Feb 2001 New UltraFast Comparators: Rail-to-Rail Inputs and 2.4V Operation Allow Use on Low Supplies

DESIGN FEATURES
New UltraFast Comparators: Rail-toRail Inputs and 2.4V Operation Allow
Use on Low Supplies
by Glen Brisebois
Introduction
The new LT1711 family of UltraFast
comparators has fully differential railto-rail inputs and outputs and
operates on supplies as low as 2.4V,
allowing unfettered application on low
voltages. The LT1711 (single) and
LT1712 (dual) are specified at 4.5ns
of propagation delay and 100MHz
toggle frequency. The low power
LT1713 (single) and LT1714 (dual)
are specified at 7ns of propagation
delay and 65MHz toggle frequency.
All of these comparators are fully
equipped to support multiple-supply
applications, and have latch-enable
pins and complementary outputs like
the popular LT1016, LT1671 and
LT1394. They are available in MSOP
and SSOP packages, fully specified
over commercial and industrial temperature ranges on 2.7V, 5V and ±5V
supplies.
rent sources and sinks feeding the
NPN and PNP differential pairs formed
by Q3–Q4 (protected by fast diodes
D11–D12) and Q1–Q2 (protected by
D1–D2). This approach makes the
inputs truly fully differential and
noninteracting, unlike approaches
that resort to resistors and diode
clamps. Even with the inputs at
opposite rails, the input bias currents
are still a simple function of the input
transistor base currents and remain
in the µA region. Both input stages
feed the level shifting transistors Q5–
Q6, and the remainder of the
differential voltage gain circuit flows
with a delightful symmetry towards
the output. Note that the channels
are identical, with polarity yet
unassigned, and are therefore interchangeable in layout. The symmetry,
broken only by the latch-enable circuit, is enhanced by the fact that all of
the transistors are well matched,
complementary 6GHz fT BJTs. Each
output stage ends in two Bakerclamped common emitter transistors,
Circuit Description
Figure 1 shows a simplified schematic of the LT1711 through LT1714.
The front end consists of eight cur-
allowing full rail-to-rail output swing.
All the comparators guarantee full 5V
TTL output capability over temperature, even when supplied with only
3V. Output rise and fall times are
fast, at 2ns for the LT1711 and LT1712
and 4ns for the LT1713 and LT1714.
Jitter is among the lowest for any
monolithic comparator, at 11psRMS
for the LT1711 and LT1712 and
15psRMS for the LT1713 and LT1714.
Some Applications
Simultaneous Full-Duplex
75MBaud Interface
with Only Two Wires
The circuit of Figure 2 shows a simple,
fully bidirectional, differential 2-wire
interface that gives good results
to 75MBaud, using the low
power LT1714. Eye diagrams under
conditions of unidirectional and
bidirectional communication are
shown in Figures 3 and 4. Although
not as pristine as the unidirectional
VCC
I1
I2
I9
R1
D4
R2
D35
Q5
Q6
Q10
D15
GND
VCC
Q34
Q22
Q13
D38
D14
Q4
OUTB1
I5
Q3
D9
D10
D11
Q8
Q42
D18
I14
Q14
R3
I6
I7
R4
D39
Q41
Q15
I11
Q7
D12
GND
D33
OUTA1
D37
D13
D32
D36
Q35
D30
VEE
VCC
VCC
Q29
Q23
Q17
BIAS
INB1
VCC
Q30
Q28
Q16
Q12
D31
R9
Q19
Q9
VEE
VEE
VCC
Q31
R8
Q11
Q2
VEE
Q1
Q18
LE1
D29
I25
I18
D34
D28
INA1
R6
I13
VCC
I10
I4
R5
I12
VEE
VCC
D3
I24
I23
I3
D2
D1
D27
D17
R11
Q43
R12
I16
I8
VEE
GND
Q40
I15
I21
I22
I20
Figure 1. LT1711–LT1714 simplified schematic
Linear Technology Magazine • February 2001
21
DESIGN FEATURES
3V
4
+
14
2
2
3V
1/2 LT1714
RXD
ALL DIODES = BAV99
–
13 16
1
–
3
R2a
2.55k
3V
15
+
11
R3a
124Ω
1/2 LT1714
TXD
8
–
6
10
12
9
R3b
124Ω
R2b
2.55k
R1b
499Ω
RXD
16 13
3
R1a
499Ω
R1c
499Ω
R2c
2.55k
R0a
140Ω
R0b
140Ω
R3c
124Ω
3V
5
5
7
14
1/2 LT1714
1
15
3V
4
+
3V
SIX FEET
TWISTED PAIR
ZO 120Ω
+
11
7
TXD
1/2 LT1714
R3d
124Ω
R1d
499Ω
12
–
6
8
10
9
R2d
2.55k
Figure 2. 75MBaud full-duplex interface on two wires
per for mance of Figure 3, the
performance under simultaneous
bidirectional operation is still excellent. Because the LT1714 input
voltage range extends 100mV beyond
both supply rails, the circuit works
with a full ±3V of ground potential
difference.
