LM5019 datasheet(12/25 Pages) TI1

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VIN - VOUT

ûIL =

L1 x CSW

VOUT

VIN

VOUT

CSW =

K x RON

DMIN

CSW(MAX) =

TON(MIN)

10/95

100 ns

= 1.05 MHz

1 - DMAX

CSW(MAX) =

TOFF(MIN)

1 - 10/12.5

200 ns

= 1 MHz

LM5019

SNVS788E – JANUARY 2012 – REVISED DECEMBER 2013

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APPLICATION INFORMATION

Selection of External Components

Selection of external components is illustrated through a design example. The design example specifications are:

Buck Converter Design Specifications

Input Voltage Range

12.5 V to 95 V

Output Voltage

10 V

Maximum Load Current

100 mA

Switching Frequency

400 kHz

RFB1, RFB2:

VOUT = VFB x (RFB2/RFB1 + 1), and since VFB = 1.225 V, the ratio of RFB2 to RFB1 calculates as 7:1. Standard

values of 6.98 k

Ω and 1.00 kΩ are chosen. Other values could be used as long as the 7:1 ratio is maintained.

Frequency Selection:

At the minimum input voltage, the maximum switching frequency of LM5019 is restricted by the forced minimum

off-time (TOFF(MIN)) as given by:

(6)

Similarly, at maximum input voltage, the maximum switching frequency of LM5019 is restricted by the minimum

TON as given by:

(7)

Resistor RON sets the nominal switching frequency based on the following equations:

where

•

K = 1 x 10

–10

(8)

Operation at high switching frequency results in lower efficiency while providing the smallest solution. For this

example a 400 kHz was selected, resulting in RON = 246 kΩ. Selecting a standard value for RON = 237 kΩ results

in a nominal frequency of 416 kHz.

Inductor Selection:

The inductance selection is a compromise between solution size, output ripple, and efficiency. The peak inductor

current at maximum load current should be smaller than the minimum current limit of 150 mA. The maximum

permissible peak to peak inductor ripple is:

ΔIL = 2*(ILIM(min) - IOUT(max) = 2*50 = 100mA

(9)

The minimum inductance is given by:

(10)

Resulting in L = 215 µH. A standard value of 22 0µH is selected. For proper operation the inductor saturation

current should be higher than the peak encountered in the application. For robust short circuit protection, the

inductor saturation current should be higher than the maximum current limit of 300 mA.

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