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LT1398C

Low Cost Dual and Triple 300MHz Current Feedback Amplifiers with Shutdown

Linear Technology 

LT1398/LT1399/LT1399HV

APPLICATI 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).

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 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

trace lengths at the inverting input. The ground plane was

also pulled away from RF and RG on both sides of the

board to minimize stray capacitance.

EN A

EN B

OUTPUT

VS = ±5V

VINA = VINB = 2VP-P

at 3.58MHz

20ns/DIV

1398/99 F05

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

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

16.9Ω 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.

VIN

VOUT

OUTPUT

200mV/DIV

RL = 150Ω

RF = RG = 324Ω

f = 10MHz

5ns/DIV

1398/99 F04

Figure 4. Square Wave Response

VS = ±5V

RF = 324Ω

RG = 162Ω

RS = 16.9Ω

CL = 330pF

20ns/DIV

1398/99 AI06

Figure 6. LT1399/LT1399HV Large-Signal Pulse Response

11

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