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LSF0102-Q1

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SDLS969 – MAY 2018

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Typical Application (continued)

(1)

Vref_A is required to be the lowest voltage level across all inputs and outputs.

Table 2. Application Operating Condition

PARAMETER

MIN

TYP

MAX

UNIT

reference voltage (A)

0.95

4.5

V

Vref_B

reference voltage (B)

Vref_A + 0.8

5.5

V

input voltage on EN pin

Vref_A + 0.8

Vref_B

5.5

V

Vpu

pull-up supply voltage

0

Vref_B

V

(1)

(2)

(3)

The 200 k

Ω, pull-up resistor is required to allow Vref_B to regulate the EN input. A filter capacitor on Vref_B is

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 k

Ω), as shown in Figure 7. This allows Vref_B to regulate the EN input. A filter capacitor on Vref_B

is recommended. The master output driver can be push-pull or open-drain (pull-up resistors may be required)

and the slave 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 7, the reference supply voltage (Vref_A) is connected to the processor core power supply voltage.

When Vref_B is connected through a 200 k

Ω resistor to a 3.3 V Vpu power supply, and Vref_A is set 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 Vpu.

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

(1)

The appropriate pull up resistor will depend on the current requirements of the application. Table 3 summarizes

resistor values, reference voltages, and currents at 15 mA, 10 mA, and 3 mA. The resistor value shown in the

+10% column (or a larger value) should be used to ensure that the pass voltage of 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 LSF0102-

Q1 device at 0.175 V, although the 15 mA applies only to current flowing through the LSF0102-Q1.

15 mA

10 mA

3 mA

NOMINAL (Ω)

NOMINAL (Ω)

NOMINAL (Ω)

5 V

310

341

465

512

1550

1705

3.3 V

197

217

295

325

983

1082

2.5 V

143

158

215

237

717

788

1.8 V

97

106

145

160

483

532

1.5 V

77

85

115

127

383

422

1.2 V

57

63

85

94

283

312