R
2
=
(R
7
+R
1
)(V
PU
-V
REFA
) - R
1
V
TH
R
7
R
1
V
REFA
3.3V
Vref_A
Vref_B
EN
A1
A2
B1
B2
GND
LSF0102
200 k
3.3V
1 M
1.8-5.5V
379 k
3.3V
Device
1.8-5.5V
Device
This node sets
~1.8V threshold
R1
R2
R7
Setup
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SLVUB53 – July 2017
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LSF-EVM Hardware User's Guide
3.3.2
Single-Supply Operation
Single-supply operation enables the use of a single supply to bias the LSF to enable translation. The
primary application for this setup is when the A side has a variable supply. Because the VREF_A voltage
must be 0.8 V less than the VREF_B voltage, this setup enables the user to bias the device and allow for
a wider range of voltages on the A side.
NOTE:
One limitation of this setup is that it does not allow for a pullup to be placed between the A
side output and the VREF_A pin. A separate supply is required for up translation on the A
side.
Figure 10. LSF0102 Single-Supply Operation Setup
To enable single-supply operation, resistors R
1
and R
2
must be populated, see
Figure 12
.
Use
Equation 2
to determine the value of R
1
to set the threshold of the device:
where
•
V
REFA
is the lowest voltage that is desired on the A side.
•
V
PU
is the supply voltage provided at the J2 jumper and must be 0.85 V higher than V
REFA
.
•
V
TH
is the threshold drop of the device, assumed to be 0.85 V.
•
R7 is the pullup resistor between V
REFB
and V
PU
, typically 200 k
Ω
.
•
R1 is the resistor that connects V
PU
to V
REFA
, recommended 1 M
Ω
.
•
R2 is the resistor that connects V
REFA
to ground.
(2)
Figure 11. LSF-EVM Without Single-Supply Resistors
Figure 12. LSF-EVM With Single-Supply Resistors
Populated