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LM20134MH Datasheet

  • LM20134MH

  • National Semiconductor [4A, PowerWise㈢ Synchronous Buck Re...

  • NSC

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LM20134
30030426
FIGURE 6. Sequencing LM20134 with Precision Enable
The value for resistor R
B
can be selected by the user to control
the current through the divider. Typically this resistor will be
selected to be between 10 k鈩?and 1 M鈩? Once the value for
R
B
is chosen the resistor R
A
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
IH_EN
), and
external resistors needs to be considered to insure proper
turn-on of the device.
The LM20134 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鈩?to 100 k鈩?
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
LM20134 can be configured to track an external voltage
source to obtain a simultaneous or ratiometric start up.
30030421
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
OUT1
, usually powers the I/O, and the lower
voltage, V
OUT2
, 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 1ms regardless of the rise time of the tracking voltage.
When using the track feature the final voltage seen by the SS/
TRACK pin should exceed 1V to provide sufficient overdrive
and transient immunity.
THERMAL CONSIDERATIONS
The thermal characteristics of the LM20134 are specified us-
ing the parameter
胃
JA
, which relates the junction temperature
to the ambient temperature. Although the value of
胃
JA
is de-
pendant on many variables, it still can be used to approximate
the operating junction temperature of the device.
To obtain an estimate of the device junction temperature, one
may use the following relationship:
T
J
= P
D
胃
JA
+ T
A
and
P
D
= P
IN
x (1 - Efficiency) - 1.1 x I
OUT
2 x DCR
Where:
T
J
is the junction temperature in 擄C.
P
IN
is the input power in Watts (P
IN
= V
IN
x I
IN
).
胃
JA
is the junction to ambient thermal resistance for the
LM20134.
30030420
FIGURE 7. Tracking an External Supply
Since the Soft-Start charging current I
SS
is always present on
the SS/TRK pin, the size of R2 should be less than 10 k鈩?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.
www.national.com
16

LM20134MH 產(chǎn)品屬性

  • 5.5V

  • 1

  • 5V

  • 4A

  • TSSOP

  • 16

  • -40°C 到 +125°C

  • No SVHC (19-Dec-2011)

  • TSSOP

  • 4A

  • Synchronous Buck

  • 5.5V

  • 2.95V

  • 表面安裝

  • 5.5V

  • 0.8V

  • 5V

  • 5V

  • 4A

  • 1.5MHz

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