Master Power

Supply

SS/TRACK

LM2747

1 k:

150:

10 k:

5 k:

FB

LM2747

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SNVS370B – MARCH 2006 – REVISED MARCH 2013

OUTPUT PRE-BIAS STARTUP

If there is a pre-biased load on the output of the LM2747 during startup, the IC will disable switching of the low-

side MOSFET and monitor the SW node voltage during the off-time of the high-side MOSFET. There is no load

current sensing while in pre-bias mode because the low-side MOSFET never turns on. The IC will remain in this

pre-bias mode until it sees the SW node stays below 0V during the entire high-side MOSFET's off-time. Once it

is determined that the SW node remained below 0V during the high-side off-time, the low-side MOSFET begins

switching during the next switching cycle. Figure 19 shows the SW node, HG, and LG signals during pre-bias

ensure that the High-Side MOSFET will be able to switch during startup.

Figure 19. Output Pre-Bias Mode Waveforms

TRACKING A VOLTAGE LEVEL

The LM2747 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 LM2747 will be controlled by the master

supply for loads that require precise sequencing. When the tracking function is used no soft-start capacitor

soft-start pin and ground should be used.

Figure 20. Tracking Circuit

One way to use the tracking feature is to design the tracking resistor divider so that the master supply’s output

explicitly showing the power components) both rise together and reach their target values at the same time. For

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Product Folder Links: LM2747