LT6700HDCB-3 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT6700HDCB-3' PDF : 20 Pages View PDF
LT6700/LT6700HV
applications information
using a supply filter, the start-up time of the LT6700 will
increase to:
tSTART = (0.17ms + 0.25 • tRC) • ΔVS
Where tSTART and tRC are in milliseconds and ΔVS 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 VL trip-point and the section B
comparator provides the VH 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 >> IB of inputs)
R2 = R1 • (0.98 • VH/VL – 1)
R3 = R1 • (2.5 • VH – 0.98 • VH/VL)
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),
the nominal resistor values are selected as follows:
R1 ≤ 400k (this sets the divider current >> IB of inputs)
R2 = R1 • (1.02 • VH/VL – 1)
R3 = R1 • (2.54 • VH – 1.02 • VH/VL)
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 RTOL (e.g. 0.01 for 1%), the worst-
case trip-point voltage (either VH or VL) deviations can be
predicted as follows (italicized values are taken from the
data sheet, expressed in volts):
Max dev VTRIP↑ = ±VTRIPnom • {2 • RTOL • [(VTRIPnom – 0.4)
/ VTRIPnom] + 1.25 • (VTH(R)max – VTH(R)min)}
Max dev VTRIP↓ = ±VTRIPnom • {2 • RTOL • [(VTRIPnom – 0.39)
/ VTRIPnom] + 1.27 • (VTH(F)max – VTH(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
3.3V Supply Monitor
5V Supply Monitor
0.1µF
R3
301k
VS
+INA OUTA
R2 LT6700-1
6.04k
–INB OUTB
R1
40.2k
GND
3.3V
VL
VH
VOUT
33k
VS
VOUT
HIGH = (3.1V < VS < 3.5V)
HYSTERESIS ZONES
APPROXIMATELY
2% OF TRIP VOLTAGE
0.1µF
487k
VS
+INA OUTA
6.04k LT6700-1
–INB OUTB
40.2k GND
Figure 1. Simple Window Comparator
5V
33k
VOUT
HIGH = (4.7V < VS < 5.3V)
6700123 F01
6700123fg
15
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