參數(shù)資料
型號(hào)： LM2744
廠商： National Semiconductor Corporation
英文描述： Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller with External Reference
中文描述： 低壓N溝道MOSFET同步降壓穩(wěn)壓控制器與外部參考
文件頁(yè)數(shù)： 10/22頁(yè)
文件大?。?/td> 663K
代理商： LM2744
Application Information
(Continued)
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 mas-
ter 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
value of at least 1nF
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
) and the LM2744’s output voltage (represented sym-
bolically in
Figure 1
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 3-4 mA for precise
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
example, then the value of R
needed to give the two
supplies identical soft-start times would be 150
if V
was
set to 0.6V. A timing diagram for the equal soft-start time
case is shown in
Figure 2
.
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 LM2744 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
= 5V and V
= 1.8V, with
R
set to 150
as before, R
is calculated from the above
equation to be 267
. 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
.
20106007
FIGURE 1. Tracking Circuit
20106008
FIGURE 2. Tracking with Equal Soft-Start Time
20106010
FIGURE 3. Tracking with Equal Slew Rates
L
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10
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