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PDF RT8479C Data sheet ( Hoja de datos )
| Número de pieza | RT8479C | |
| Descripción | Two-Stage Hysteretic LED Driver | |
| Fabricantes | Richtek | |
| Logotipo |  |
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®
RT8479C
Two-Stage Hysteretic LED Driver with Internal MOSFETs
General Description
The RT8479C is a two-stage controller with dual
MOSFETs and consists of a Boost converter (first stage)
and a Buck converter (second stage). The advantage of
two-stage topology is highly compatible with ET (Electronic
Transformer) and extremely high Power Factor
performance in MR16 / AR111 lighting market fields
applications.
The first stage is a Boost converter for constant voltage
output with inductor peak current over-current protection.
The second stage is a Buck converter for constant output
current by typical constant peak current regulation.
The RT8479C is equipped with dual output gate drivers
for internal power MOSFETs.
The RT8479C is available in the SOP-8 (Exposed Pad)
package.
Applications
MR16 Lighting
Signage and Decorative LED Lighting
Architectural Lighting
High Power LED Lighting
Low Voltage Industrial Lighting
Indicator and Emergency Lighting
Automotive LED Lighting
Features
Two-Stage Topology (Boost + Buck)
Dual MOSFETs Inside
Wide Input Voltage Range : 4.5V to 36V
Excellent Power Factor
Programmable Boost Output Voltage
Independent Dual Stage Function
Programmable LED Current with ±5% LED Current
Accuracy
Input Under-Voltage Lockout Detection
Thermal Shutdown Protection
Ordering Information
RT8479C
Package Type
SP : SOP-8 (Exposed Pad-Option 2)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :
RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
Simplified Application Circuit
L1 D5
VL
AC 12V
VN
D1 D2
CIN
D3 D4
R1
R2
C1
To
Dimming
RT8479C
OVP
VCC
ISN
LX1 CREG
VCOMP
ACTL
LX2
GND
VCC
COUT
RSENSE
LED+
C3
C2 LED-
L2
D6
Copyright ©2014 Richtek Technology Corporation. All rights reserved.
DS8479C-04 August 2014
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
1 page  
RT8479C
Parameter
Symbol
ACTL Sense Threshold High
ACTL Input Bias Current
ISN Threshold
Stage 2 Peak to Peak Sense
Voltage
VISN
LX1 Internal Switch RDS(ON) RDS(ON)_1
LX2 Internal Switch RDS(ON) RDS(ON) _2
Test Conditions
Sense threshold percentage
at VACTL = 2.7V
VACTL = 3V
(dV1 + dV2) / 2
Sink = 100mA
Sink = 100mA
Min Typ Max Unit
94 97 100 %
-- -- 1 A
123.5 130 136.5 mV
-- 15 -- %
-- 0.15 --
-- 0.2 --
Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in
the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may
affect device reliability.
Note 2. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is
measured at the exposed pad of the package.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Copyright ©2014 Richtek Technology Corporation. All rights reserved.
DS8479C-04 August 2014
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
5 Page 
Application Information
The RT8479C consists of a constant output current Buck
controller and a fixed off-time controlled Boost controller.
The Boost controller is based on a peak current, fixed off-
time control architecture and designed to operate up to
1MHz to use a very small inductor for space constrained
applications. A high-side current sense resistor is used
to set the output current of the Buck controller. A 1%
sense resistor performs a ±5% LED current accuracy for
the best performance.
Under-Voltage Lockout (UVLO)
The RT8479C includes an under-voltage lockout function
with 100mV hysteresis. The internal MOSFET turns off
when VCC falls below 4.2V (typ.).
CREG Regulator
The CREG pin requires a capacitor for stable operation
and to store the charge for the large GATE switching
currents. Choose a 10V rated low ESR, X7R or X5R,
ceramic capacitor for best performance. A 4.7μF capacitor
will be adequate for many applications. Place the capacitor
close to the IC to minimize the trace length to the CREG
pin and to the IC ground.
An internal current limit on the CREG output protects the
RT8479C from excessive on-chip power dissipation.
The CREG pin has set the output to 4.3V (typ.) to protect
the internal FETs from excessive power dissipation
caused by not being fully enhanced. If the CREG pin is
used to drive extra circuits beside RT8479C, the extra
loads should be limited to less than 10mA.
Internal MOSFET
There are two drivers, LX1 and LX2, in the RT8479C.
The driver consists of a CMOS buffer designed to drive
the internal power MOSFET.
It features great sink capabilities to optimize switch on
and off performance without additional external
components. Whenever the IC supply voltage is lower than
the under voltage threshold, the internal MOSFET is turned
off.
RT8479C
Average Output Current Setting
The output current that flows through the LED string is
set by an external resistor, RSENSE, which is connected
between the VCC and ISN terminal. The relationship
between output current, IOUT, and RSENSE is shown below:
IOUT = 130mV / RSENSE
LED Current Ripple Reduction
Higher LED current ripple will shorten the LED life time
and increase heat accumulation of LED. To reduce the
LED current ripple, an output capacitor in parallel with the
LED should be added. The typical value of output capacitor
is 4.7μF.
VCC Voltage Setting
The VCC voltage setting is equipped with an Over-Voltage
Protection (OVP) function. When the voltage at the OVP
pin exceeds threshold approximately 1.9V, the power
switch is turned off. The power switch can be turned on
again once the voltage at the OVP pin drops below 1.6V.
For Boost applications, the output voltage can be set by
the following equation :
VCC(MAX) = 1.9 x (1 + R4 / R5)
R4 and R5 are the voltage divider resistors from VOUT to
GND with the divider center node connected to the OVP
pin. For MR16 LED lamp application, the minimum voltage
of VCC should maintain above 25V for stable operation.
Step-Down Converter Inductor Selection
The RT8479C implemented a simple high efficiency,
continuous mode inductive step-down converter. The
inductance L2 in Buck converter is determined by the
following factors : inductor ripple current, switching
frequency, VOUT/VIN ratio, internal MOSFET, topology
specifications, and component parameter. The inductance
L2 is calculated according to the following equation :
L2 ≥ [VCC(MAX) − VOUT − VISN − (RDS2(ON) x IOUT)] x D / [fSW
x ΔIOUT]
where
fsw is switching frequency (Hz).
Copyright ©2014 Richtek Technology Corporation. All rights reserved.
DS8479C-04 August 2014
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
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| Páginas | Total 15 Páginas | |
| PDF Descargar | [ Datasheet RT8479C.PDF ] | |
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