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PDF MT7968 Data sheet ( Hoja de datos )
| Número de pieza | MT7968 | |
| Descripción | High Accuracy Dual Winding PSR AC-DC LED Driver | |
| Fabricantes | Maxic Technology | |
| Logotipo |  |
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MT7968
High Accuracy, Dual Winding PSR AC-DC LED Driver
DESCRIPTION
MT7968 is a PWM controller for AC-DC LED
lighting. With primary side sensing and
regulation technology, no secondary side
feedback circuit is needed. Further, the loop
compensation components are also eliminated
while maintaining system stability. Integrated
with 600V power switch and adjustable LED
open-circuit protection (OVP) function simplifies
the system design and improves the reliablity.
With Maxic’s proprietary current regulation
technique, the MT7968 achieves ±3% accuracy
of LED current along with excellent line
regulation and load regulation.
FEATURES
Build in 600V power MOSFET
No auxiliary winding needed
Supporting 85V to265V AC line voltage
range
Primary side sensing and regulation, no
need of secondary side feedback
High precision constant LED current (+/-3%)
Adjustable LED open-circuit voltage
protection threshold
LED short circuit protection
Cycle-by-cycle peak current control
VDD under voltage lock-out protection
Over temperature protection
Available in DIP8 package
MT7968 provides plenty of protections, such as
over current protection(OCP), short circuit
protection (SCP)，over voltage protection (OVP)
and over temperature protection(OTP),etc, to
enhance system reliability.
APPLICATION
LED bulb, Spotlight
LED lighting application
General purpose constant current source
Typical Application Circuit
Vin_ac
MT7968
3 VCC
4
SW
8
CS
CS 7
2 OVP
DRAIN 6
15
RCS
RSET
GND
DRAIN
NP:NS
LED+
LED-
MT7968 Rev. 1.35
www.maxictech.com
Copyright © 2014 Maxic Technology Corporation
Page 1
1 page  
MT7968
High Accuracy, Dual Winding PSR AC-DC LED Driver
I P _ PK
500 (mA)
RCS
where RCS is the peak current sensing resistor
(refer to the Typical Application Circuit),
The LED current can be calculated by the
following equation:
I LED
I P _ PK
4
NP
NS
500
4 RCS
NP (mA)
NS
where NP is the turns of the primary winding, NS
is the turns of the secondary winding, IP_PK is the
primary side peak current. Shown in the above
equation, the output current is determined by the
turns ratio of the transformer and the current
sense resistor value, insensitive to the
inductance of the transformer.
Switching Frequency
MT7968 is designed to operating in
discontinuous conduction mode and no external
loop compensation is needed for stability. The
maximum duty cycle is limited to 42%. It’s highly
recommended to limit the maximum switching
frequency less than 100kHz and the minimum
switching frequency more than 20kHz.
The switching frequency can be set by formula:
fSW
8
N
2
P
VLED
NS2 Lp ILED
where, NP is the turns of the primary winding, NS
is the turns of the secondary winding, Lp is the
transformer primary winding inductance.
Designing the switching frequency between
40kHz to 80kHz by properly set the transformer
parameters.
LED Open Circuit Protection (OVP)
By detecting the secondary inductor
demagnetization time, MT7968 implements LED
open circuit protection function.
Figure 2 shows the transformer primary and
secondary current waveform while MT7968 is
working:
I S _ PK
0.5
RCS
NP
NS
I P _ PK
0.5
RCS
VOVP
LS
Tdemag_ov
Fig.2 Transformer primary and secondary
current waveform
Refer to Fig 2, when LED is open-circuited, the
secondary inductor demagnetization time is:
Tdemag
_OV
I S _ PK
VOVP
LS
LS
0.5
RCS
NP
NS
VOVP
（us）
where, NP is the turns of the primary winding, NS
is the turns of the secondary winding, Ls is the
secondary inductance in uH, 0.5 is voltage
threshold for VCS detection in unit volt. From the
above formula, secondary inductor
demagnetization time contains VOVP voltage
information. By detecting secondary inductor
demagnetization time when LED is
open-circuited, OVP protection is achieved:
VOVP
LS
0.5
RCS
NP
NS
Tdemag _OV
（V）
A resistor RSET at OVP pin (see Figure 3) sets the
demagnetization time when LED is
open-circuited :
Tdemag_OV 0.1 RSET （us）
RSET units is kΩ.
GND 1
RSET OVP 2
VCC 3
SW 4
8 CS
7 CS
6 DRAIN
5 DRAIN
Fig.3 OVP pin schematic
MT7968 Rev. 1.35
www.maxictech.com
Copyright © 2014 Maxic Technology Corporation
Page 5
5 Page | ||
| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet MT7968.PDF ] | |
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