L1 =

LM5008A

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SNVS583F – MARCH 2009 – REVISED MARCH 2013

APPLICATIONS INFORMATION

SELECTION OF EXTERNAL COMPONENTS

A guide for determining the component values will be illustrated with a design example. Refer to the Block

Diagram. The following steps will configure the LM5008A for:

•

Input voltage range (Vin): 12V to 95V

•

•

Load current (for continuous conduction mode): 100 mA to 300 mA

Standard values of 3.01 k

Ω and 1.00 kΩ are chosen. Other values could be used as long as the 3:1 ratio is

maintained.

application requires a specific frequency, the choice of frequency is generally a compromise since it affects the

size of L1 and C2, and the switching losses. The maximum allowed frequency, based on a minimum on-time of

400 ns, is calculated from:

(6)

will be used to allow for tolerances in Equation 2, resulting in a frequency of 223 kHz.

L1: The main parameter affected by the inductor is the output current ripple amplitude. The choice of inductor

value therefore depends on both the minimum and maximum load currents, keeping in mind that the maximum

ripple current occurs at maximum Vin.

a) Minimum load current: To maintain continuous conduction at minimum Io (100 mA), the ripple amplitude

using Equation 7.

(7)

At Vin = 95V, L1(min) calculates to 200 µH. The next larger standard value (220 µH) is chosen and with this

b) Maximum load current: At a load current of 300 mA, the peak of the ripple waveform must not reach the

minimum value of the LM5008A’s current limit threshold (410 mA). Therefore the ripple amplitude must be less

than 220 mAp-p, which is already satisfied in the above calculation. With L1 = 220 µH, at maximum Vin and Io,

the peak of the ripple will be 391 mA. While L1 must carry this peak current without saturating or exceeding its

temperature rating, it also must be capable of carrying the maximum value of the LM5008A’s current limit

threshold (610 mA) without saturating, since the current limit is reached during startup.

The DC resistance of the inductor should be as low as possible. For example, if the inductor’s DCR is one ohm,

the power dissipated at maximum load current is 0.09W. While small, it is not insignificant compared to the load

power of 3W.

µF.

C2, and R3: When selecting the output filter capacitor C2, the items to consider are ripple voltage due to its

ESR, ripple voltage due to its capacitance, and the nature of the load.

ESR and R3: A low ESR for C2 is generally desirable so as to minimize power losses and heating within the

capacitor. However, the regulator requires a minimum amount of ripple voltage at the feedback input for proper

loop operation. For the LM5008A the minimum ripple required at pin 5 is 25 mVp-p, requiring a minimum ripple at

than this, R3 is inserted as shown in the Block Diagram. R3’s value, along with C2’s ESR, must result in at least

25 mVp-p ripple at pin 5. Generally, R3 will be 0.5 to 3.0

Ω.

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Product Folder Links: LM5008A