8.2.1.1 Design Requirements

8.2.1.1.1 Enable and Disable Guidelines

In the previous figure, VBIAS is referenced to the lower power supply (VEXT,A) of 1.8V power supply and VEXT,B

is set to 3.3V. The A1 and A2 channels have a maximum output voltage equal to VEXT,A and the B1 and B2

channels have a maximum output voltage equal to VEXT,B.

The LSF0002 has an VBIAS input that is used to disable the device by driving this node LOW to GND, placing

all I/Os in the high-impedance state. Since the LSF family of devices are switch-type voltage translators, the

power consumption is very low. TI recommends always enabling the LSF family for bidirectional applications

(I2C, SMBus, PMBus, or MDIO).

Table 8-2. Application Operating Condition

PARAMETER

MIN

TYP

MAX

UNIT

VEXT,A (1)

reference voltage (A)

0.9

5.5

V

VEXT,B

reference voltage (B)

VEXT,A + 0.8

5.5

V

VBIAS

input voltage on VBIAS pin

VEXT,A

VEXT,A + 0.8

V

VPU

pull-up supply voltage

0

VEXT,B

V

(1)

VEXT,A is required to be the lowest voltage level across all inputs and outputs.

Note

The 200kΩ, bias resistor is not required for the LSF0002.

8.2.1.2 Detailed Design Procedure

8.2.1.2.1 Bidirectional Translation

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

the VBIAS input must be connected to VEXT,A. This allows VBIAS to regulate the bias of the I/O channels for proper

translation. A filter capacitor on VBIAS 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.

8.2.1.2.2 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

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

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

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

following equation:

Rpu = Vpu − 0.35 V

0.015 A  

(1)

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

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

is 350mV 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 0.175V, although the 15mA applies only to current flowing through the LSF family

device. The device driving the low state at 0.175V 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.

LSF0002

SDLS975 – APRIL 2024

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