LM20333EVAL National Semiconductor, LM20333EVAL Datasheet - Page 14

• Manufacturer Part Number: LM20333EVAL
• Description: EVALUATION BOARD FOR THE LM20333
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM20333EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• Current - Output: 3A
• Voltage - Input: 4.5 ~ 25V
• Regulator Topology: Buck
• Board Type: Fully Populated
• Utilized Ic / Part: LM20333
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Frequency - Switching: -

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SETTING THE START UP TIME (C
The addition of a capacitor connected from the SS pin to
ground sets the time at which the output voltage will reach the
final regulated value. Larger values for C
start up times. Table 3, shown below provides a list of soft
start capacitors and the corresponding typical start up times.
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
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 1 ms 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 capac-
itor 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
resulting in start up times less than predicted. To aid in dis-
charging of soft-start capacitor during long disable periods an
external 1MΩ 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 LM20333 can be used to address many sequencing re-
quirements. The turn-on of the LM20333 can be controlled
with the precision enable pin by using two external resistors
as shown in Figure 5 .
The value for resistor R
the current through the divider. Typically this resistor will be
selected to be between 10 kΩ and 49.9 kΩ. Once the value
for R
tion below to set the desired turn-on voltage.
FIGURE 5. Sequencing LM20333 with Precision Enable
TABLE 3. Start Up Times for Different Soft-Start
B
Start Up Time (ms)
is chosen the resistor R
SS
.
10
15
20
1
5
B
can be selected by the user to control
Capacitors
SS
A
can be solved using the equa-
and the 4.5 µA soft-start pin
SS
SS
to completely discharge
)
SS
C
will result in longer
SS
none
100
120
33
68
(nF)
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14
When designing for a specific turn-on threshold (V
erance on the input supply, enable threshold (V
external resistors need to be considered to ensure proper
turn-on of the device.
The LM20333 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 when the output is within the PGOOD tolerance
window. Typical values for this resistor range from 10 kΩ to
100 kΩ.
TRACKING AN EXTERNAL SUPPLY
By using a properly chosen resistor divider network connect-
ed to the SS/TRK pin, as shown in Figure 6, the output of the
LM20333 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 7, to give the desired start up.
Figure 6 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.
FIGURE 6. Tracking an External Supply
SS
is always present on
IH_EN
TO
PG
) the tol-
) to pull
30051661
), and