LM5019 datasheet(16/30 Pages) TI

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RUV2

VIN (UVLO,rising) = 1.225V x

RUV1

+ 1

(

)

VIN(HYS) = IHYS x RUV2

IOUT(MAX)

CIN

8 x CSW x ûVIN

ûIL(MIN)

25 mV

Csw(RFB2||RFB1)

C >

LM5019

SNVS788F – JANUARY 2012 – REVISED DECEMBER 2014

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Using type II ripple circuit equations with minimum FB pin ripple of 25 mV, the values of the series resistor RC

and ac coupling capacitor Cac can calculated.

(11)

Assuming RFB2 = 6.98 kΩ and RFB1 = 1 kΩ, the calculated minimum value of Cac is 0.013 µF. A standard value of

0.1 µF is selected for Cac = C8. The value of the series output resistor RC is calculated for the minimum input

voltage condition when the inductor ripple current as at a minimum. Using Equation 9 and assuming VIN = 12.5

V, the minimum inductor ripple current is 27 mA. The calculated minimum value of RC is 0.93 Ω. A standard

value of 1.5

Ω is selected for RC = R2 to provide additional ripple for stable switching at low VIN.

8.2.1.2.6

VCC and Bootstrap Capacitor

The VCC capacitor provides charge to bootstrap capacitor as well as internal circuitry and low side gate driver.

The bootstrap capacitor provides charge to high side gate driver. The recommended value for CVCC = C7 is 1 μF.

A good value for CBST = C1 is 0.01 μF.

8.2.1.2.7

Input Capacitor

Input capacitor should be large enough to limit the input voltage ripple shown in Equation 12.

(12)

Choosing a

ΔVIN = 0.5 V gives a minimum CIN = 0.06 μF. A standard value of 1.0 μF is selected for CIN = C4.

The input capacitor should be rated for the maximum input voltage under all conditions. A 50-V, X7R dielectric

should be selected for this design.

Input capacitor should be placed directly across VIN and RTN (pin 2 and 1) of the IC. If it is not possible to place

all of the input capacitor close to the IC, a 0.1-

μF capacitor should be placed near the IC to provide a bypass

path for the high frequency component of the switching current. This helps limit the switching noise.

8.2.1.2.8

UVLO

The UVLO resistors RUV1 and RUV2 set the UVLO threshold and hysteresis according to Equation 13 and

Equation 14.

(13)

(14)

Where IHYS = 20 µA. For UVLO hysteresis of 2.5 Vand UVLO rising threshold of 12 V the calculated values of the

UVLO resistors are RUV2 = 127 k

Ω and RUV1 = 14.5 kΩ. Selecting standard values for RUV1 = R7 = 14 kΩ and

RUV2 = R5 = 127 kΩ results in UVLO rising threshold of 12.5 V and hysteresis of 2.5 V.

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