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PDF MT7812 Data sheet ( Hoja de datos )
| Número de pieza | MT7812 | |
| Descripción | High Efficiency High Precision Buck Constant Current Driver | |
| Fabricantes | Maxic Technology | |
| Logotipo |  |
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MT7812
High Efficiency High Precision Buck Constant Current Driver
DESCRIPTION
MT7812 is a high precision LED constant current
control chip operating at critical conduction mode
(CRM) with zero current switched-on and peak
current switched-off. It’s mainly targeted for
non-isolated buck LED power systems.
Critical conduction mode ensures that the
MT7812 turns on the internal power MOSFET
when the inductor current reaches zero, reducing
the power MOSFET switching loss, the system
achieves more than 95% efficiency. With critical
conduction mode, and the line compensation,
MT7812 outputs high accuracy LED current, and
further achieves good line regulation and load
regulation.
protection(OCP), short circuit protection (SCP)，
over voltage protection (OVP) and over
temperature protection(OTP),etc, to ensure
system reliability.
FEATURES
Critical Conduction Mode, not sensitive to
the inductance.
Up to 95% of efficiency
Highly accurate constant LED current
Cycle-by-cycle current limitation
LED Short Circuit Protection
LED Over Voltage Protection
Leading edge blanking technique
Under-voltage lockout (UVLO) protection
Over temperature protection
SOP8 package
MT7812 embedded with internal power MOSFET,
simplify the peripheral circuits. Low BOM cost is
achieved.
MT7812 has wide working voltage range, which
is suitable for full-range AC input or 10V-400V
DC input voltage. MT7812 provides various
kinds of protections, such as over current
APPLICATIONS
LED bulb, LED tube, LED signal and
landscape lamp
LED stage light, LED candle light, LED corn
light, etc
General purpose constant current source
Typical Application Circuit
MT7812 Rev. 1.30
www.maxictech.com
Copy right © 2014 Maxic Technology Corporation
Page 1
1 page  
MT7812
High Efficiency High Precision Buck Constant Current Driver
Switching Frequency
MT7812 operates at critical conduction mode.
When the inductor current is zero, the system
turns on the internal power MOSFET, inductor
current rises from the ground up. The power
MOSFET conduction time is：
TON
L I LPK
VIN VLED
（3）
where L is the inductance of the inductor; ILPK is
peak current through the inductor; VIN is the DC
voltage of the rectified input voltage；VLED is the
forward voltage drop on the LED.
When the CS voltage increases to the 400mV,
the internal switch is turned off, in result, the
internal power MOSFET is also switched off. The
inductor current will discharge the LED through
the free-wheeling diode. The internal power
MOSFET won’t turned on until the current in the
inductor drops to zero. The power MOSFET
turn-off time is：
TOFF
L I LPK
VLED
（4）
Operating frequency of the system is：
f
1
VLED
(1
VLED
VIN
)
（5）
TON TOFF
L I LPK
From the above equation, it’s showing that
MT7812 operating frequency is determined by
the input voltage VIN, the LED forward voltage
drop VLED and the inductance L. The higher the
input voltage VIN ， the higher the operating
frequency. For both EMI and efficiency
consideration, the operating frequency located
between 30KHz -80KHz is recommended. To
meet this requirement, appropriate inductance
value should be determined at minimum input
voltage condition.
MT7812 sets the maximum off-time
TOFFMAX=400uS ， the minimum off-time
TOFFMIN=1.5uS. Shown in TOFF equation, if the
inductance is large, TOFF maybe longer than
TOFFMAX, the system will turn on the switch before
the inductor current falls to zero, after five times
consecutive TOFFMAX cycles, the system enters
hiccup and re-start status. On the contrary, if
inductance is small, TOFF may be shorter than
TOFFMIN，then the inductor current has already
been reduced to zero before the next cycle starts,
the system enters into discontinuous mode. The
actual LED current will be smaller than the target
value.
Maxic Technology Corporation provides software
design tool of this chip, it helps customer to come
out proper solution and to choose the right
inductance value.
LED Over voltage protection setup
MT7812 sets the Over voltage protection
threshold by VOVP pin (Pin2). The external R1
and R2 form a resistive voltage divider.
Over voltage protection threshold is：
VOVP
0.90 *
R1
R2
（6）
R2 resistance is between 30Kohm ~ 50Kohm
（Refer to the following schematic. Equation (6)
is based on the following assumptions:
Rst1=Rst2，R1 >> Rst1. Rst1 and Rst2 is about
200K ohms. R1 resistance is in mega ohm
range）. At the LED open circuit condition, once
the output voltage exceeds the OVP threshold,
MT7812 stops switching. Until the VCC voltage
drops to UVLO threshold, the system restarts.
MT7812 Rev. 1.30
www.maxictech.com
Copy right © 2014 Maxic Technology Corporation
Page 5
5 Page | ||
| Páginas | Total 9 Páginas | |
| PDF Descargar | [ Datasheet MT7812.PDF ] | |
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