LP3907 Datasheet, PDF (41/58 Pages) National Semiconductor (TI) – Dual High-Current Step-Down DC/DC and Dual Linear Regulator with I2C Compatible Interface

www.ti.com
Typical Application (continued)
9.2.2 Detailed Design Procedure
LP3907
SNVS511S – JUNE 2007 – REVISED APRIL 2016
9.2.2.1 Component Selection
9.2.2.1.1 Inductors for SW1 And SW2
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 25ºC, 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:
9.2.2.1.1.1 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 follows:
I_sat > I_outmax + I_ripple
where
I_ripple
=
§¹
©f
·
¹
x
§^VIN ^{- V}OUT·
© 2L ¹
x
§^VOUT·
© VIN ¹
where
• I_RIPPLE = Maximum load current
• I_OUTMAX = Average to peak inductor current
• V_IN = Maximum input voltage to the buck
• L = Min inductor value including worse case tolerances (30% drop can be considered for method 1)
• f = Minimum switching frequency (1.6 MHz)
• V_OUT = Buck output voltage
(6)
9.2.2.1.1.2 Method 2:
A more conservative and recommended approach is to choose an inductor that has saturation current rating
greater than the maximum current limit of 1250 mA for Buck1 and 1750 mA for Buck2.
Given a peak-to-peak current ripple (I_PP) the inductor needs to be at least
L
t
§^VIN ^{- V}OUT·
© IPP ¹
x
§^VOUT·
© VIN ¹
x
§¹
©f
·
¹
(7)
space
INDUCTOR
L_SW1,2
Table 8. Suggested Inductor Values
VALUE (µH)
2.2
DESCRIPTION
SW1,2 inductor
NOTES
DCR: 70 mΩ
9.2.2.1.2 External Capacitors
The regulators on the LP3907 require external capacitors for regulator stability. These are specifically designed
for portable applications requiring minimum board space and smallest components. These capacitors must be
correctly selected for good performance.
Copyright © 2007–2016, Texas Instruments Incorporated
Product Folder Links: LP3907
Submit Documentation Feedback
41