SNVS276I April 2004 – February 2019 LM2743
PRODUCTION DATA.
The LM2743 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 LM2743 will be controlled by the master supply for loads that require precise sequencing. When the tracking function is used no soft-start capacitor should be connected to the SS/TRACK pin. Otherwise, a CSS value of at least 1 nF 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 (VOUT1) and the LM2743’s output voltage (represented symbolically in Figure 19 as VOUT2, that is, 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 RT1 and RT2 is:
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.