LM2642MTCX/NOPB National Semiconductor, LM2642MTCX/NOPB Datasheet - Page 16

• Manufacturer Part Number: LM2642MTCX/NOPB
• Description: IC CTLR SW SYNC STPDN 28-TSSOP
• Manufacturer: National Semiconductor

Specifications of LM2642MTCX/NOPB

• Applications: Embedded systems, Console/Set-Top boxes
• Current - Supply: 1mA
• Voltage - Supply: 4.5 V ~ 30 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP
• For Use With: LM2642REVD EVAL - BOARD EVALUATION LM2642
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM2642MTCX

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Component Selection
Where 200nA is the maximum current drawn by FBx pin.
Example: Vnom=5V, Vfb=1.238V, Ifbmax=200nA.
Choose 60K
The output voltage is limited by the maximum duty cycle as
well as the minimum on time. Figure 9 shows the limits for
input and output voltages. The recommended maximum out-
put voltage is approximately 1V less than the nominal input
voltage. At 30V input, the minimum output is approximately
2.3V and the maximum is approximately 27V.
For input voltages below 5.5V, VLIN5 must be connected to
Vin through a small resistor (approximately 4.7 ohm). This
will ensure that VLIN5 does not fall below the UVLO thresh-
old.
FIGURE 9. Available Output Voltage Range
FIGURE 8. Output Voltage Setting
20046211
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20046213
(2)
(3)
(4)
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Output Capacitor Selection
In applications that exhibit large and fast load current
swings, the slew rate of such a load current transient may be
beyond the response speed of the regulator. Therefore, to
meet voltage transient requirements during worst-case load
transients, special consideration should be given to output
capacitor selection. The total combined ESR of the output
capacitors must be lower than a certain value, while the total
capacitance must be greater than a certain value. Also, in
applications where the specification of output voltage regu-
lation is tight and ripple voltage must be low, starting from the
required output voltage ripple will often result in fewer design
iterations.
ALLOWED TRANSIENT VOLTAGE EXCURSION
The allowed output voltage excursion during a load transient
(∆Vc_s) is:
Where
±
Example: Vnom = 5V, δ% = 7%, e% = 3.4%, Vrip = 40mV
peak to peak.
Since the ripple voltage is included in the calculation of
∆Vc_s, the inductor ripple current should not be included in
the worst-case load current excursion. That is, the worst-
case load current excursion should be simply maximum load
current change specification, ∆Ic_s.
MAXIMUM ESR CALCULATION
Unless the rise and fall times of a load transient are slower
than the response speed of the control loop, if the total
combined ESR (Re) is too high, the load transient require-
ment will not be met, no matter how large the capacitance.
The maximum allowed total combined ESR is:
Example: ∆Vc_s = 160mV, ∆Ic_s = 3A. Then Re_max =
53.3mΩ.
Maximum ESR criterion can be used when the associated
capacitance is high enough, otherwise more capacitors than
the number determined by this criterion should be used in
parallel.
MINIMUM CAPACITANCE CALCULATION
In a switch mode power supply, the minimum output capaci-
tance is typically dictated by the load transient requirement.
If there is not enough capacitance, the output voltage excur-
sion will exceed the maximum allowed value even if the
maximum ESR requirement is met. The worst-case load
transient is an unloading transient that happens when the
input voltage is the highest and when the present switching
cycle has just finished. The corresponding minimum capaci-
tance is calculated as follows:
e% is the output voltage initial accuracy.
±
δ% is the output voltage regulation window and