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參數(shù)資料
| 型號(hào)： | LM20154MHX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 4A, 1MHz PowerWise㈢ Synchronous Buck Regulator with SYNCOUT |
| 中文描述： | 6.6 A SWITCHING REGULATOR, 1150 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | TSSOP-16 |
| 文件頁數(shù)： | 16/22頁 |
| 文件大?。?/td> | 631K |
| 代理商： | LM20154MHX |
mately 615
. To insure a specific logic low level is met the
pull-up resistor should be selected using the formula below.
Where, V
PULLUP
is the high pull up voltage for the resistor
R
and V
pin. For most applications the SYNCOUT pin will be pulled up
to the input voltage.
USING PRECISION ENABLE AND POWER GOOD
The precision enable (EN) and power good (PGOOD) pins of
the LM20154 can be used to address many sequencing re-
quirements. The turn-on of the LM20154 can be controlled
with the precision enable pin by using two external resistors
as shown in
Figure 6
30030826
FIGURE 6. Sequencing LM20154 with Precision Enable
The value for resistor R
can be selected by the user to control
the current through the divider. Typically this resistor will be
selected to be between 10 k
and 1 M
. Once the value for
R
is chosen the resistor R
can be solved using the equation
below to set the desired turn on voltage.
When designing for a specific turn-on threshold (V
TO
) the tol-
erance on the input supply, enable threshold (V
), and
external resistors needs to be considered to insure proper
turn on of the device.
The LM20154 features an open drain power good (PGOOD)
pin to sequence external supplies or loads and to provide fault
detection. This pin requires an external resistor (R
PG
) to pull
PGOOD high while when the output is within the PGOOD tol-
erance window. Typical values for this resistor range from 10
k
to 100 k
.
TRACKING AN EXTERNAL SUPPLY
By using a properly chosen resistor divider network connect-
ed to the SS/TRK pin, as shown in
Figure 7
, the output of the
LM20154 can be configured to track an external voltage
source to obtain a simultaneous or ratiometric start up.
30030820
FIGURE 7. Tracking an External Supply
Since the Soft-Start charging current I
is always present on
the SS/TRK pin, the size of R2 should be less than 10 k
to
minimize the errors in the tracking output. Once a value for
R2 is selected the value for R1 can be calculated using ap-
propriate equation in
Figure 8
, to give the desired start up.
Figure 8
shows two common start up sequences; the top
waveform shows a simultaneous start up while the waveform
at the bottom illustrates a ratiometric start up.
30030821
FIGURE 8. Common Start Up Sequences
A simultaneous start up is preferred when powering most FP-
GAs, DSPs, or other microprocessors. In these systems the
higher voltage, V
, usually powers the I/O, and the lower
voltage, V
, powers the core. A simultaneous start up pro-
vides a more robust power up for these applications since it
avoids turning on any parasitic conduction paths that may ex-
ist between the core and the I/O pins of the processor.
The second most common power on behavior is known as a
ratiometric start up. This start up is preferred in applications
where both supplies need to be at the final value at the same
time.
Similar to the Soft-Start function, the fastest start up possible
is 1 ms regardless of the rise time of the tracking voltage.
When using the track feature the final voltage seen by the SS/
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