RS1

SRSD = SROUT1

RS1 + RS2

Master Power

Supply

VOUT1

LM2745/8

RS1

RFB2

RFB1

FB

VOUT2

VFB

SD

RS2

5V

1.8V

1.8V

VOUT1

VOUT2

LM2745, LM2748

SNOSAL2E – APRIL 2005 – REVISED APRIL 2013

www.ti.com

For the example case of VOUT1 = 5V and VOUT2 = 1.8V, with RT1 set to 150Ω 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

SEQUENCING

The start up/soft-start of the LM2745/8 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.

Figure 22. Sequencing Circuit

A desired delay time tDELAY between the startup of the master supply output voltage and the LM2745/8 output

voltage can be set based on the SD pin low-to-high threshold VSD-IH and the slew rate of the voltage at the SD

pin, SRSD:

tDELAY = VSD-IH / SRSD

Note again, that in Figure 22, the LM2745/8’s output voltage has been represented symbolically as VOUT2, i.e.

without explicitly showing the power components.

VSD-IH is typically 1.08V and SRSD is the slew rate of the SD pin voltage. The values of the sequencing divider

resistors RS1 and RS2 set the SRSD based on the master supply output voltage slew rate, SROUT1, using the

following equation:

12

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