Datasheet

0.65 =
V
OUT2
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
LM2745/8
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
LM2745, LM2748
www.ti.com SNOSAL2E –APRIL 2005–REVISED APRIL 2013
Figure 19. 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 LM2745/8’s output voltage (represented symbolically in Figure 19 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 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 LM2745/8 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 20.
Figure 20. 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 LM2745/8
always reaches regulation before the output voltage of the master supply. In this case, the tracking resistors can
be determined based on the following equation:
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