LT6550/LT6551
11
65501fa
APPLICATIO S I FOR ATIO
WUU
U
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 ±1.4V. R
IN
limits the
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
The input voltage range is V
EE
to (V
CC
– 1.75V) over
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
the minimum input voltage is (V
OUT
Min)/2 and the maxi-
mum input voltage is (V
OUT
Max)/2 or (V
CC
– 1.75V),
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
V
+
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
R
IN
225Ω
RG
450Ω
RF
450Ω
R1
R2 R3
R4 R5
CM
V
+
GND
V
+
GND
+–
