LM2746MXAX datasheet(10/24 Pages) NSC

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Application Information (Continued)

Where f

SW is in Hz and RFADJ is in k

Ω.

TRACKING A VOLTAGE LEVEL

The LM2746 can track the output of a master power supply

during soft-start by connecting a resistor divider to the SS/

TRACK pin. In this way, the output voltage slew rate of the

LM2746 will be controlled by the master supply for loads that

require precise sequencing. Because the output of the mas-

ter supply is divided down, in order to track properly the

output voltage of the LM2746 must be lower than the voltage

of the master supply. When the tracking function is used no

soft-start capacitor should be connected to the SS/TRACK

pin. However in all other cases, a C

SS value of at least 1nF

between the soft-start pin and ground should be used.

One way to use the tracking feature is to design the tracking

resistor divider so that the master supply’s output voltage

OUT1) and the LM2746’s output voltage (represented sym-

bolically in Figure 1 as V

OUT2, i.e. without explicitly showing

the power components) both rise together and reach their

target values at the same time. For this case, the equation

governing the values of the tracking divider resistors R

T1 and

T2 is:

The current through R

T1 should be about 3-4 mA for precise

tracking. The final voltage of the SS/TRACK pin should be

set higher than the feedback voltage of 0.6V (say about

0.65V as in the above equation). If the master supply voltage

were 5V and the LM2746 output voltage were 1.8V, for

example, then the value of R

T1 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

TRACKING A 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 LM2746 al-

ways reaches regulation before the output voltage of the

master supply. In this case, the tracking resistors can be

determined based on the following equation:

For the example case of V

OUT1 = 5V and VOUT2 = 1.8V, with

T1 set to 150

Ω as before, R

T2 is calculated from the above

equation to be 265

Ω. A timing diagram for the case of equal

slew rates is shown in Figure 3.

SEQUENCING

The start up/soft-start of the LM2746 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 4.

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FIGURE 1. Tracking Circuit

20147708

FIGURE 2. Tracking with Equal Soft-Start Time

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FIGURE 3. Tracking with Equal Slew Rates

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