LP3910 Datasheet, PDF (62/72 Pages) Texas Instruments – Power Management IC for Hard-Drive-Based Portable Media Players

LP3910
SNVS481M – NOVEMBER 2006 – REVISED DECEMBER 2015
Typical Application (continued)
9.2.1 Design Requirements
For typical PMU applications, use the parameters listed in Table 15.
DESIGN PARAMETER
Minimum input voltage
Maximum input voltage
LDO1 output voltage
LDO2 output voltage
Buck1 output voltage
Buck2 output voltage
Buck-Boost output voltage
Charge current
Table 15. Design Parameters
EXAMPLE VALUE
2.7 V
5.5 V
2.5 V
3V
1.6 V
1.8 V
3.3 V
100 mA
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9.2.2 Detailed Design Procedure
9.2.2.1 Inductors for Buck1, Buck2 and Buck-Boost
There are two main considerations when choosing an inductor; the inductor must not saturate and the inductor
current ripple is small enough to achieve the desired output voltage ripple. Care must be taken when reviewing
the different saturation current ratings that are specified by different manufacturers.
Saturation current ratings are typically specified at 25°C, so ratings at maximum ambient temperature of the
application must be requested from the manufacturer.
There are two methods to choose the inductor saturation current rating:
9.2.2.1.1 Method 1
The saturation current is greater than the sum of the maximum load current and the worst-case average-to-peak
inductor current. This can be written as Equation 4:
I_SAT ! I_OUTMAX I_RIPPLE
where
I_RIPPLE
1
¦
§
¨©
V_IN
V_OUT
2L
·
¸¹
§
¨
V_OUT
·
¸
© ^VIN ¹
(4)
Considered when using the Buck-Boost in boost mode, use Equation 5:
I_SAT ! I_RIPPLE I_AVE
where
I_RIPPLE
§
¨¹
V_IN
·
¸
V_IN ^©
V_{OUT ¹}
2L¦
I_AVE ! I_OUTMAX
V_OUT
V_IN
where
• I_RIPPLE: Average-to-peak inductor current
• I_OUTMAX: Maximum load current
• V_IN: Maximum input voltage in application
• L: Minimum inductor value including worst case tolerances (30% drop can be considered for Method 1)
• ƒ: Minimum switching frequency
• V_OUT: Output voltage
(5)
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