LM25019 datasheet(11/18 Pages) TI

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ûIL

COUT =

8 x gsw x ûVripple

VIN

BST

RON

RTN

VCC

VIN

VOUT

RFB1

RUV1

RON

COUT

CBST

CIN

RFB2

RUV2

UVLO

CVCC

LM25019

9V-48V

Shutdown

CBYP

VIN - VOUT

ûIL =

L1 x gSW

VOUT

VIN

VOUT

gSW =

K x RON

DMIN

gSW(MAX) =

TON(MIN)

10/48

100 ns

= 2.1 MHz

1 - DMAX

gSW(MAX) =

TOFF(MIN)

1 - 10/12.5

200 ns

= 1 MHz

LM25019

www.ti.com

SNVS952 – DECEMBER 2012

RFB1, RFB2:

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

values of 6.98k

Ω and 1.00kΩ 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 LM25019 is restricted by the forced minimum

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

(6)

Similarly, at maximum input voltage, the maximum switching frequency of LM25019 is restricted by the minimum

TON as given by:

(7)

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

(8)

where K = 1 x 10–10. Operation at high switching frequency results in lower efficiency while providing the smallest

solution. For this example a 400kHz was selected, resulting in RON = 246kΩ. Selecting a standard value for RON

= 237k

Ω results in a nominal frequency of 416kHz.

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 150mA. The maximum

permissible peak to peak inductor ripple is:

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

The minimum inductance is given by:

(9)

Resulting in L=190µH. A standard value of 220µH 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 300mA.

Figure 15. Reference Schematic for Selection of External Components

Output Capacitor:

The output capacitor is selected to minimize the capacitive ripple across it. The maximum ripple is observed at

maximum input voltage and is given by:

(10)

where

ΔVripple is the voltage ripple across the capacitor. Substituting ΔVripple = 10mV gives COUT = 2.94μF. A

4.7

μF standard value is selected. An X5R or X7R type capacitor with a voltage rating 16V or higher should be

selected.

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