lm2746mxax National Semiconductor Corporation, lm2746mxax Datasheet - Page 10

• Manufacturer Part Number: lm2746mxax
• Description: Low Voltage N-channel Mosfet Synchronous Buck Regulator Controller
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
Where f
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
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
(V
bolically in Figure 1 as V
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
R
The current through R
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
supplies identical soft-start times would be 150Ω. A timing
diagram for the equal soft-start time case is shown in Figure
2.
T2
OUT1
is:
) and the LM2746’s output voltage (represented sym-
SW
is in Hz and R
FIGURE 1. Tracking Circuit
T1
OUT2
should be about 3-4 mA for precise
FADJ
, i.e. without explicitly showing
is in kΩ.
T1
needed to give the two
SS
value of at least 1nF
(Continued)
20147707
T1
and
10
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
R
equation to be 265Ω. 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.
T1
set to 150Ω as before, R
FIGURE 2. Tracking with Equal Soft-Start Time
FIGURE 3. Tracking with Equal Slew Rates
OUT1
T2
= 5V and V
is calculated from the above
OUT2
= 1.8V, with
20147708
20147710