9.2.1.1.2 Bias Circuitry

For proper operation, VCCA must always be at least 0.8 V less than VCCB (VCCA + 0.8 ≦ VCCB). The 200 kΩ bias

resistor is required to allow Vref_B to regulate the EN input and properly bias the device for translation. A 0.1 µF

capacitor is recommended for providing a path from Vref_B to ground for high frequency noise. Vref_B and VI(EN)

are recommended to be 1.0 V higher than Vref_A for best signal integrity.

Attempting to drive the EN pin directly with a push-pull output device is a very common design error with the

LSF0102-Q1 series of devices. It is also very important to note that current does flow into the A-side voltage

supply during normal operation. Not all voltage sources can sink current, so be sure that applicable designs can

handle this current. For more design details, see the Understanding the Bias Circuit for the LSF Family video.

1.8 V

Vref_A

VCCB

EN

200 kΩ

Vref_B

3.3 V

A1

B1

0.1 µF

Figure 9-2. Bias Circuitry Inside the LSF010x-Q1 Device

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 VCCB through a bias resistor

(typically 200 kΩ), as shown in Figure 9-1. 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).

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 contention in either direction. If both outputs are

open-drain, no direction control is needed.

In Figure 9-1, the reference supply voltage Vref_A is connected to the processor core power supply voltage. Vref_B

is connected through a 200 kΩ resistor to a 3.3 V VB_Pullup power supply and Vref_A is set to 1.2 V. The output of

A1 has a maximum output voltage equal to Vref_A, and the bidirectional interface on channel 2 has a maximum

output voltage equal to VPU1.

9.2.1.2.2 Pull-Up Resistor Sizing

To maintain an appropriate output low voltage, the pull-up resistor value should limit the current through the pass

transistor when it is in the ON state to less than 15 mA. This ensures a pass voltage of 260 mV to 350 mV.

To set the current through each pass transistor at 15 mA, the pull-up resistor value can be calculated using the

following equation:

Rpu = Vpu − 0.35 V

0.015 A  

(3)

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LSF0102-Q1

SDLS969B – MAY 2018 – REVISED MAY 2023

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