9.2.1.2 Detailed Design Procedure

9.2.1.2.1 Bidirectional Translation

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

the EN input must be connected to Vref_B and both pins pulled to HIGH side Vpu through a pull-up resistor

(typically 200 kΩ). This allows Vref_B to regulate the EN input. A filter capacitor on Vref_B is recommended.

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, then data must be unidirectional or the outputs must be tri-state and

be controlled by some direction-control mechanism to prevent HIGH-to-LOW contentions in either

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

Figure 9-1 shows how the reference supply voltage (Vref_A) is connected to the processor core power supply

voltage. When Vref_B is connected through a 200 kΩ resistor to a 3.3 V Vpu power supply, and Vref_A is set

1 V. The output of A3 and B4 has a maximum output voltage equal to Vref_A, and the bidirectional interface

(Ch1/2, MDIO) has a maximum output voltage equal to Vpu.

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. This ensures 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 summarizes 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 to ensure 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

VDPU (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

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LSF0108-Q1

SDLS967E – MAY 2016 – REVISED NOVEMBER 2022

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