LM2743MTCSLASHNOPB datasheet(12/44 Pages) TI2

12 / 44 page

1.8V

VOUT1

VOUT2

VOUT2 = VOUT1

RT1

RT1 + RT2

1.8V

VOUT1

VOUT2

LM2743

SNVS276I – APRIL 2004 – REVISED FEBRUARY 2019

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Feature Description (continued)

The current through RT1 should be about 3 mA to 4 mA for precise tracking. The final voltage of the SS/TRACK

pin should be set higher than the feedback voltage of 0.6 V (say about 0.65 V as in the above equation). If the

master supply voltage was 5 V and the LM2743 output voltage was 1.8 V, for example, then the value of RT1

needed to give the two supplies identical soft-start times would be 150

Ω. A timing diagram for the equal soft-

start time case is shown in Figure 20.

Figure 20. Tracking with Equal Soft-Start Time

7.3.4 Tracking Voltage Slew Rate

The tracking feature can alternatively be used not to make both rails reach regulation at the same time but rather

to have similar rise rates (in terms of output dV/dt). This method ensures that the output voltage of the LM2743

always reaches regulation before the output voltage of the master supply. Because the output of the master

supply is divided down, in order to track properly the output voltage of the LM2743 must be lower than the

voltage of the master supply. In this case, the tracking resistors can be determined based on the following

equation:

(5)

For the example case of VOUT1 = 5 V and VOUT2 = 1.8 V, with RT1 set to 150 Ω as before, RT2 is calculated from

the above equation to be 265

Ω. A timing diagram for the case of equal slew rates is shown in Figure 21.

Figure 21. Tracking with Equal Slew Rates

7.3.5 Sequencing

The start up/soft-start of the LM2743 can be delayed for the purpose of sequencing by connecting a resistor

divider from the output of a master power supply to the SD pin, as shown in Figure 22.

LM2743MTC/NOPB Datasheet(PDF) 12 Page - Texas Instruments