Datasheet

LT6700/LT6700HV
15
6700123fh
For more information www.linear.com/LT6700
APPLICATIONS INFORMATION
using a supply filter, the start-up time of the LT6700 will
increase to:
t
START
= (0.17ms + 0.25 • t
RC
) • ΔV
S
Where t
START
and t
RC
are in milliseconds and ΔV
S
is the
change in supply in volts. The low supply current of the
LT6700 should not cause significant voltage drop due to
a 2k maximum series R.
Flexible Window Comparator
Using the LT6700-1/LT6700HV-1 as shown in the circuits
of Figure 1, the wire-AND configuration permits high
accuracy window functions to be implemented with a
simple 3-resistor voltage divider network. The section A
comparator provides the V
L
trip-point and the section B
comparator provides the V
H
trip-point, with the built-in
hysteresis providing about 1.7% recovery level at each
trip point to prevent output chatter.
For designs that are to be optimized to detect
departure
from a window limit, the nominal resistor divider values
are selected as follows (refer to the resistor designators
shown on the first circuit of Figure 1):
R1 ≤ 400k (this sets the divider current >> I
B
of inputs)
R2 = R1 • (0.98 • V
H
/V
L
– 1)
R3 = R1 • (2.5 • V
H
– 0.98 • V
H
/V
L
)
To create window functions optimized for detecting
entry
into a window (i.e., where the output is to indicate a “com-
ing into spec” condition, as with the examples in Figure 1),
Figure 1. Simple Window Comparator
the nominal resistor values are selected as follows:
R1 ≤ 400k (this sets the divider current >> I
B
of inputs)
R2 = R1 • (1.02 • V
H
/V
L
– 1)
R3 = R1 • (2.54 • V
H
– 1.02 • V
H
/V
L
)
The worst-case variance of the trip-points is related to
the specified threshold limits of the LT6700/LT6700HV
device and the basic tolerance of divider resistors used.
For resistor tolerance R
TOL
(e.g. 0.01 for 1%), the worst-
case trip-point voltage (either V
H
or V
L
) deviations can be
predicted as follows (italicized values are taken from the
data sheet, expressed in volts):
Max dev V
TRIP↑
= ±V
TRIPnom
• {2 • R
TOL
• [(V
TRIPnom
– 0.4)
/ V
TRIPnom
] + 1.25 • ( V
TH(R)max
– V
TH(R)min
)}
Max dev V
TRIP↓
= ±V
TRIPnom
• { 2 • R
TOL
• [(V
TRIPnom
– 0.39)
/ V
TRIPnom
] + 1.27 • ( V
TH(F)max
– V
TH(F)min
)}
Generating an External Reference Signal
In some applications, it would be advantageous to have
access to a signal that is directly related to the internal
400mV reference, even though the reference itself is not
available externally. This can be accomplished to a reason-
able degree by using an inverting comparator section as
a “bang-bang” servo, establishing a nominal voltage, on
an integration capacitor, that is scaled to the reference.
This method is used in Figure 2, where the reference level
has been doubled to drive a resistor bridge. The section
B output cycles on and off to swing the section B input
between its hysteresis trip points as the load capacitor
LT6700-1
V
S
GND
R3
301k
R2
6.04k
33k
+INA
–INB OUTB
OUTA
V
OUT
V
S
V
L
V
H
3.3V
3.3V Supply Monitor 5V Supply Monitor
R1
40.2k
V
OUT
HIGH = (3.1V < V
S
< 3.5V)
HYSTERESIS ZONES
APPROXIMATELY
2% OF TRIP VOLTAGE
LT6700-1
V
S
GND
487k
6.04k
33k
+INA
–INB OUTB
OUTA
5V
40.2k
V
OUT
HIGH = (4.7V < V
S
< 5.3V)
6700123 F01
0.1µF 0.1µF