Datasheet

L1 =
V
OUT1
x (V
IN
- V
OUT1
)
I
OR
x F
s
x V
IN
LM5008
www.ti.com SNVS280G –APRIL 2004–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 Figure 2. The
following steps will configure the LM5008 for:
• Input voltage range (Vin): 12V to 95V
• Output voltage (V
OUT1
): 10V
• Load current (for continuous conduction mode): 100 mA to 300 mA
• Maximum ripple at V
OUT2
: 100 mVp-p at maximum input voltage
R1 and R2: From Figure 2, V
OUT1
= V
FB
x (R1 + R2) / R2, and since V
FB
= 2.5V, the ratio of R1 to R2 calculates
as 3:1. Standard values of 3.01 kΩ (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.
F
s
and R
ON
: The recommended operating frequency range for the LM5008 is 50kHz to 600 kHz. 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
400 ns, is calculated from:
F
MAX
= V
OUT
/ (V
INMAX
x 400ns) (6)
For this exercise, Fmax = 263kHz. From Equation 2, R
ON
calculates to 304 kΩ. A standard value 357 kΩ resistor
will be used to allow for tolerances in Equation 2, resulting in a frequency of 224kHz.
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
(I
OR
) must be less than 200 mAp-p so the lower peak of the waveform does not reach zero. L1 is calculated
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
value I
OR
calculates to 181 mAp-p at Vin = 95V, and 34 mAp-p at Vin = 12V.
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 LM5008’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 LM5008’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.
C3: The capacitor on the V
CC
output provides not only noise filtering and stability, but its primary purpose is to
prevent false triggering of the V
CC
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.
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