LM2744EVAL National Semiconductor, LM2744EVAL Datasheet - Page 10

• Manufacturer Part Number: LM2744EVAL
• Description: BOARD EVALUATION LM2744
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM2744EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.2V
• Current - Output: 3.5A
• Voltage - Input: 1.8 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 1MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2744
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

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Where F
TRACKING A VOLTAGE LEVEL
The LM2744 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
LM2744 will be controlled by the master supply for loads that
require precise sequencing. Because the output of the master
supply is divided down, in order to track properly the output
voltage of the LM2744 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
tween 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
slightly higher than the reference voltage (say about 50mV
higher as in the above equation). If the master supply voltage
was 5V and the LM2744 output voltage was 1.8V, for exam-
ple, then the value of R
identical soft-start times would be 150Ω if V
0.6V. A timing diagram for the equal soft-start time case is
shown in Figure 2.
T2
OUT1
is:
) and the LM2744’s output voltage (represented sym-
SW
is in Hz and R
FIGURE 1. Tracking Circuit
T1
T1
should be about 3-4 mA for precise
OUT2
FADJ
needed to give the two supplies
, i.e. without explicitly showing
is in kΩ.
SS
value of at least 1nF be-
REF
was set to
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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 en-
sures that the output voltage of the LM2744 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:
For the example case of V
R
equation to be 267Ω. A timing diagram for the case of equal
slew rates is shown in Figure 3.
SEQUENCING
The start up/soft-start of the LM2744 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
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