LP3905 Datasheet, PDF (11/15 Pages) National Semiconductor (TI) – Power Management Unit For Low Power Handheld Applications

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.
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_FB at 0.5V, the current through the
resistor feedback network will be 2.5µA.
The formula for output voltage selection is
V_OUT - output voltage (Volts)
V_FB - feedback voltage (0.5V)
R_1FB - feedback resistor from V_OUT to FB
R_2FB - feedback resistor from FB to GND
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:
20152958
Adjustable Buck Converter Components
V_OUT will be adjusted to make the voltage at FB equal to
0.5V. The resistor from FB to ground (R_FB2) should be
For recommended component values see Table 1
TABLE 1. Buck Component Configurations for Various Output Voltage Values
V_OUT (V)
1.0
1.2
1.4
1.5
1.6
1.85
2.5
2.8
3.3
RFB1 (kΩ)
200
280
360
360
442
540
402
464
562
RFB2 (kΩ)
200
200
200
180
200
200
100
100
100
C1 (pF)
18
12
10
10
8.2
6.8
8.2
8.2
6.8
C2 (pF)
none
none
none
none
none
none
none
33
33
L (µH)
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
C_OUT (µF)
10
10
10
10
10
10
10
10
10
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_OUTMAX: maximum load current (600mA)
• V_IN: maximum input voltage in application
• L : min inductor value including worst case tolerances
(30% drop can be considered for method 1)
• f : minimum switching frequency (1.6Mhz)
• V_OUT: output voltage
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_RIPPLE: average to peak inductor current
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