L1 =

LM5009

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SNVS402G – FEBRUARY 2006 – REVISED FEBRUARY 2013

APPLICATIONS INFORMATION

SELECTION OF EXTERNAL COMPONENTS

A guide for determining the component values will be illustrated with a design example. Refer to Figure 8. The

following steps will configure the LM5009 for:

•

Input voltage range (Vin): 12V to 90V

•

•

Load current (for continuous conduction mode): 100mA to 150mA

as 3:1. Standard values of 3.01 k

Ω (R1) and 1.00 kΩ (R2) are chosen. Other values could be used as long as

the 3:1 ratio is maintained. The selected values, however, provide a small amount of output loading (2.5 mA) in

the event the main load is disconnected. This allows the circuit to maintain regulation until the main load is

reconnected.

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 250 ns, is calculated from:

(6)

will be used to allow for tolerances in Equation 2, resulting in a nominal frequency of 337 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

calculated using the following equation:

(7)

At Vin = 90V, L1(min) calculates to 132 µH. The next larger standard value (150 µH) is chosen and with this

b) Maximum load current: At a load current of 150 mA, the peak of the ripple waveform must not reach the

minimum value of the LM5009’s current limit threshold (250 mA). Therefore the ripple amplitude must be less

than 200 mA peak-to-peak, which is already satisfied in the above calculation. With L1 = 150 µH, at maximum

Vin and Io, the peak of the ripple will be 238 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 LM5009’s current

limit threshold (370 mA) without saturating, since the current limit is reached during startup.

0.1 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.

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

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

loop operation. For the LM5009 the minimum ripple required at pin 5 is 25 mV peak-to-peak, requiring a

considerably less than this, R3 is inserted as shown in Figure 8. R3’s value, along with C2’s ESR, must result in

at least 25 mV peak-to-peak ripple at pin 5. Generally, R3 will be 0.5

Ω to 5.0 Ω.

low ripple (3 mV to 13 mV) but lower regulation due to R3.

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Product Folder Links: LM5009