LT1398/LT1399/LT1399HV
O U
W
U
PPLICATI
A
S I FOR ATIO
The enable/disable times are very fast when driven from
standard 5V CMOS logic. Each amplifier enables in about
30ns (50% point to 50% point) while operating on ±5V
supplies (Figure 2). Likewise, the disable time is approxi-
mately 40ns (50% point to 50% point) (Figure 3).
EN A
EN B
OUTPUT
Differential Input Signal Swing
To avoid any breakdown condition on the input transis-
tors, thedifferentialinputswingmustbelimitedto±5V. In
normal operation, the differential voltage between the
input pins is small, so the ±5V limit is not an issue. In the
disabled mode however, the differential swing can be the
same as the input swing, and there is a risk of device
breakdown if input voltage range has not been properly
considered.
1398/99 F05
VS = ±5V
INA = VINB = 2VP-P
at 3.58MHz
20ns/DIV
V
Figure 5. 3-Input Video MUX Switching Response (AV = 2)
Using the LT1399 to Drive LCD Displays
Driving the current crop of XGA and UXGA LCD displays
can be a difficult problem because they require drive
voltagesofupto12V, areusuallyacapacitiveloadofover
300pF, and require fast settling. The LT1399HV is par-
ticularly well suited for driving these LCD displays be-
cause it is capable of swinging more than ±6V on ±7.5V
supplies, and it can drive large capacitive loads with a
small series resistor at the output, minimizing settling
time. As seen in Figures 6 and 7, at a gain of +3 with a
16.9Ω output series resistor and a 330pF load, the
LT1399HV is capable of settling to 0.1% in 30ns for a 6V
step. Similarly, a 12V output step settles in 70ns.
3-Input Video MUX Cable Driver
The application on the first page of this data sheet shows
a low cost, 3-input video MUX cable driver. The scope
photo below (Figure 4) displays the cable output of a
30MHz square wave driving 150Ω. In this circuit the
active amplifier is loaded by the sum of RF and RG of each
disabled amplifier. Resistor values have been chosen to
keep the total back termination at 75Ω while maintaining
a gain of 1 at the 75Ω load. The switching time between
any two channels is approximately 32ns when both
enable pins are driven.
When building the board, care was taken to minimize
tracelengthsattheinvertinginput. Thegroundplanewas
also pulled away from RF and RG on both sides of the
board to minimize stray capacitance.
VIN
VOUT
OUTPUT
200mV/DIV
1398/99 AI06
VS = ±5V
20ns/DIV
RF = 324Ω
R
G = 162Ω
RS = 16.9Ω
CL = 330pF
Figure 6. LT1399/LT1399HV Large-Signal Pulse Response
1398/99 F04
RL = 150Ω
5ns/DIV
RF = RG = 324Ω
f = 10MHz
Figure 4. Square Wave Response
11
Linear [ Linear ]
