LP3905SD-A3/NOPB National Semiconductor, LP3905SD-A3/NOPB Datasheet - Page 11

• Manufacturer Part Number: LP3905SD-A3/NOPB
• Description: IC REG LDO DUAL 600MA BUCK 14LLP
• Manufacturer: National Semiconductor

Specifications of LP3905SD-A3/NOPB

• Applications: Handheld/Mobile Devices
• Voltage - Supply: 3 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 14-LLP
• For Use With: LP3905SD-A3EV - BOARD EVALUATION LP3905SD-A3
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Supply: -
• Other names: LP3905SD-A3; LP3905SD-A3TR

Application Information
DC/DC CONVERTORS
Adjustable Buck - Output Voltage Selection
The buck converter output voltage of the adjustable version
device can be set via the selection of the external feedback
resistor network forming the output feedback between the
output voltage side of the Inductor and the FB pin and the FB
Pin and GND.
V
0.5V. The resistor from FB to ground (R
Buck Inductor Selection
There are two main considerations when choosing an induc-
tor; the inductor should not saturate, and the inductor current
ripple is small enough to achieve the desired output voltage
ripple. Different saturation current rating specs are followed
by different manufacturers so attention must be given to
details. Saturation current ratings are typically specified at
25˚C so ratings at max ambient temperature of application
should be requested from manufacturer.
There are two methods to choose the inductor saturation
current rating.
Method 1:
The saturation current is greater than the sum of the maxi-
mum load current and the worst case average to peak
inductor current. This can be written as
• I
OUT
V
OUT
RIPPLE
1.85
1.0
1.2
1.4
1.5
1.6
2.5
2.8
3.3
will be adjusted to make the voltage at FB equal to
Adjustable Buck Converter Components
(V)
: average to peak inductor current
RFB1 (kΩ)
TABLE 1. Buck Component Configurations for Various Output Voltage Values
200
280
360
360
442
540
402
464
562
RFB2 (kΩ)
FB2
20152958
200
200
200
180
200
200
100
100
100
) should be
C1 (pF)
11
8.2
6.8
8.2
8.2
6.8
18
12
10
10
around 200kΩ to keep the current drawn through the resistor
network well below the 16µA quiescent current level (PFM
mode) but large enough that it is not susceptible to noise. If
R2 is 200kΩ and with V
resistor feedback network will be 2.5µA.
The formula for output voltage selection is
V
V
R
R
For any out voltage greater than or equal to 1.0V a zero
should be added around 45 kHz by the addition of a capaci-
tor C1. The formula for the calculation of C1 is:
For recommended component values see Table 1
Method 2:
A more conservative and recommended approach is to
choose an inductor that has saturation current rating greater
than the max current limit of 1220mA.
A 2.2µH inductor with a saturation current rating of at least
1250mA is recommended for most applications.The induc-
tor’s resistance should be less than 0.3Ω for good efficiency.
For low-cost applications, an unshielded bobbin inductor
could be considered. For noise critical applications, a toroi-
dal or shielded-bobbin inductor should be used. A good
practice is to lay out the board with overlapping footprints of
both types for design flexibility. This allows substitution of a
low-noise shielded inductor, in the event that noise from
low-cost bobbin models is unacceptable.
• I
• V
• L : min inductor value including worst case tolerances
• f : minimum switching frequency (1.6Mhz)
• V
OUT
FB
1FB
2FB
(30% drop can be considered for method 1)
OUTMAX
IN
OUT
- output voltage (Volts)
- feedback voltage (0.5V)
- feedback resistor from V
- feedback resistor from FB to GND
: maximum input voltage in application
: output voltage
: maximum load current (600mA)
C2 (pF)
none
none
none
none
none
none
none
33
33
FB
at 0.5V, the current through the
L (µH)
OUT
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
to FB
C
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OUT
10
10
10
10
10
10
10
10
10
(µF)