LM5009 datasheet(11/22 Pages) TI

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L1 =

VOUT1 x (VIN - VOUT1)

IOR x Fs x VIN

LM5009

www.ti.com

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

•

Output voltage (VOUT1): 10V

•

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

R1 and R2: From Figure 8, VOUT1 = VFB x (R1 + R2) / R2, and since VFB = 2.5V, the ratio of R1 to R2 calculates

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.

Fs and RON: Unless the 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 250 ns, is calculated from:

FMAX = VOUT / (VINMAX x 250 ns)

(6)

For this exercise, Fmax = 444 kHz. From Equation 2, RON calculates to 180 kΩ. A standard value 237 kΩ resistor

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

(IOR) must be less than 200 mA peak-to-peak so the lower peak of the waveform does not reach zero. L1 is

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

value IOR calculates to 176 mA peak-to-peak at Vin = 90V, and 33 mA peak-to-peak at Vin = 12V.

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.

C3: The capacitor on the VCC output provides not only noise filtering and stability, but also prevents false

triggering of the VCC UVLO at the buck switch on/off transitions. For this reason, C3 should be no smaller than

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

minimum ripple at VOUT1 of 100 mV. Since the minimum ripple current (at minimum Vin) is 33 mA peak-to-peak,

the minimum ESR required at VOUT1 is 3 Ω. Since quality capacitors for SMPS applications have an ESR

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.

b) Nature of the Load: The load can be connected to VOUT1 or VOUT2. VOUT1 provides good regulation, but with a

ripple voltage which ranges from 100 mV (at Vin = 12V) to 580 mV (at Vin = 90V). Alternatively, VOUT2 provides

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

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