LD7835

10

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7835-DS-01 July 2015

VCC drops to UVLO (OFF) level. LD7835 will shut down

its operation until VCC rises up to next UVLO (ON) level.

If the open situation remains still, the system will enter to

hiccup mode.

ZCD

t

ZCD OVP

Tripped

t

OUT

Switching

Switching

Non-Switching

Output

Voltage

t

VCC

t

LED Opened

Vout

UVLO(ON)

UVLO(OFF)

OVP

Level

Fig. 16

Programming Maximum ON-time

LD7835 features adjustable maximum ON-time to limit

power output in abnormal operation. The selection of

maximum ON-time is subject to ZCD resistance as

shown in Fig. 17. ZCD resistance can be obtained from

below:

ZCD

Fig. 17

The following table is a suggestion for maximum

ON-time setting.

(Ω)

Max. Ton

(Typ.)

Suggestion

(Ω)

15

s

46k

21

s

30k

29

s

18k

Current Ripple Reduction (C.R.R) Function

LD7835 features current ripple reduction function to

minimum the output capacitor. The C.R.R rate is subject

is a suggestion for CRR rate setting.

R

F

(Ω)

CRR

Effective Rate

Suggestion

(Ω)

16k<R

F

Heavy

18k

7k<R

F

<8k

Light

7.5k

R

F

<0.2k

Disable

0

The C.R.R effect is dependent on C.R.R rate, AC line

following table for tuning reference.

C.R.R Rate

AC Line

Rup

Current Ripple

Disable

NA

NA

Original

Light

↓

↑

Lightest

Heavy

↑

↓

Heaviest

If C.R.R function is enabled, the output capacitance and

the saturation knee point of inductor can be minimize.

However, power factor and THD will be slightly lower

than disabled condition

. It’s a trade-off factor between

power factor and current ripple.

LED Short Protection

If some LED is in short condition, VCC will drop to UVLO

(OFF) level and disable the LD7835. The LD7835 will

not resume operation until VCC rises up to next UVLO

(ON) level. If the short condition remains still, the system

will enter to hiccup mode.