to use the high value feedback resistors to set the gain.
However, care must be taken to insure that the pole formed
by the feedback resistors and the total input capacitance
at the inverting input does not degrade the stability of the
amplifi er. For instance, the LT1498/LT1499 in a noninvert-
ing gain of 2, set with two 30k resistors, will probably
oscillate with 10pF total input capacitance (5pF input
capacitance + 5pF board capacitance). The amplifi er has
a 2.5MHz crossing frequency and a 60° phase margin at
6dB of gain. The feedback resistors and the total input
capacitance create a pole at 1.06MHz that induces 67° of
phase shift at 2.5MHz! The solution is simple, either lower
the value of the resistors or add a feedback capacitor of
10pF of more.
