LM2744 datasheet(12/33 Pages) TI

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RS1

SRSD = SROUT1

RS1 + RS2

Master Power

Supply

VOUT1

LM2744

RS1

RFB2

RFB1

VOUT2

VFB

RS2

1.8V

VOUT1

VOUT2

LM2744

SNVS292F – SEPTEMBER 2004 – REVISED MARCH 2013

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Figure 23. Tracking with Equal Slew Rates

SEQUENCING

The start up/soft-start of the LM2744 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 24.

Figure 24. Sequencing Circuit

A desired delay time tDELAY between the startup of the master supply output voltage and the LM2744 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

(6)

Note again, that in Figure 24, the LM2744’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:

(7)

For example, if the master supply output voltage slew rate was 1V/ms and the desired delay time between the

startup of the master supply and LM2744 output voltage was 5ms, then the desired SD pin slew rate would be

(1.08V/5ms) = 0.216V/ms. Due to the internal impedance of the SD pin, the maximum recommended value for

RS2 is 1kΩ. To achieve the desired slew rate, RS2 would then be 274Ω. A timing diagram for this example is

shown in Figure 25.

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