LM20333MH National Semiconductor, LM20333MH Datasheet - Page 14

• Manufacturer Part Number: LM20333MH
• Description: BUCK REG, 3A, ADJ, POWERWISE, 20TSSOP
• Manufacturer: National Semiconductor

Specifications of LM20333MH

• Primary Input Voltage: 36V
• No. Of Outputs: 1
• Output Voltage: 32V
• Output Current: 3A
• Voltage Regulator Case Style: TSSOP
• No. Of Pins: 20
• Operating Temperature Range: -40°C To +125°C
• Svhc: No SVHC
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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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.
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.
FIGURE 5. Sequencing LM20333 with Precision Enable
B
is chosen the resistor R
B
can be selected by the user to control
A
can be solved using the equa-
SS
to completely discharge
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IH_EN
TO
PG
) the tol-
) to pull
), and
14
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.
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.
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.
FIGURE 7. Common Start Up Sequences
FIGURE 6. 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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