LT6550/LT6551

11

65501f

APPLICATIO S I FOR ATIO

Amplifier Characteristics

Figure 1 shows a simplified schematic of one channel of

the LT6551 quad. Resistors RF and RG provide an internal

gain of 2. (The LT6550 triple is a slight variation with the

gain setting resistor, RG, connected to a separate ground

pin). The input stage consists of transistors Q1 to Q8 and

resistor R1. This topology allows for high slew rates at low

supply voltages. There are back-to-back series diodes, D1

to D4, across the + and – inputs of each amplifier to limit

the differential input voltage to

current through these diodes if the input differential volt-

age exceeds

±1.4V. The input stage drives the degenera-

tion resistors of PNP and NPN current mirrors, Q9 to Q12,

that convert the differential signals into a single-ended

output. The complementary drive generator supplies cur-

rent to the output transistors that swing from rail-to-rail.

Input Voltage Range

temperature. If the device is operated on a single 3V supply

Figure 1. LT6551 Simplified Schematic

the maximum input is (3V – 1.75V) or 1.25V, and the

internal gain of two will set the output voltage to 2.5V.

Increasing the input beyond 1.25V will force the device out

of its linear range, no longer a gain of 2, and the output will

not increase beyond 2.5V. At a higher supply voltage, i.e.

5V, the maximum input voltage is 5V – 1.75V or 3.25V.

However, due to the internal gain of 2, the output will clip

with a lower input voltage. For linear unclipped operation

whichever is less.

ESD

The LT6550/LT6551 have reverse-biased ESD protection

diodes on all inputs and outputs as shown in Figure 1. If

these pins are forced beyond either supply, unlimited

current will flow through these diodes. If the current is

limited to 10mA or less, no damage to the device will

occur.

COMPLEMENTARY

DRIVE

GENERATOR

6551 F01

OUT

GND

DESD3

DESD4

DESD1

DESD2

IN

D3

D4

D1

D2

I1

I2

I3

I4

Q1

Q2

Q3

Q4

Q5

Q10

Q13

Q9

Q12

Q14

Q11

Q6

Q7

Q8

225

Ω

RG

450

Ω

RF

450

Ω

R1

R2

R3

R4

R5

CM

GND

GND

+–