MT-7933 Просмотр технического описания (PDF) - Unspecified
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MT-7933 Datasheet PDF : 7 Pages
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Maximizing IC Performance
MT7933
Single Stage, High PFC, AC-DC LED Driver
APPLICATION INFORMATION
Charged
By Resistor
18V
MT7933 is a primary-side controller for AC-DC
LED driver. The LED current can be accurately
regulated through sensing the primary side
information to realize real current control.
MT7933 integrates power factor correction
function to eliminate pollution to the AC line and
works in CRM (Critical Conduction Mode).
Real Current Control
MT7933 accurately regulate LED current through
sensing the primary side information. The LED
current can be easily set as following (refer to the
application circuit in page 1):
1
ILED = 2
NP VFB
NS RS
Where NP is primary winding, NS is secondary
winding; VFB (=400mV) is the internal voltage
reference and RS is an external current sensing
resistor.
Start Up
During start-up process, VDD is charged through
a start-up resistor. As VDD reaches 18V, COMP
is pre-charged by internal circuit. After COMP
reaches 0.8V, the internal control loop is well
settled, which is considered as “LoopOK”. Then
PWM signal is send to DRV Pin to power up the
convertor.
The power supply is taken over by the auxiliary
winding once the voltage 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-up procedure is shown in Fig.1.
VDD
7.2V
0V
0.8V
COMP
0V
DRV
Fig.1 Start up sequence
Power Factor Correction
The primary side current increases linearly from
zero to peak value, as sensed by the current
sensing pin CS, during the power MOSFET
on-time. When the primary current reaches the
threshold, MT7933 turns off the power MOSFET
immediately. MT7933 turns on the power
MOSFET again after the drain of the power
MOSFET damps to its valley or nearby. The peak
current threshold is regulated by MAXIC
proprietary technique and follows the rectified
sinusoidal-shape of input voltage. As a result, the
envelope of the inductor current is
sinusoidal-shaped, high power factor is therefore
achieved in this way.
Fig.2 Power Factor Correction
MT7933 Rev. 1.20
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Copyright © 2012 Maxic Technology Corporation
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