LM76005 datasheet(24/51 Pages) TI2

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The choice of switching frequency is a compromise between conversion efficiency and overall solution size.

Lower switching frequency implies reduced switching losses and usually results in higher system efficiency.

However, higher switching frequency allows for the use of smaller inductors and output capacitors, hence, a

more compact design. When choosing operating frequency, the most important consideration is thermal

limitations. This constraint typically dominates frequency selection. For the LM76005, the safe operating area is

controlled by the thermal performance (RθJA=18.8 °C/W); see Figure 8-3.

Ambient Temperature (°C)

100

125

150

lm76

Figure 8-3. LM76005 Safe Operating Area (5 VOUT, 400 kHz, RθJA=18.8 °C/W)

8.2.2.4 Input Capacitors

The LM76005 device requires an input decoupling and, depending on the application, a bulk input capacitor. The

typical recommended value for the high frequency decoupling capacitor is 10 μF to 22 μF. TI recommends a

high-quality ceramic type X5R or X7R with sufficiency voltage rating. The voltage rating must be greater than the

maximum input voltage. To compensate the derating of ceramic capacitors, a voltage rating of twice the

maximum input voltage is recommended.

Many times, it is desirable and necessary to use an electrolytic capacitor on the input in parallel with the

ceramics. This is especially true if the LM76005 circuit is not located within approximately 5 cm from the input

voltage source. This capacitor can help damp any ringing on the input supply caused by the long power leads

and the associated inductance. The use of this additional capacitor also helps with momentary voltage dips

caused by input supplies with unusually high impedance. The optimum value for this capacitor is four times the

ceramic input capacitance with ESR close to the characteristic impedance of the LC filter formed by the

application input inductance and ceramic input capacitors.

For this design, two 4.7-μF, X7R dielectric capacitors rated for 100 V are used for the input decoupling

capacitance. A single capacitor can aslo be used that has an equivalent series resistance (ESR) of

approximately 3 mΩ, and an RMS current rating of 5 A. Include a capacitor with a value of 47 nF for high-

frequency filtering and place it as close as possible to the device pins.

Note

DC-Bias Effect: High capacitance ceramic capacitors have a DC-bias derating effect, which has a

strong influence on the final effective capacitance. Therefore, choose the right capacitor value

carefully. Package size and voltage rating in combination with dielectric material are responsible for

differences between the rated capacitor value and the effective capacitance.

8.2.2.5 Inductor Selection

The first criterion for selecting an output inductor is the inductance itself. In most buck converters, this value is

based on the desired peak-to-peak ripple current, ΔiL that flows in the inductor along with the load current. As

with switching frequency, the selection of the inductor is a tradeoff between size and cost. Higher inductance

means lower ripple current and hence lower output voltage ripple. Lower inductance results in smaller, less

expensive devices. An inductance that gives a ripple current of 20% to 40% of the maximum output current is a

LM76005

SNVSBK5A – FEBRUARY 2020 – REVISED JULY 2020

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