RON =

5V x (8V - 1.4V)

8V x 175 kHz x 1.18 x 10

-10 - 1.4 k: = 198 k:

15

LM5010A, LM5010A-Q1

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8.2.2 Detailed Design Procedure

The procedure for calculating the external components is illustrated with a design example. Configure the circuit

in Figure 11 according to the components listed in Table 2.

Table 2. List of Components for LM5010A Example Circuit

ITEM

DESCRIPTION

VALUE

C1

Ceramic capacitor

(2) 2.2 µF, 100 V

C2

Ceramic capacitor

22 µF, 16 V

C3

Ceramic capacitor

0.47 µF, 16 V

C4, C6

Ceramic capacitor

0.022 µF, 16 V

C5

Ceramic capacitor

0.1 µF, 100 V

D1

Schottky diode

100V, 6 A

L1

Inductor

100 µH

R1

Resistor

1 k

Ω

R2

Resistor

1 k

Ω

R3

Resistor

1.5

Ω

RON

Resistor

200 k

Ω

U1

LM5010Ax

—

8.2.2.1 Component Selection

8.2.2.1.1

R1 and R2

These resistors set the output voltage, and calculate their ratio with Equation 10.

R1/R2 = (VOUT / 2.5 V) – 1

(10)

R1 and R2 calculates to 1. The resistors should be chosen from standard value resistors in the range of 1 k

Ω to

10 k

Ω. A value of 1 kΩ is used for R1 and R2.

8.2.2.1.2

RON, FS

RON can be chosen using Equation 7 to set the nominal frequency, or from Equation 6 if the ON-time at a

particular VIN is important. A higher frequency generally means a smaller inductor and capacitors (value, size and

cost), but higher switching losses. A lower frequency means a higher efficiency, but with larger components.

Generally, if PC board space is tight, a higher frequency is better. The resulting ON-time and frequency have a

±25% tolerance. Using Equation 7 at a nominal VIN of 8 V, RON is calculated with Equation 11.

(11)

A value of 200 k

Ω will be used for RON, yielding a nominal frequency of 161 kHz at VIN = 6 V, and 205 kHz at VIN

= 60 V.

8.2.2.1.3

L1

The guideline for choosing the inductor value in this example is that it must keep the circuit’s operation in

continuous conduction mode at minimum load current. This is not a strict requirement since the LM5010Ax

regulates correctly when in discontinuous conduction mode, although at a lower frequency. However, to provide

an initial value for L1 the above guideline will be used. See Figure 12.