Datasheet

L1 =
V
OUT1
x (V
IN
- V
OUT1
)
I
OR
x F
s
x V
IN
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 (V
OUT1
): 10V
• Load current (for continuous conduction mode): 100mA to 150mA
R1 and R2: From Figure 8, 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
: 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:
F
MAX
= V
OUT
/ (V
INMAX
x 250 ns) (6)
For this exercise, Fmax = 444 kHz. From Equation 2, R
ON
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
(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 132 µH. The next larger standard value (150 µH) is chosen and with this
value I
OR
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 V
CC
output provides not only noise filtering and stability, but also prevents 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.
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 V
OUT1
of 100 mV. Since the minimum ripple current (at minimum Vin) is 33 mA peak-to-peak,
the minimum ESR required at V
OUT1
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 V
OUT1
or V
OUT2
. V
OUT1
provides good regulation, but with a
ripple voltage which ranges from 100 mV (at Vin = 12V) to 580 mV (at Vin = 90V). Alternatively, V
OUT2
provides
low ripple (3 mV to 13 mV) but lower regulation due to R3.
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