LM34923 Datasheet, PDF (15/26 Pages) National Semiconductor (TI) – EVAL evaluation board provides the design engineer with a fully

LM34923
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SNVS695A – MARCH 2011 – REVISED FEBRUARY 2013
APPLICATIONS INFORMATION
SELECTION OF EXTERNAL COMPONENTS
A guide for determining the component values is illustrated with a design example. Refer to the Block Diagram.
The following steps will configure the LM34923 for:
• Input voltage range (Vin): 15V to 75V
• Output voltage (V_OUT): 10V
• Load current (for continuous conduction mode): 100 mA to 400 mA
• Switching Frequency: 300 kHz
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: Unless the application requires a specific frequency, the choice of frequency is generally a
compromise. A higher frequency allows for a smaller inductor, input capacitor, and output capacitor (both in value
and physical size), while providing a lower conversion efficiency. A lower frequency provides higher efficiency,
but generally requires higher values for the inductor, input capacitor and output capacitor. The maximum allowed
switching frequency for the LM34923 is limited by the minimum on-time (200 ns) at the maximum input voltage,
and by the minimum off-time (260 ns) at the minimum input voltage. The maximum frequency limit for each
application is defined by the following two calculations:
V_OUT
F_S(max)1 = _{VIN(max) x 200 ns}
(7)
V_IN(min) - V_OUT
F_S(max)2 = _{VIN(min) x 260 ns}
(8)
The maximum allowed frequency is the lesser of the two above calculations. See the graph “Maximum Switching
Frequency”. For this exercise, F_s(max)1 calculates to 667 kHz, and F_s(max)2 calculates to 1.28 MHz. Therefore the
maximum allowed frequency for this example is 667 kHz, which is greater than the 300 kHz specified for this
design. Using Equation 1, RT calculates to 258 kΩ. A standard value 261 kΩ resistor is used. The minimum on-
time calculates to 469 ns, and the maximum on-time calculates to 2.28 µs.
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) if a flyback diode is
used, the ripple amplitude (I_OR) must be less than 200 mA p-p so the lower peak of the waveform does not reach
zero. L1 is calculated using the following equation:
V_OUT x (V_IN - V_OUT)
L1 =
I_OR x F_s x V_IN
(9)
At Vin = 75V, L1(min) calculates to 146µH. The next larger standard value (150 µH) is chosen and with this value
I_OR calculates to 195 mA p-p at Vin = 75V, and 75 mA p-p at Vin = 15V.
b) Maximum load current: At a load current of 400 mA, the peak of the ripple waveform must not reach the
minimum specified value of the LM34923’s current limit threshold (700 mA). Therefore the ripple amplitude must
be less than 600 mA p-p, which is already satisfied in the above calculation. With L1 = 150 µH, at maximum Vin
and Io, the peak of the ripple is 498 mA. While L1 must carry this peak current without saturating or exceeding its
temperature rating, it also must be capable of carrying the maximum specified value of the LM34923’s current
limit threshold 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 0.5 ohm,
the power dissipated at maximum load current is 0.08W. While small, it is not insignificant compared to the load
power of 4W.
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