The circuit works well with the
resistor values shown, but other sets
of values can be used. The starting
point is the characteristic impedance,
ZO, of the twisted-pair cable. The input
impedance of the resistive network
should match the characteristic
impedance and is given by:
RIN = 2 • RO • (R1||(R2 + R3)
(RO + 2 • (R1||(R2 + R3)))
This comes out to 120Ω for the
values shown. The Thevenin equivalent source voltage is given by:
5ns/DIV
5ns/DIV
Figure 3. Performance of Figure 1’s
circuit operated unidirectionally; eye is
wide open (cursors show bit interval of
13.3ns or 75MBaud).
Figure 4. Performance of Figure 1’s circuit
operated simultaneous-bidirectionally;
crosstalk appears as noise. Eye is slightly
shut but performance is still excellent.
C4
100pF
R5
7.5k
V • (R2 + R3 – R1)
VTh = S
•
(R2 + R3 + R1)
R6
162Ω
22
C3
100pF
R7 15.8k
VS
RO
(RO + 2 • (R1||(R2 + R3)))
This amounts to an attenuation
factor of 0.0978 with the values
shown. (The actual voltage on the
lines will be cut in half again due to
the 120Ω ZO.) This attenuation factor
is important because it is the key to
deciding the ratio of the R2, R3 resistor divider in the receiver path. This
divider allows the receiver to reject
the large signal of the local transmitter and instead sense the attenuated
signal of the remote transmitter. Note
The author having already designed
R2 + R3 to be 2.653kΩ (by allocating
input impedance across RO, R1, and
R2 + R3 to get the requisite 120Ω), R2
and R3 then become 2529Ω and
123.5Ω, respectively. The nearest 1%
value for R2 is 2.55k and that for R3
is 124Ω.
that in the above equations, R2 and
R3 are not yet fully determined
because they only appear as a sum.
This allows the designer to now place
an additional constraint on their values. The R2, R3 divider ratio should
be set to one-half of the attenuation
factor mentioned above or R3/R2 =
1/2 • 0.0976.1
2
–
R9
2k
3
+
C2
0.1µF
R8
2k
VS
R1
1k
2
R2
1k
R4
210Ω
VS
6
SINE
VS = 2.7V TO 6V
1
+
7
LT1713
3
LT1806
4
1MHz AT CUT
VS
7
–
4
SQUARE
8
5
6
C1
0.1µF
R3
1k
Figure 5. LT1713 comparator configured as a series-resonant crystal oscillator;
the LT1806 op amp is configured as a bandpass filter with a Q of 5 and fC of 1MHz.
Linear Technology Magazine • February 2001
DESIGN FEATURES
1MHz Series-Resonant Crystal
Oscillator with Square and
Sinusoid Outputs
Figure 5 shows a classic 1MHz seriesresonant crystal oscillator. At series
resonance, the crystal is a low impedance and the positive feedback
connection brings about oscillation
at the series resonance frequency.
