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PDF RT8476 Data sheet ( Hoja de datos )
| Número de pieza | RT8476 | |
| Descripción | Two-Stage Hysteretic LED Driver Controller | |
| Fabricantes | Richtek | |
| Logotipo |  |
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®
RT8476
Two-Stage Hysteretic LED Driver Controller
General Description
The RT8476 is a two-stage controller with dual gate drivers
consist of a Boost converter (first stage) and a Buck
converter (second stage). The advantage of the two-stage
topology is highly compatible with ET (Electronic
Transformer) in MR16 / AR111 lighting market field
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 RT8476 is equipped with dual output gate drivers for
external power MOSFETs, suitable for higher power
applications.
The RT8476 is available in the SOP-8 and SOP-8 (Exposed
pad) packages.
Applications
z MR16 Lighting
z Signage and Decorative LED Lighting
z Architectural Lighting
z High Power LED Lighting
z Low Voltage Industrial Lighting
z Indicator and Emergency Lighting
z Automotive LED Lighting
Features
z Two-Stage Topology (Boost + Buck)
z Wide Input Voltage Range : 4.5V to 40V
z Adjustable Peak Input Current Control
z Adjustable Boost Output Voltage
z Independent Dual Stage Function
z Adjustable LED Current with ±5% LED Current
Accuracy
z Input Under Voltage Lockout Detection
z Thermal Shutdown Protection
z SOP-8 and SOP-8 (Exposed Pad) Packages
z RoHS Compliant and Halogen Free
Ordering Information
RT8476
Package Type
S : SOP-8
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
VL
AC 12V
VN
L1 D6
D1 D2
CIN
D3 D4
COUT R4
RT8476
OVP
VCC
R5
M1
ISN
CREG
GATE1
CS
R6 GATE2
GND
RSENSE
D7
C3 L2
M2
LED+
C6
LED-
Copyright ©2013 Richtek Technology Corporation. All rights reserved.
DS8476-00 April 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
1 page  
RT8476
Parameter
Symbol
OVP Threshold
OVP High Level
VO V P_ H
OVP Low Level
VO V P_ L
OVP Pin Leakage Current
Gate Driver
IOVP
GATE1 Duty Off-Time
UGATE1 Drive Sink
RUGATE1sk
LGATE1 Drive Source
RL GATE 1s r
GATE1 Default Pull Down Resistor
Buck Converter
ISN Threshold
ISN Hysteresis
VI SN
ΔVISN
ISN Pin Leakage Current
IISN
UGATE2 Drive Sink
RUGATE2sk
LGATE2 Drive Source
RL GATE 2s r
GATE2 Default Pull Down Resistor
Temperature Protection
Thermal Shutdown Temperature
Thermal Shutdown Hysteresis
TSD
ΔTSD
Test Conditions
Sink = 50mA
Source = −50mA
Sink = 50mA
Source = −50mA
Min Typ Max Unit
1.71 1.9 2.09 V
1.44 1.6 1.76 V
-- 1 -- μA
-- 1.5 --
-- 2 --
-- 1.25 --
-- 90 --
μs
Ω
Ω
kΩ
123.5 130 136.5
10 15 20
-- 1 --
-- 2 --
-- 1.25 --
-- 90 --
mV
%
μA
Ω
Ω
kΩ
140 155 170
-- 35 --
°C
°C
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.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Copyright ©2013 Richtek Technology Corporation. All rights reserved.
DS8476-00 April 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
5 Page 
Application Information
The RT8476 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
800kHz 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 RT8476 includes an under voltage lookout 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 RT8476 from excessive on-
chip power dissipation.
The CREG pin has set the output to 4.3V (typ.) to protect
the external FETs from excessive power dissipation
caused by not being fully enhanced. If the CREG pin is
used to drive extra circuits beside RT8476, the extra loads
should be limited to less than 10mA.
Gate Driver
There are two gate drivers, GATE1 and GATE2, in the
RT8476. The Gate driver consists of a CMOS buffer
designed to drive the external power MOSFET. It features
unbalanced source and 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 Gate Driver is pulled low.
RT8476
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 RT8476 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]
Copyright ©2013 Richtek Technology Corporation. All rights reserved.
DS8476-00 April 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet RT8476.PDF ] | |
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