LM2642MTC National Semiconductor, LM2642MTC Datasheet - Page 17

• Manufacturer Part Number: LM2642MTC
• Description: Voltage Regulator IC
• Manufacturer: National Semiconductor

Specifications of LM2642MTC

• No. Of Pins: 28
• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Package / Case: 28-TSSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Inductor Selection
The size of the output inductor can be determined from the
desired output ripple voltage, Vrip, and the impedance of the
output capacitors at the switching frequency. The equation to
determine the minimum inductance value is as follows:
In the above equation, Re is used in place of the impedance
of the output capacitors. This is because in most cases, the
impedance of the output capacitors at the switching fre-
quency is very close to Re. In the case of ceramic capaci-
tors, replace Re with the true impedance.
Example: Vin (max)= 30V, Vnom = 5.0V, Vrip = 40mV, Re
=20mΩ, f = 300kHz
Lmin = 7µH
The actual selection process usually involves several itera-
tions of all of the above steps, from ripple voltage selection,
to capacitor selection, to inductance calculations. Both the
highest and the lowest input and output voltages and load
transient requirements should be considered. If an induc-
tance value larger than Lmin is selected, make sure that the
Cmin requirement is not violated.
Priority should be given to parameters that are not flexible or
more costly. For example, if there are very few types of
capacitors to choose from, it may be a good idea to adjust
the inductance value so that a requirement of 3.2 capacitors
can be reduced to 3 capacitors.
Since inductor ripple current is often the criterion for select-
ing an output inductor, it is a good idea to double-check this
value. The equation is:
Where D is the duty cycle, defined by V
Also important is the ripple content, which is defined by Irip
/Inom. Generally speaking, a ripple content of less than 50%
is ok. Larger ripple content will cause too much loss in the
inductor.
Example: Vin = 12V, Vnom = 5.0V, f = 300kHz, L = 8µH
Given a maximum load current of 3A, the ripple content is
1.2A / 3A = 40%.
When choosing the inductor, the saturation current should
be higher than the maximum peak inductor current and the
RMS current rating should be higher than the maximum load
current.
Input Capacitor Selection
The fact that the two switching channels of the LM2642 are
180˚ out of phase will reduce the RMS value of the ripple
nom
/V
in
.
17
current seen by the input capacitors. This will help extend
input capacitor life span and result in a more efficient sys-
tem. Input capacitors must be selected that can handle both
the maximum ripple RMS current at highest ambient tem-
perature as well as the maximum input voltage. In applica-
tions in which output voltages are less than half of the input
voltage, the corresponding duty cycles will be less than 50%.
This means there will be no overlap between the two chan-
nels’ input current pulses. The equation for calculating the
maximum total input ripple RMS current for duty cycles
under 50% is:
where I1 is maximum load current of Channel 1, I2 is the
maximum load current of Channel 2, D1 is the duty cycle of
Channel 1, and D2 is the duty cycle of Channel 2.
Example: Imax_1 = 3.6A, Imax_2 = 3.6A, D1 = 0.42, and D2
= 0.275
Choose input capacitors that can handle 1.66A ripple RMS
current at highest ambient temperature. In applications
where output voltages are greater than half the input voltage,
the corresponding duty cycles will be greater than 50%, and
there will be overlapping input current pulses. Input ripple
current will be highest under these circumstances. The input
RMS current in this case is given by:
Where, again, I1 and I2 are the maximum load currents of
channel 1 and 2, and D1 and D2 are the duty cycles. This
equation should be used when both duty cycles are ex-
pected to be higher than 50%.
Input capacitors must meet the minimum requirements of
voltage and ripple current capacity. The size of the capacitor
should then be selected based on hold up time require-
ments. Bench testing for individual applications is still the
best way to determine a reliable input capacitor value. The
input capacitor should always be placed as close as possible
to the current sense resistor or the drain of the top FET.
MOSFET Selection
BOTTOM FET SELECTION
During normal operation, the bottom FET is switching on and
off at almost zero voltage. Therefore, only conduction losses
are present in the bottom FET. The most important param-
eter when selecting the bottom FET is the on resistance
(Rdson). The lower the on resistance, the lower the power
loss. The bottom FET power loss peaks at maximum input
voltage and load current. The equation for the maximum
allowed on resistance at room temperature for a given FET
package, is:
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