11

VCC

OVP

8

LM3447

VCC

GND

AUX

NA

RDAMP

CVCC

10

GATE

S

Q

Q

R

FAULT

TIMER

(812ms)

LOGIC & CONTROL

VREC

VBIAS

VCC

10.5V

7.5V

17.7V

13.5V

VOUT

t1

t2

t3

t4

time

t5

t0

LM3447

SNOSC65 A – APRIL 2012 – REVISED MAY 2012

www.ti.com

Figure 20. Typical Waveforms and Start-up Sequence Under Dimming Conditions

The value of capacitor CVCC is critical design parameter as it determines VCC ripple voltage during dimming

operation. A X7R ceramic capacitor with value ranging from 22

μF to 47μF and 25V voltage rating is

recommended for CVCC as trade-off between size and performance in space constraint applications. At low

dimming levels, large VCC voltage ripple and QPASS threshold voltage variations can intefere with smooth

dimming performance. An external zener doide, DZBS, can be placed in series with BIAS to boost VCC voltage

and eliminate any observable dimming discontinuities. A low power zener diode ( 200mW) with reverse

breakdown voltage ranging from 1.8V to 4.5V is recommended for most dimming application.

VCC OVERVOLTAGE PROTECTION

Figure 21. VCC Overvoltage Protection Circuit

The LM3447 has a built-in overvoltage protection (OVP) mode to protect VCC from exceeding its ABS MAX

rating under fault conditions. The VCC voltage is monitored by a comparator with a rising threshold of 18.9V and

175mV of hysteresis. Upon detecting an overvoltage condition, GATE is pulled low for duration of 812ms,

determined by the internal fault timer. On clearance of the fault, the timer is disabled and normal device

operation resumes. An optional damping resistor, RDAMP in series with the auxiliary winding can be used to

prevent transformer leakage current from peak charging CVCC and false triggering the OVP circuit. Based on the

magnitude of leakage inductance a resistor of 10

Ω to 47Ω should provide proper damping.

12

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