mt7933 
maximizing ic performance           single stage, high pfc, ac-dc led driver 
mt7933 rev. 1.00                        
copyright © 2012 maxic technology corporation                        page4
application information 
mt7933 is a primary-side controller for ac-dc led driver. the ledcurrent can be accurately regulated through sensing the primaryside information to realize real current control. mt7933 integratespower factor correction function to eliminate pollution to the acline and works in crm (critical conduction mode).  
real current control 
mt7933 accurately regulate led current through sensing the primaryside information. the led current can be easily set as following(refer to the application circuit in page 1): 
s
fbspledrvnn2
1=
i where np is primary winding, ns is secondary winding; vfb(=400mv) is the internal voltage reference and rs is an externalcurrent sensing resistor.   
start up 
during start-up process, vdd is charged through a start-upresistor. as vdd reaches 18v, comp is pre-charged by internalcircuit. after comp reaches 0.8v, the internal control loop is wellsettled, which is considered as “loopok”. then pwm signal is sendto drv pin to power up the convertor. 
the power supply is taken over by the auxiliary winding once thevoltage of this winding is high enough. 
the pwm signal is shut down after vdd goes below 
7.2v 
(uvlo 
threshold 
voltage). 
meanwhile, comp is discharged to ground. the whole start-upprocedure is shown in fig.1. 
>18v
vdd
loopok<7.2v
comp
drv
18v
7.2v
0v
0.8v
0v
chargedby resistor
fig.1  start up sequence
power factor correction 
the primary side current increases linearly from zero to peakvalue, as sensed by the current sensing pin cs, during the powermosfet on-time. when the primary current reaches the threshold,mt7933 turns off the power mosfet immediately. mt7933 turns on thepower mosfet again after the drain of the power mosfet damps to itsvalley or nearby. the peak current threshold is regulated by maxicproprietary technique and follows the rectified sinusoidal-shape of main line voltage. as a result, theenvelope of the inductor current is sinusoidal-shaped, high powerfactor is therefore achieved in this way.

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