LD5523K

05/16/2018

13

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD5523K-DS-00 May 2018

Brown-In / Brown-Out Protection (BNI/BNO)

and Bulk Cap OVP - Auto Recovery

The LD5523K integrates the brown in/out and bulk cap

protections and valley detection into FB pin. The auxiliary

voltage reflects a proportional bulk voltage during the on

time. Fix the internal current at the BNI and BNO, the BNI

level could be set by modulating the FB divided resistors

and auxiliary voltage, as shown in Fig. 18. For preventing

the abnormal condition of line voltage to causing damage,

BNO function is implemented, while turns off the gate

signal after de-bounce time 75ms as BNO occurring, as

shown in Fig. 19. The relationship of input voltage and

BNI/BNO is expressed in following equation.

VDC_BNI =

NP

Na

∙ IBNI ∙ R1

VDC_BNO =

NP

Na

∙ IBNO ∙ R1

VDC_BULK _OVP =

NP

Na

∙ IBULK _OVP ∙ R1

, where

VDC_BNI is predicted BNI DC value of input voltage.

VDC_BNO is predicted BNO DC value of input voltage.

VDC_BNULK_OVP is predicted BULK OVP DC value of input

voltage.

IBNI is BNI trip current.

IBNO is BNO trip current.

IBULK_OVP is BULK OVP trip current.

Np is turns ration of primary-side winding.

Na is turns ration of auxiliary winding.

LD5523K

R1

R2

VCC

FB

GND

Va

VFB

Fig. 18

t

Vcc

t

OUT

Switching

Non-Switching

t

IFB

t

Line Voltage

Non-

Switching

Normal Range

UVLO(on)

UVLO(off)

95µA

85µA

Fig. 19

Over Voltage Protection on FB Pin (FB OVP)

- Auto Recovery

An output overvoltage protection is implemented in the

LD5523K. The auxiliary winding voltage can be reflected

from secondary winding, in which the FB pin voltage is

proportional to output voltage during the gate off time.

OVP is worked by sensing the auxiliary voltage via the

divided resistors R2, referring to Fig. 18. The equation of

FB OVP is shown as follows.

R2 =

R1 ∙ VFB_OVP

Va − VFB_OVP

Va =

Na

NS

VO + VF

VFB_OVP is the FB pin OVP trip voltage level. Va is the

auxiliary winding voltage which reflects from the forward

voltage VF of Schottky diode and output voltage VO. NS is

turns ration of secondary-side winding.