Datasheet

L1 =
V
OUT
x (V
IN
- V
OUT
)
I
OR
x F
s
x V
IN
LM5009A
www.ti.com SNVS608G –JUNE 2009–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 the Block
Diagram. The following steps will configure the LM5009A for:
• Input voltage range (Vin): 12V to 90V
• Output voltage (V
OUT1
): 10V
• Load current (for continuous conduction mode): 100 mA to 150 mA
R
FB1
, R
FB2
: V
OUT
= V
FB
x (R
FB1
+ R
FB2
) / R
FB1
, and since V
FB
= 2.5V, the ratio of R
FB2
to R
FB1
calculates as 3:1.
Standard values of 3.01 kΩ and 1.00 kΩ are chosen. Other values could be used as long as the 3:1 ratio is
maintained.
F
s
and R
T
: The recommended operating frequency range for the LM5009A is 50 kHz to 1.1 MHz. 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 400 ns) (6)
For this exercise, Fmax = 277 kHz. From Equation 2, R
T
calculates to 260 kΩ. A standard value 309 kΩ resistor
will be used to allow for tolerances in Equation 2, resulting in a frequency of 234 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
(I
OR
) 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 190 µH. The next larger standard value (220 µH) is chosen and with this
value I
OR
calculates to 173 mA peak-to-peak at Vin = 90V, and 32 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 LM5009A’s current limit threshold (240 mA). Therefore the ripple amplitude must be less
than 180 mA peak-to-peak, which is already satisfied in the above calculation. With L1 = 220 µH, at maximum
Vin and Io, the peak of the ripple will be 236 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 LM5009A’s
current limit threshold (360 mA) without saturating, since the current limit is reached during startup.
The DC resistance of the inductor should be as low as possible to minimize its power loss.
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 and off transitions. C3 should be no smaller than
0.47 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 LM5009A the minimum ripple required at pin 5 is 25 mV peak-to-peak, requiring a
minimum ripple at V
OUT
of 100 mV. Since the minimum ripple current (at minimum Vin) is 32 mA peak-to-peak,
the minimum ESR required at V
OUT
is 100 mV/32 mA = 3.12Ω. Since quality capacitors for SMPS applications
have an ESR considerably less 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 mV peak-to-peak ripple at pin 5. Generally, R3 will be 0.5 to 4.0Ω.
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