LM2830XQMF/NOPB National Semiconductor, LM2830XQMF/NOPB Datasheet - Page 11

• Manufacturer Part Number: LM2830XQMF/NOPB
• Description: IC REG BUCK 1A 1.6MHZ SOT23-5
• Manufacturer: National Semiconductor
• Type: Step-Down (Buck)r

Specifications of LM2830XQMF/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 4.5 V
• Current - Output: 1A
• Frequency - Switching: 1.6MHz
• Voltage - Input: 3 ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: SOT-23-5, SC-74A, SOT-25
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: LM2830XQMF

Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (V
The catch diode (D1) forward voltage drop and the voltage
drop across the internal PMOS must be included to calculate
a more accurate duty cycle. Calculate D by using the following
formula:
V
The diode forward drop (V
pending on the quality of the diode. The lower the V
higher the operating efficiency of the converter. The inductor
value determines the output ripple current. Lower 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 (1.2A) 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 1.2A. One can either reduce Δi
neering judgment that zero margin will be safe enough. The
typical current limit is 1.75A.
The LM2830 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-
SW
L
can be approximated by:
= 20% of 1A, the peak current in the inductor will be 1.2A.
Δi
FIGURE 3. Inductor Current
L
= 0.1 x (I
V
SW
I
LPK
O
= I
LPK
) to input voltage (V
D
= I
OUT
) can range from 0.3V to 0.7V de-
OUT
) in the inductor is calculated by:
OUT
)
x R
→
+ Δi
0.2 x (I
DSON
L
L
OUT
, or make the engi-
)
IN
):
D
, the
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11
age ripple. Now that the ripple current is determined, the
inductance is calculated by:
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. Because
of the speed of the internal current limit, the peak current of
the inductor need only be specified for the required maximum
output current. For example, if the designed maximum output
current is 1.0A and the peak current is 1.25A, then the induc-
tor should be specified with a saturation current limit of >
1.25A. There is no need to specify the saturation or peak cur-
rent of the inductor at the 1.75A typical switch current limit.
The difference in inductor size is a factor of 5. Because of the
operating frequency of the LM2830, ferrite based inductors
are preferred to minimize core losses. This presents little re-
striction since the variety of ferrite-based inductors is huge.
Lastly, inductors with lower series resistance (R
vide better operating efficiency. For recommended inductors
see Example Circuits.
INPUT CAPACITOR
An input capacitor is necessary to ensure that V
drop excessively during switching transients. The primary
specifications of the input capacitor are capacitance, voltage,
RMS current rating, and ESL (Equivalent Series Inductance).
The recommended input capacitance is 22 µF.The input volt-
age rating is specifically stated by the capacitor manufacturer.
Make sure to check any recommended deratings and also
verify if there is any significant change in capacitance at the
operating input voltage and the operating temperature. The
input capacitor maximum RMS input current rating (I
must be greater than:
Neglecting inductor ripple simplifies the above equation to:
It can be shown from the above equation that maximum RMS
capacitor current occurs when D = 0.5. Always calculate the
RMS at the point where the duty cycle D is closest to 0.5. The
ESL of an input capacitor is usually determined by the effec-
tive cross sectional area of the current path. A large leaded
capacitor will have high ESL and a 0805 ceramic chip capac-
itor will have very low ESL. At the operating frequencies of the
LM2830, leaded capacitors may have an ESL so large that
the resulting impedance (2
quired to provide stable operation. As a result, surface mount
capacitors are strongly recommended.
Sanyo POSCAP, Tantalum or Niobium, Panasonic SP, and
multilayer ceramic capacitors (MLCC) are all good choices for
π
fL) will be higher than that re-
DCR
www.national.com
IN
) will pro-
does not
RMS-IN
)