LD9852

02/17/2015

10

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD9852-DS-00 February 2015

Fig. 15 illustrates the operation of over voltage

protection, where ZCD divider resistors continue to

detect the output voltage as the gate switches in off

state and return the information to ZCD. If the output

voltage increases to trip the threshold, it will signal that

there may be some LED(s) open.

The LD9852 will

VCC drops to UVLO(OFF) level. LD9852 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.

Fig. 15

Programming Maximum ON-time

LD9852 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. 16. ZCD resistance can be obtained from

below:

ZCD

Fig. 16

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

LED Short Protection

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

(OFF) level and disable the LD9852. The LD9852 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.

Over Current Protection (OCP)

The LD9852 detects the MOSFET current from the

Source pin, which is for the pulse-by-pulse current limit

and output current feedback. The maximum voltage

threshold of the Source pin is set at 1.2V. From above,

the MOSFET peak current can be obtained from below.

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