LM5017 datasheet(21/31 Pages) TI

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VIN(UVLO, rising) = 1.225V x

RUV2

+ 1

)

(

RUV1

VIN (HYS) = IHYS x RUV2

OUT(MA

VD1 =

VIN

LM5017

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SNVS783I – JANUARY 2012 – REVISED OCTOBER 2015

The calculated value for Rr is 66 k

Ω. This value provides the minimum ripple for stable operation. A smaller

resistance should be selected to allow for variations in TON, COUT1 and other components. For this design, Rr

value of 46.4 k

Ω is selected.

8.2.2.2.9

Secondary Diode

The reverse voltage across secondary-rectifier diode D1 when the high-side buck switch is off can be calculated

using Equation 31.

(31)

For a VIN_MAX of 95 V and the 1:1 turns ratio of this design, a 100 V Schottky is selected.

8.2.2.2.10

VCC and Bootstrap Capacitor

A 1-µF capacitor of 16 V or higher rating is recommended for the VCC regulator bypass capacitor.

A good value for the BST pin bootstrap capacitor is 0.01-µF with a 16 V or higher rating.

8.2.2.2.11

Input Capacitor

The input capacitor is typically a combination of a smaller bypass capacitor located near the regulator IC and a

larger bulk capacitor. The total input capacitance should be large enough to limit the input voltage ripple to a

desired amplitude. For input ripple voltage

ΔVIN, CIN can be calculated using Equation 32.

(32)

Choosing a

ΔVIN of 0.5 V gives a minimum CIN of 0.2 μF. A standard value of 0.47 μF is selected for CBYP in this

design. A bulk capacitor of higher value reduces voltage spikes due to parasitic inductance between the power

source to the converter. A standard value of 2.2

μF is selected for CIN in this design. The voltage ratings of the

two input capacitors should be greater than the maximum input voltage under all conditions.

8.2.2.2.12

UVLO Resistors

UVLO resistors RUV1 and RUV2 set the undervoltage lockout threshold and hysteresis according to Equation 33

and Equation 34.

(33)

(34)

Where IHYS = 20 μA, typical.

For a UVLO hysteresis of 2.5 V and UVLO rising threshold of 20 V, Equation 33 and Equation 34 require RUV1 of

8.25 k

Ω and RUV2 of 127 kΩ and these values are selected for this design example.

8.2.2.2.13

VCC Diode

Diode D2 is an optional diode connected between VOUT1 and the VCC regulator output pin. When VOUT1 is more

than one diode drop greater than the VCC voltage, the VCC bias current is supplied from VOUT1. This results in

reduced power losses in the internal VCC regulator which improves converter efficiency. VOUT1 must be set to a

voltage at least one diode drop higher than 8.55 V (the maximum VCC voltage) if D2 is used to supply bias

current.

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