LM5019 datasheet(20/30 Pages) TI

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VD1 =

VIN

Cr = 1000 pF

Cac = 0.1 2F

50 mV

RrCr @

(VIN (MIN) - VOUT) x TON

GND

Cac

To FB

FB1

FB2

C OUT

OUT

COUT2

VOUT2 =

IOUT2 x TON (MAX)

LM5019

SNVS788F – JANUARY 2012 – REVISED DECEMBER 2014

www.ti.com

The secondary output current (IOUT2) is sourced by COUT2 during on-time of the buck switch, TON. Ignoring the

current transition times in the secondary winding, the secondary output capacitor ripple voltage can be calculated

using Equation 23.

(23)

For a 1:1 transformer turns ratio, the primary and secondary voltage ripple equations are identical. Therefore,

COUT2 is chosen to be equal to COUT1 (1 µF) to achieve comparable ripple voltages on primary and secondary

outputs.

If lower output voltage ripple is required, a higher value should be selected for COUT1 and/or COUT2.

8.2.2.2.8

Type III Feedback Ripple Circuit

Type III ripple circuit as described in Ripple Configuration is required for the Fly-Buck topology. Type I and Type

II ripple circuits use series resistance and the triangular inductor ripple current to generate ripple at VOUT and the

FB pin. The primary ripple current of a Fly-Buck is the combination or primary and reflected secondary currents

as illustrated in Figure 18. In the Fly-Buck topology, Type I and Type II ripple circuits suffer from large jitter as the

reflected load current affects the feedback ripple.

Figure 20. Type III Ripple Circuit

Selecting the Type III ripple components using the equations from Ripple Configuration will guarantee that the FB

pin ripple is be greater than the capacitive ripple from the primary output capacitor COUT1. The feedback ripple

component values are chosen as shown in Equation 24.

(24)

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 25.

(25)

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.

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