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PDF RT8476A Data sheet ( Hoja de datos )
| Número de pieza | RT8476A | |
| Descripción | Two-Stage Hysteretic LED Driver Controller | |
| Fabricantes | Richtek | |
| Logotipo |  |
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®
RT8476A
Two-Stage Hysteretic LED Driver Controller
General Description
The RT8476A 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 RT8476A is equipped with dual output gate drivers for
external power MOSFETs, suitable for higher power
applications.
The RT8476A 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)
Wide Input Voltage Range : 4.5V to 40V
Adjustable Peak Input Current Control
Adjustable Boost Output Voltage
Independent Dual Stage Function
Adjustable LED Current with ±5% LED Current
Accuracy
Input Under-Voltage Lockout Detection
Thermal Shutdown Protection
SOP-8 (Exposed Pad) Package
RoHS Compliant and Halogen Free
Ordering Information
RT8476A
Package Type
SP : SOP-8 (Exposed Pad-Option 1)
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 D6
VL
AC 12V
VN
D1 D2
CIN
D3 D4
COUT R4
RT8476A
OVP
VCC
R5
M1
ISN
CREG
GATE1
CS
R6 ACTL GATE2
GND
RSENSE
D7
C3 L2
M2
LED+
C6
LED-
Copyright ©2015 Richtek Technology Corporation. All rights reserved.
DS8476A-02 March 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
1 page  
RT8476A
Typical Application Circuit
VL
AC 12V
VN
L1
22µH D6
RSENSE
280m
D1 D2
R4 RT8476A
3 OVP
VCC 6
COUT
R5
CIN M1
ISN 5
1µF
D3 D4
1 GATE1
2 CS
CREG 7
C3
4.7µF
M2
D7
L2
120µH
R6
120m
4 ACTL
GATE2 8
GND
9 (Exposed Pad)
LED+
C6
4.7µF
LED-
Copyright ©2015 Richtek Technology Corporation. All rights reserved.
DS8476A-02 March 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
5 Page 
RT8476A
Step-Down Converter Inductor Selection
The RT8476A 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).
RDS2(ON) is the low-side switch on-resistance of external
MOSFET (M2). The typical value is 0.35Ω.
D is the duty cycle = VOUT / VIN
IOUT is the required LED current (A)
ΔIOUT is the inductor peak-peak ripple current (internally
set to 0.3 x IOUT)
VCC is the supply input voltage (V)
VOUT is the total LED forward voltage (V)
VISN is the voltage cross current sense resistor (V)
L2 is the inductance (H)
The selected inductor must have saturation current higher
than the peak output LED current and continuous current
rating above the required average output LED current. In
general, the inductor saturation current should be 1.5
times the LED current. In order to minimize output current
ripple, higher values of inductance are recommended at
higher supply voltages. Because high values of inductance
has high line resistance, it will cause lower efficiency.
Step-Up Converter Inductor Selection
The RT8476A uses a constant off-time control to provide
high efficiency step-up converter. The resistor, R6, between
the Source of the external N-MOSFET and GND should
be selected to provide adequate switch maximum current
to drive the application. The current limit threshold on the
CS pin of the RT8476A is 240mV (typ.). When the CS pin
voltage is higher than the 240mV reference, the
comparator will disable the power section. The GATE1
Copyright ©2015 Richtek Technology Corporation. All rights reserved.
DS8476A-02 March 2015
will pull low after fixed delay time 1.5μs (typ.) and then
turn on again after OVP operation is removed. This cycle
repeats, keeping the output voltage within a small window.
Following the constant off-time mechanism, the inductance
L1 is calculated according to the following equation :
L1 > tOFF x (VCC(MAX) − VIN(MIN) + VF) / ILIM
where
tOFF is Off-Time. The typical value is 1.5μs.
ILIM is the input current. The typical value is 2A for MR16
application.
VCC is the supply input voltage (V)
VIN is the input voltage after bridge diodes (V)
VF is the forward voltage (V)
L1 is the inductance (H)
D = 1 − (VIN / VOUT)
fSW = (1 − D) / tOFF
where
D is the operation duty
fSW is the switching frequency of Boost controller.
Check the ILIM setting satisfied the output LED current
request by the following equation :
(IOUT + ΔIOUT) < [2 x L1 x ILIM + tOFF x (VIN − VOUT − VF)] x
VIN / [2 x L1 x (VCC)]
Diode Selection
To obtain better efficiency, the Schottky diode is
recommended for its low reverse leakage current, low
recovery time and low forward voltage. With its low power
dissipation, the Schottky diode outperforms other silicon
diodes and increases overall efficiency.
Input Capacitor selection
Input capacitor has to supply peak current to the inductor
and flatten the current ripple on the input. The low ESR
condition is required to avoid increasing power loss. The
ceramic capacitor is recommended due to its excellent
high frequency characteristic and low ESR, which are
suitable for the RT8476A. For maximum stability over the
entire operating temperature range, capacitors with better
dielectric are suggested.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
11 Page | ||
| Páginas | Total 14 Páginas | |
| PDF Descargar | [ Datasheet RT8476A.PDF ] | |
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