Datasheet

V
OUT2
=
V
OUT1
R
T1
R
T1
+ R
T2
5V
1.8V
V
OUT1
V
OUT2
0.65 = V
OUT1
R
T1
R
T1
+ R
T2
Master Power
Supply
V
OUT1
= 5V
SS/TRACK
LM2743
R
T2
1 k:
R
T1
150:
R
FB2
10 k:
R
FB1
5 k:
FB
V
OUT2
= 1.8V
V
SS
= 0.65V
V
FB
LM2743
www.ti.com SNVS276G –APRIL 2004–REVISED MARCH 2013
Figure 23. Tracking Circuit
One way to use the tracking feature is to design the tracking resistor divider so that the master supply’s output
voltage (V
OUT1
) and the LM2743’s output voltage (represented symbolically in Figure 23 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 R
T2
is:
The current through R
T1
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 0.6V (say about 0.65V as in the above equation). If the
master supply voltage was 5V and the LM2743 output voltage was 1.8V, for example, then the value of R
T1
needed to give the two supplies identical soft-start times would be 150Ω. A timing diagram for the equal soft-start
time case is shown in Figure 24.
Figure 24. Tracking with Equal Soft-Start Time
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 LM2743
always reaches regulation before the output voltage of the master supply. Because the output of the master
supply is divided down, in order to track properly the output voltage of the LM2743 must be lower than the
voltage of the master supply. In this case, the tracking resistors can be determined based on the following
equation:
Copyright © 2004–2013, Texas Instruments Incorporated Submit Documentation Feedback 11
Product Folder Links: LM2743