LM20145EVAL National Semiconductor, LM20145EVAL Datasheet - Page 15

• Manufacturer Part Number: LM20145EVAL
• Description: BOARD EVAL 5A POWERWISE LM20145
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM20145EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.2V
• Current - Output: 5A
• Voltage - Input: 2.95 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 500kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM20145
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

If different start up times are needed the equation shown be-
low can be used to calculate the start up time.
As shown above, the start up time is influenced by the value
of the Soft-Start capacitor C
current I
istics table.
While the Soft-Start capacitor can be sized to meet many start
up requirements, there are limitations to its size. The Soft-
Start time can never be faster than 1ms due to the internal
default 1 ms start up time. When the device is enabled there
is an approximate time interval of 50 µs when the Soft-Start
capacitor will be discharged just prior to the Soft-Start ramp.
If the enable pin is rapidly pulsed or the Soft-Start capacitor
is large there may not be enough time for C
discharge resulting in start up times less than predicted. To
aid in discharging of Soft-Start capacitor during long disable
periods an external 1 MΩ resistor from SS/TRK to ground can
be used without greatly affecting the start-up time.
USING PRECISION ENABLE AND POWER GOOD
The precision enable (EN) and power good (PGOOD) pins of
the LM20145 can be used to address many sequencing re-
quirements. The turn-on of the LM20145 can be controlled
with the precision enable pin by using two external resistors
as shown in Figure 6.
The value for resistor R
the current through the divider. Typically this resistor will be
selected to be between 10 kΩ and 1 MΩ. Once the value for
R
below to set the desired turn-on voltage.
When designing for a specific turn-on threshold (V
erance on the input supply, enable threshold (V
external resistors needs to be considered to insure proper
turn-on of the device.
The LM20145 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
PGOOD high while when the output is within the PGOOD tol-
erance window. Typical values for this resistor range from 10
kΩ to 100 kΩ.
FIGURE 6. Sequencing LM20145 with Precision Enable
B
is chosen the resistor R
SS
(A). that may be found in the electrical character-
B
can be selected by the user to control
A
SS
can be solved using the equation
(F) and the 5 µA Soft-Start pin
SS
to completely
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IH_EN
TO
PG
) the tol-
) to pull
), and
15
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
LM20145 can be configured to track an external voltage
source to obtain a simultaneous or ratiometric start up.
Since the Soft-Start charging current I
the SS/TRK pin, the size of R2 should be less than 10 kΩ 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.
A simultaneous start up is preferred when powering most FP-
GAs, DSPs, or other microprocessors. In these systems the
higher voltage, V
voltage, V
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..
FIGURE 8. Common Start Up Sequences
FIGURE 7. Tracking an External Supply
OUT2
, powers the core. A simultaneous start up pro-
OUT1
, usually powers the I/O, and the lower
SS
is always present on
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