lt6700hdcb-3-trpbf Linear Technology Corporation, lt6700hdcb-3-trpbf Datasheet - Page 10

• Manufacturer Part Number: lt6700hdcb-3-trpbf
• Description: Micropower, Low Voltage, Dual Comparator With 400mv Reference
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT6700-1/LT6700-2/LT6700-3
To create window functions optimized for detecting entry
into a window (i.e. where the output is to indicate a
“coming into spec” condition, as with the examples in
Figure 1), the nominal resistor values are selected as
follows:
The worst-case variance of the trip-points is related to the
specified threshold limits of the LT6700 device and the
basic tolerance of divider resistors used. For resistor
tolerance R
point voltage (either V
as follows (italicized values are taken from the datasheet,
expressed in volts):
Max dev V
/ V
Max dev V
/ V
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
10
TRIPnom
R1 ≤ 400k (this sets the divider current >> I
R2 = R1 • (1.02 • V
R3 = R1 • (2.54 • V
TRIPnom
TRIP↓
] + 1.27 • ( V
TRIP↑
] + 1.25 • ( V
TOL
= ±V
= ±V
(e.g. 0.01 for 1%), the worst-case trip-
U
TRIPnom
TRIPnom
H
TH(F)max
H
H
TH(R)max
/V
or V
– 1.02 • V
U
L
– 1)
L
• {2 • R
• {2 • R
) deviations can be predicted
3.3V Supply Monitor
R3
301k
R2
6.04k
R1
40.2k
– V
– V
+INA
–INB OUTB
TH(F)min
LT6700-1
H
TOL
W
TOL
GND
TH(R)min
/V
V
S
OUTA
L
• [(V
)
• [(V
)}
TRIPnom
Figure 1. Simple Window Comparator
TRIPnom
)}
3.3V
0.1µF
33k
B
U
V
of inputs)
OUT
V
HIGH = (3.1V < V
2% OF TRIP VOLTAGE
OUT
HYSTERESIS ZONES
– 0.39)
APPROXIMATELY
– 0.4)
V
L
V
S
V
S
H
< 3.5V)
charges and discharges in a shallow, controlled fashion.
The multiplied reference signal also contains ripple that is
the hysteresis multiplied by the same factor, so additional
filtering is performed at the sense node of the bridge to
prevent comparator chatter in the section A comparator,
which is performing the actual conditional decision for the
circuit.
Instrumentation Grade Pulse Width Modulator (PWM)
Comparators with hysteresis are frequently employed to
make simple oscillator structures, and the LT6700 lends
itself nicely to forming a charge-balancing PWM function.
The circuit shown in Figure 3 forms a PWM that is intended
to transmit an isolated representation of a voltage differ-
ence, rather like an isolated instrumentation amplifier. The
section B comparator is used to generate a 2V reference
supply level for the CMOS NOT gate (inverter), which
serves as the precision switch element for the charge
balancer. The heart of the charge balancer is the section A
comparator, which is detecting slight charge or discharge
487k
6.04k
40.2k
Figure 2. Micropower Thermostat/Temperature Alarm
5V Supply Monitor
3.3µF
+INA
–INB OUTB
LT6700-1
GND
V
R
R
*RESISTANCE MAY REQUIRE OPTIMIZATION FOR OPERATION
HYSTERESIS ZONE ≈0.4°C
S
OUTA
OVER INTENDED R
TH
SET
R
= 1M (e.g., YSI 44015, 1.00MΩ AT 25°C)
TH
= R
2 • V
TH
AT T
REF
R
T
SET
5V
SET
6700123 F01
0.1µF
33k
TH
499k
499k
AND V
V
HIGH = (4.7V < V
OUT
0.1µF
SUPPLY
+INA
–INB OUTB
LT6700-1
GND
V
S
OUTA
RANGES
S
< 5.3V)
10k
6700123 F02
220k
220k*
1.4V TO 18V
(I
T < T
3.3µF
S
≈ 10µA)
SET
6700123fd