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Hardware Description
5
SLVUB53 – July 2017
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Copyright © 2017, Texas Instruments Incorporated
LSF-EVM Hardware User's Guide
Figure 7. LSF Pass Transistor LOW-State Operation
The pullup resistors that come populated on the board are 310
Ω
.
The pullup resistor must be selected to limit the current through the pass transistor to 64 mA maximum.
To ensure a pass voltage of 260 mV to 350 mV, the pullup resistor should be sized to limit the current
through the pass transistor to 15 mA, see
Figure 7
. To set the current through each pass transistor at 15
mA, calculate the pullup resistor value using
Equation 1
:
Rpu = (Vpu – 0.35 V) / 0.015 A
(1)
Table 3
summarizes resistor values, reference voltages, and currents at 15 mA, 10 mA, and 3 mA. The
resistor value shown in the +10% column (or a larger value) must be used to ensure that the pass voltage
of the transistor is 350 mV or less.
(1)
Calculated for V
OL
= 0.35 V
(2)
Assumes output driver V
OL
= 0.175 V at stated current
(3)
+10% to compensate for V
DD
range and resistor tolerance
Table 3. Pullup Resistor Values
(1) (2)
V
PU
15 mA
10 mA
3 mA
NOMINAL (
Ω
)
+10%
(3)
(
Ω
)
NOMINAL (
Ω
)
+10%
(3)
(
Ω
)
NOMINAL (
Ω
)
+10%
(3)
(
Ω
)
5 V
310 (Populated
Resistance)
341
465
512
1550
1705
3.3 V
197
217
295
325
983
1082
2.5 V
143
158
215
237
717
788
1.8 V
97
106
145
160
483
532
1.5 V
77
85
115
127
383
422
1.2 V
57
63
85
94
283
312
2.3.3
J5 - J8 - Channel 1-2 Edge-Mounted SMB Connectors
Connectors J5 and J6 are provided for channel 1 of the device to enable optimal conditions for high-speed
operation of the LSF devices. J7 and J8 are spaces left for additional edge-mount connectors to connect
channel 2. See
Section 4.3
for orderable part numbers.
For input signals that are being driven by a 50-
Ω
source, TI recommends to have a 50-
Ω
termination
either at the device, see
Figure 8
.
2.3.4
J9, J10 - Channel 3-8 Header Connectors
Connectors J9 and J10 are provided for channels 3-8 of the device to enable the remaining connections
for the LSF0108 device. Each output has a corresponding ground connection to enable measurement of
the output using a differential probe for optimal signal integrity.
2.3.5
J11, J12 - Additional Headers for LSF0204 and LSF0204D Operation
J11 and J12 are provided as connectors for use when the LSF0204 device is populated. See
Section 3.2
for additional information.