9.2.1.2 Detailed Design Procedure

9.2.1.2.1 Bidirectional Translation

For the bidirectional translation configuration (higher voltage to lower voltage or lower voltage to higher voltage),

the EN input must be connected to Vref_B and both pins must be pulled up to the HIGH side VCCB through a

bias resistor (typically 200 kΩ). This allows Vref_B to regulate the EN input and bias the channels for proper

translation. A filter capacitor on Vref_B is recommended for a stable supply at the device. The controller output

driver can be push-pull or open-drain (pull-up resistors may be required) and the peripheral device output can be

push-pull or open-drain (pull-up resistors are required to pull the Bn outputs to VPU).

Note

If either output is push-pull, data must be unidirectional or the outputs must be tri-state and be

controlled by some direction-control mechanism to prevent HIGH-to-LOW bus contention in either

direction. If both outputs are open-drain, no direction control is needed.

9.2.1.2.2 Pull-Up Resistor Sizing

The pull-up resistor value needs to limit the current through the pass transistor when it is in the ON state to about

15 mA. Doing this causes a voltage drop of 260 mV to 350 mV to have a valid LOW signal on the downstream

channel. If the current through the pass transistor is higher than 15 mA, the voltage drop is also higher in the

ON state. To set the current through each pass transistor at 15 mA, calculate the pull-up resistor value using the

following equation:

Rpu = Vpu − 0.35 V

0.015 A  

(3)

Table 9-3 provides resistor values, reference voltages, and currents at 8 mA, 5 mA, and 3 mA. The resistor value

shown in the +10% column (or a larger value) should be used so that the voltage drop across the transistor is

350 mV or less. The external driver must be able to sink the total current from the resistors on both sides of

the LSF family device at 0.175 V, although the 15 mA applies only to current flowing through the LSF family

device. The device driving the low state at 0.175 V must sink current from one or more of the pull-up resistors

and maintain VOL. A decrease in resistance will increase current, and thus result in increased VOL.

Table 9-3. Pull-Up Resistor Values

VPU (1) (2)

8 mA

5 mA

3 mA

NOMINAL (Ω)

+10%(3) (Ω)

NOMINAL (Ω)

+10%(3) (Ω)

NOMINAL (Ω)

+10%(3) (Ω)

5 V

581

639

930

1023

1550

1705

3.3 V

369

406

590

649

983

1082

2.5 V

269

296

430

473

717

788

1.8 V

181

199

290

319

483

532

1.5 V

144

158

230

253

383

422

1.2 V

106

117

170

187

283

312

(1)

Calculated for VOL = 0.35 V

(2)

Assumes output driver VOL = 0.175 V at stated current

(3)

+10% to compensate for VDD range and resistor tolerance

9.2.1.2.3 Single Supply Translation

Sometimes, an external device will have an unknown voltage that could be above or below the desired

translation voltage, preventing a normal connection of the LSF. Resistors are added on the A side in place

of the second supply in this case – this is an example of when LSF single supply operation is utilized, shown in

Figure 9-5. In the following figure, a single 3.3 V supply is used to translate between a 3.3 V device and a device

that can change between 1.8 V and 5.0 V. R1 and R2 are added in place of the second supply. Note that due to

some current coming out of the Vref_A pin, this cannot be treated as a simple voltage divider.

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