lt6700hvcs6-3-trpbf Linear Technology Corporation, lt6700hvcs6-3-trpbf Datasheet - Page 17

• Manufacturer Part Number: lt6700hvcs6-3-trpbf
• Description: Micropower, Low Voltage, Dual Comparator With 400mv Reference
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Instrumentation Grade Pulse Width Modulator (PWM)
Comparators with hysteresis are frequently employed
to make simple oscillator structures, and the LT6700/
LT6700HV 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 difference, rather like an isolated instrumentation
amplifi er. 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
states on the 0.22μF “integration” capacitor as it remains
balanced at ≈400mV by feedback through the NOT gate.
The input sense voltage, V
0V TO 2V
V
22μF
+
–
IN
**
††
*
†
10k**
1% METAL FILM
DELETE FOR PWM MODE
CONNECT FOR PWM MODE
OPTIMIZED FOR 2kHz ΔΣ SAMPLING, f
IN
NC7S14
309k*
, is converted to an imbal-
5 • V
REF
= 2V
0.22μF
309k*
††
Figure 3. Isolated PWM or Δ∑ Converter
470Ω
100k*
COIN CELL
Lithium
412k*
100k*
PWM(MAX)
3V NOM (I
+
+INA
–INB OUTB
LT6700-1
≈ 0.6kHz
GND
V
S
OUTA
S
LT6700-1/LT6700-2/LT6700-3/
< 3mA)
ance current that the NOT gate duty cycle is continually
correcting for, thus the digital waveform at the section A
comparator output is a PWM representation of V
respect to the 2V “full scale.” In this particular circuit, the
PWM information drives the LED of an optocoupler, allow-
ing the V
barrier. As an additional option to the circuit, the feedback
loop can be broken and a second optocoupler employed to
provide the charge balance management. This confi gura-
tion allows for clocking the comparator output (externally
to this circuit) and providing synchronous feedback such
that a simple Δ∑ voltage-to-frequency conversion can be
formed if desired. Approximately 11-bit accuracy and noise
performance was observed in a one second integration
period for duty factors from 1% to 99%.
0.1μF
10k
0.1μF
LT6700HV-1/LT6700HV-2/
10k
IN
6700123 F03
information to be coupled across a dielectric
10k
750Ω
†
6
5
1
2
SAMPLE
3V/5V
3V/5V
ΔΣ
IN
6
5
1
2
10k
750**
MOC-207
MOC-207**
PWM OUT
(OR ΔΣ SENSE)
LT6700HV-3
17
IN
6700123ff
with