The RC feedback to the – input ensures
that the circuit does not find a stable
DC operating point and refuse to
oscillate. The comparator output is a
1MHz square wave (top trace of Figure 6), with jitter measured at 28psRMS
on a 5V supply and 40 psRMS on a 3V
supply. At pin 2 of the comparator, on
the other side of the crystal, is a clean
sine wave except for the presence of
the small high frequency glitch (middle
trace of Figure 6). This glitch is caused
by the fast edge of the comparator
output feeding back through crystal
capacitance. Amplitude stability of
the sine wave is maintained by the
is the bottom trace of Figure 6. Distortion was measured at –70dBc and
–55dBc on the second and third harmonics, respectively.
A
3V/DIV
B
1V/DIV
Conclusion
C
1V/DIV
200ns/DIV
Figure 6. Oscillator waveforms with VS = 3V:
Trace A = comparator output; Trace B =
crystal feedback to pin 2 of the LT1713;
Trace C = buffered, inverted and bandpass
filtered output of LT1806
fact that the sine wave is basically a
filtered version of the square wave.
Hence, the usual amplitude-control
loops associated with sinusoidal
oscillators are not necessary.2 The
sine wave is filtered and buffered by
the fast, low noise LT1806 op amp. To
remove the glitch, the LT1806 is configured as a bandpass filter with a Q
of 5 and unity gain center frequency
of 1MHz. The final sinusoidal output
The fully differential rail-to-rail inputs
of the new LT1711 family of fast comparators make them useful across a
wide variety of applications. The high
speed, low jitter performance of this
family, coupled with their small package sizes and 2.4V operation, makes
them attractive where PCB real estate
is at a premium and bandwidth-topower ratios must be optimized.
1 Using the design value of R2 + R3 = 2.653k rather
than the implementation value of 2.55k + 124Ω =
2.674k.
2 Amplitude will be a linear function of comparator
output swing, which is supply dependent and
therefore adjustable. The important difference
here is that any added amplitude stabilization or
control loop will not be faced with the classical
task of avoiding regions of nonoscillation vs
clipping.
LT1618, continued from page 7
L1
10µH
VIN
2.7V TO 5V
D1
0.619Ω
80mA
90
85
9
10kHz TO 50kHz
PWM
BRIGHTNESS
ADJUST
8
7
VIN
SW
SHDN
ISP
ISN
R3
5.1k
4
VIN = 5V
80
3
2
R1
2M
LT1618
IADJ
FB
VC
GND
5
1
C2
1µF
CC
0.1µF
Linear Technology Magazine • February 2001
VIN = 2.8V
65
50
51Ω
51Ω
51Ω
51Ω
10
20
30
40
50
60
70
80
LED CURRENT (mA)
Figure 10. High power white
LED driver efficiency
Figure 9. High power white LED driver
For larger LCD displays where a
greater amount of light output is
needed, multiple strings of LEDs can
be driven in parallel. When driving
parallel strings, ballast resistors
should be added to compensate for
LED forward voltage variations. The
amount of ballasting needed depends
on the LEDs used and how well they
70
55
R2
121k
(408) 573-4150
(408) 573-4150
(800) 282-9855
(847) 956-0666
High Power
White LED Driver
VIN = 3.3V
75
60
10
C3
0.1µF
C1: TAIYO YUDEN JMK212BJ475
C2: TAIYO YUDEN TMK316BJ105
D1: ON SEMICONDUCTOR MBR0530
L1: SUMIDA CR43-100
EFFICIENCY (%)
C1
4.7µF
are matched. The circuit in Figure 9 is
ideal for larger displays, providing
constant current drive for twenty white
LEDs from a single Li-Ion cell. Efficiency reaches a respectable 82%, as
seen in Figure 10.
Conclusion
The constant-current/constant-voltage operation of the LT1618 makes
the device an ideal choice for a variety
of constant-current designs. The
device provides accurate output current regulation or input current
limiting, along with excellent output
voltage regulation. With a wide input
voltage range and the ability to
produce outputs up to 35V, the
LT1618 works well in many different
applications.
References
1. Kim, Dave. “Tiny Regulators Drive White LED
Backlights.” Linear Technology Design Note 231
(May 2000).
23