8.3.3.1 Up and Down Translation

8.3.3.1.1 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 pull-up 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 of the device's output is open drain or its input has a leakage

greater than 1 µA.

1.8 V

Vref_A

VCCB

EN

200 k

A1

A2

B1

B2

GND

LSF010x-Q1

Vref_B

VCCA

1.8 V

Device

3.3 V

RB1

3.3 V

Device

0.1 μF

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 shows the maximum data rate formula and Equation 2

shows 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.

1

3×2RB1CB1=  16RB1CB1  bitssecond

(1)

IOL ≅ VCCARA1+ VCCBRB1 A

(2)

8.3.3.1.2 Down Translation

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. For a summary of device operation, refer to Section 8.3.3. 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.

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LSF0102-Q1

SDLS969B – MAY 2018 – REVISED MAY 2023

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