LM26420XMH/NOPB National Semiconductor, LM26420XMH/NOPB Datasheet - Page 14

IC REG SYNC BUCK DL 2A 20-TSSOP

LM26420XMH/NOPB

Manufacturer Part Number
LM26420XMH/NOPB
Description
IC REG SYNC BUCK DL 2A 20-TSSOP
Manufacturer
National Semiconductor
Series
PowerWise®r
Type
Step-Down (Buck)r
Datasheet

Specifications of LM26420XMH/NOPB

Internal Switch(s)
Yes
Synchronous Rectifier
Yes
Number Of Outputs
2
Voltage - Output
0.8 ~ 4.5 V
Current - Output
2A
Frequency - Switching
2.2MHz
Voltage - Input
3 ~ 5.5 V
Operating Temperature
-40°C ~ 125°C
Mounting Type
Surface Mount
Package / Case
20-TSSOP Exposed Pad, 20-eTSSOP, 20-HTSSOP
Primary Input Voltage
5.5V
No. Of Outputs
2
Output Voltage
4.5V
Output Current
2A
No. Of Pins
20
Operating Temperature Range
-40°C To +125°C
Msl
MSL 1 - Unlimited
Filter Terminals
SMD
Rohs Compliant
Yes
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Power - Output
-
Other names
LM26420XMH
www.national.com
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (V
The voltage drop across the internal NMOS (SW_BOT) and
PMOS (SW_TOP) must be included to calculate a more ac-
curate duty cycle. Calculate D by using the following formulas:
V
The inductor value determines the output ripple current. Low-
er inductor values decrease the size of the inductor, but
increase the output ripple current. An increase in the inductor
value will decrease the output ripple current.
One must ensure that the minimum current limit (2.4A) is not
exceeded, so the peak current in the inductor must be calcu-
lated. The peak current (I
In general,
If Δi
The minimum guaranteed current limit over all operating con-
ditions is 2.4A. One can either reduce Δi
neering judgment that zero margin will be safe enough. The
typical current limit is 3.3A.
The LM26420 operates at frequencies allowing the use of ce-
ramic output capacitors without compromising transient re-
sponse. Ceramic capacitors allow higher inductor ripple
without significantly increasing output ripple. See the output
capacitor section for more details on calculating output volt-
age ripple. Now that the ripple current is determined, the
inductance is calculated by:
SW_TOP
L
= 20% of 2A, the peak current in the inductor will be 2.4A.
and V
Δi
SW_BOT
FIGURE 3. Inductor Current
V
V
L
SW_TOP
SW_BOT
= 0.1 x (I
I
can be approximated by:
LPK
OUT
= I
= I
LPK
= I
OUT
OUT
OUT
) to input voltage (V
) in the inductor is calculated by:
OUT
)
x R
x R
+ Δi
0.2 x (I
DSON_TOP
DSON_BOT
L
L
OUT
, or make the engi-
)
IN
):
30069605
14
Where
When selecting an inductor, make sure that it is capable of
supporting the peak output current without saturating. Induc-
tor saturation will result in a sudden reduction in inductance
and prevent the regulator from operating correctly. The peak
current of the inductor is used to specify the maximum output
current of the inductor and saturation is not a concern due to
the exceptionally small delay of the internal current limit sig-
nal. For example, if the designed maximum output current is
2.0A and the peak current is 2.3A, then the inductor should
be specified with a saturation current limit of > 2.3A. There is
no need to specify the saturation or peak current of the in-
ductor at the 3.25A typical switch current limit. The difference
in inductor size is a factor of 5. Ferrite based inductors are
preferred to minimize core losses when opperating with the
frequencies used by the LM26420. This presents little restric-
tion since the variety of ferrite-based inductors is huge. Lastly,
inductors with lower series resistance (R
ter operating efficiency. For recommended inductors see Ex-
ample Circuits.
INPUT CAPACITOR SELECTION
The input capacitors provide the AC current needed by the
nearby power switch so that current provided by the upstream
power supply does not carry a lot of AC content, generating
less EMI. To the buck regulator in question, the input capac-
itor also prevents the drain voltage of the FET switch from
dipping when the FET is turned on, therefore providing a
healthy line rail for the LM26420 to work with. Since typically
most of the AC current is provided by the local input capaci-
tors, the power loss in those capacitors can be a concern. In
the case of the LM26420 regulator, since the two channels
operate 180° out of phase, the AC stress in the input capac-
itors is less than if they operated in phase. The measure for
the AC stress is called input ripple RMS current. It is strongly
recommended that at least one 10µF ceramic capacitor be
placed next to each of the VIND pins. Bulk capacitors such as
electrolytic capacitors or OSCON capacitors can be added to
help stabilize the local line voltage, especially during large
load transient events. As for the ceramic capacitors, use X7R
or X5R types. They maintain most of their capacitance over
a wide temperature range. Try to avoid sizes smaller than
0805. Otherwise significant drop in capacitance may be
caused by the DC bias voltage. See OUTPUT CAPACITOR
SELECTION section for more information. The DC voltage
rating of the ceramic capacitor should be higher than the
highest input voltage.
Capacitor temperature is a major concern in board designs.
While using a 10µF or higher MLCC as the input capacitor is
a good starting point, it is a good idea to check the tempera-
ture in the real thermal environment to make sure the capac-
itors are not over heated. Capacitor vendors may provide
curves of ripple RMS current vs. temperature rise, based on
a designated thermal impedance. In reality, the thermal
impedance may be very different. So it is always a good idea
to check the capacitor temperature on the board.
DCR
) will provide bet-

Related parts for LM26420XMH/NOPB