11

LT1398/LT1399/LT1399HV

S

APPLICATI

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).

Differential Input Signal Swing

To avoid any breakdown condition on the input transis-

tors, the differential input swing must be limited to

±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.

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 150

Ω. In this circuit the

disabled amplifier. Resistor values have been chosen to

keep the total back termination at 75

Ω while maintaining

a gain of 1 at the 75

Ω 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

trace lengths at the inverting input. The ground plane was

board to minimize stray capacitance.

20ns/DIV

at 3.58MHz

1398/99 F05

EN A

EN B

OUTPUT

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

voltages of up to 12V, are usually a capacitive load of over

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

16.9

Ω 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.

Figure 6. LT1399/LT1399HV Large-Signal Pulse Response

20ns/DIV

1398/99 AI06

Figure 4. Square Wave Response

OUTPUT

200mV/DIV

5ns/DIV

1398/99 F04

f = 10MHz