LSF0102DDFR datasheet(15/57 Pages) TI

15 / 57 page

8.4.1 Up and Down Translation

Up Translation:

When the signal is being driven from A to B and the An port is HIGH, the switch will be OFF and the Bn port

will then be driven to a voltage higher than Vref_A by the pullup resistor that is connected to the pull-up supply

voltage (Vpu#). This functionality allows seamless translation between higher and lower voltages selected by the

user, without the need for directional control. Pull-up resistors are always required on the high side, and pull-ups

are only required on the low side if the low side device's output is open drain or its input has a leakage greater

than 1 µA.

Vref_A

VCCB

200 kΩ

GND

LSF0108

Vref_B

3.3 V

Device

3.3 V

RB1

0.1 μF

RA1

1.8 V

Device

1.8 V

VCCA

Figure 8-2. Up Translation Example Schematic with Push-Pull and Open Drain Configuration

Up translation with the LSF requires attention to two important factors: maximum data rate and sink current.

Maximum data rate is directly related to the rising edge of the output signal. Sink current depends on supply

values and the chosen pull-up resistor values. Equation 1 below shows the maximum data rate formula and

equation 2 presents the maximum sink current formula, both of which are estimations. A low RC value is needed

to reach high speeds, which also require strong drivers. Please see the Up Translation with the LSF Family video

for estimated data rate and sink current calculations based on circuit components.

Down Translation:

3×2RB1CB1= 16RB1CB1 bitssecond

(1)

IOL ≅ VCCARA1+ VCCBRB1 A

(2)

When the signal is being driven HIGH from the Bn port to An port, the switch will be OFF, clamping the voltage

on the An port to the voltage set by Vref_A. A pull-up resistor can be added on either side of the device. There

are special circumstances that allow the removal of one or both of the pull-up resistors. If the signal is always

going to be down translated from a push-pull transmitter, then the resistor on the B-side can be removed. If

the leakage current into the receiver on the A-side is less than 1 µA, then the resistor on the A-side can also

be removed. This arrangement with no external pull-up resistors can be used when down translating from a

push-pull output to a low-leakage input. For an open drain transmitter, the pull-up resistor on the B-side is

necessary because an open drain output can't drive high by itself. Refer to Table 8-2 for a summary of device

operation. For additional details on the functional operation of the LSF family of devices, see the Up Translation

with the LSF Family and Down Translation with the LSF Family videos.

www.ti.com

LSF0108

SDLS966L – DECEMBER 2013 – REVISED DECEMBER 2022

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