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PDF RT8494 Data sheet ( Hoja de datos )
| Número de pieza | RT8494 | |
| Descripción | High Voltage High Current LED Driver Controller | |
| Fabricantes | Richtek | |
| Logotipo |  |
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®
RT8494
High Voltage High Current LED Driver Controller for
Buck, Boost or Buck-Boost Topology
General Description
The RT8494 is a current mode PWM controller designed
to drive an external MOSFET for high current LED
applications. With a current sense amplifier threshold of
190mV, the LED current is programmable with one
external current sense resistor. With the maximum
operating input voltage of 36V and output voltage up to
90V, the RT8494 is ideal for Buck, Boost or Buck-Boost
operation.
With the switching frequency programmable over 100kHz
to 1MHz, the external inductor and capacitors can be small
while maintaining high efficiency.
Dimming can be done by either analog or digital. The built-
in clamping comparator and filter allow easy low noise
analog dimming conversion from digital signal with only
one external capacitor.
The RT8494 is available in SOP-14 package.
Features
High Voltage Capability : VIN up to 36V, LED Sensing
Threshold Common Mode Voltage up to 90V
Buck, Boost or Buck-Boost Operation
Programmable Switching Frequency
Easy Dimming Control : Analog or Digital
Converting to Analog with One External Capacitor
Programmable Soft-Start to Avoid Inrush Current
Programmable Over-Voltage Protection
VIN Under-Voltage Lockout and Thermal Shutdown
AEC-Q100 Compliance
Applications
General Industrial High Power LED Lighting
Desk Lights and Room Lighting
Building and Street Lighting
Industrial Display Backlight
Ordering Information
RT8494
Package Type
S : SOP-14
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.
Pin Configurations
(TOP VIEW)
RSET
ISW
ISP
ISN
VC
ACTL
DCTL
2
3
4
5
6
7
14
13
12
11
10
9
8
SOP-14
GATE
GBIAS
GND
VCC
OVP
EN
SS
Marking Information
RT8494
GSYMDNN
RT8494GS : Product Number
YMDNN : Date Code
Copyright ©2015 Richtek Technology Corporation. All rights reserved.
DS8494-00 January 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
1 page  
Operation
The startup voltage of the RT8494 is around 4.5V. When
VCC voltage is greater than 4.5V, the RT8494 starts
operation and a regulated GBIAS supply voltage is
generated by an internal LDO circuit. With VCC greater
than 10V, the GBIAS supply will be regulated around 8.5V
to supply the power for the internal GATE pin driver circuit.
As the system starts, the capacitor at the soft-start pin is
slowly charged by an internal current source around 6μA.
During soft-start period, the VC pin voltage follows the
soft-start pin voltage up by one VBE and gradually ramps
up. The slowly rising VC pin voltage allows the PWM duty
to increase gradually to achieve soft-start function.
In normal operation, the GATE turns high when the
oscillator (OSC) turns high. The ISW pin voltage is the
triangular feedback signal of the sensed switch current
(which equals inductor current ramp).
The PWM comparator compares the ISW pin voltage to
the VC pin voltage. When the ISW pin voltage exceeds
the VC pin voltage, the PWM comparator resets the latch
and turns off GATE. If the ISW pin voltage does not exceed
the VC pin voltage by the end of the switching cycle, the
GATE will be turned off by the OSC circuit for a minimum
off time. The cycle repeats when the GATE is turned on
at the beginning of the next OSC cycle.
RT8494
The RT8494 features high voltage LED driver control. The
common mode operation voltage of the ISP and ISN pins
can be high up to 90V. The regulated (VISP − VISN) sense
threshold voltage is around 190mV. If the sensed (VISP −
VISN) voltage is lower than 190mV, the VC pin will be
charged higher by the internal OP AMP in the PWM
control loop and vice versa. By the PWM closed loop
control, the (VISP − VISN) voltage is regulated to 190mV.
The actual LED output current can be adjusted by the
sense resistor between the ISP and ISN pins.
The dimming can be done by varying the ACTL/DCTL pin
voltage signal. The internal sense threshold reference for
(VISP − VISN) regulation follows the ACTL/DCTL signal to
achieve dimming control.
The fault protection features of the RT8494 include (1)
VCC Under-Voltage Lockout (UVLO) (2) VOUT Over -
Voltage Protection (OVP) (3) switch Over-Current
Protection (OCP) (4) Over-Temperature Protection (OTP).
Copyright ©2015 Richtek Technology Corporation. All rights reserved.
DS8494-00 January 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
5 Page 
Applications Information
The RT8494 is a current mode PWM controller designed
to drive an external MOSFET for high current LED
applications. The LED current can be programmed by an
external resistor. The input voltage range of the RT8494
can be up to 36V and the output voltage can be up to 90V.
The RT8494 provides analog and PWM dimming to achieve
LED current control.
GBIAS Regulator and Bypass Capacitor
The GBIAS pin requires a capacitor for stable operation
and to store the charge for the large GATE switching
currents. Choose a 25V rated low ESR, X7R or X5R
ceramic capacitor for best performance. The value of a
1μF capacitor will be adequate for many applications.
Place the capacitor close to the IC to minimize the trace
length to the GBIAS pin and also to the IC ground. An
internal current limit on the GBIAS output protects the
RT8494 from excessive on-chip power dissipation.
The GBIAS pin has its own under voltage disable (UVLO)
set to 4.3V(typical) to protect the external FETs from
excessive power dissipation caused by not being fully
enhanced. If the input voltage, VIN, will not exceed 8V,
then the GBIAS pin should be connected to the input
supply. Be aware if GBIAS supply is used to drive extra
circuits besides RT8494, typically the extra GBIAS load
should be limited to less than 10mA.
Loop Compensation
The RT8494 uses an internal error amplifier whose
compensation pin (VC) allowing the loop response
optimized for specific application. The external inductor,
output capacitor and the compensation resistor and
capacitor determine the loop stability. The inductor and
output capacitor are chosen based on performance, size
and cost. The compensation resistor and capacitor at VC
are selected to optimize control loop response and
stability. For typical LED applications, a 3.3nF
compensation capacitor at VC is adequate, and a series
resistor should always be used to increase the slew rate
on the VC pin to maintain tighter regulation of LED current
during fast transients on the input supply to the converter
an external resistor in series with a capacitor is connected
RT8494
from the VC pin to GND to provide a pole and a zero for
proper loop compensation. The typical compensation for
the RT8494 is 10kΩ and 3.3nF.
Soft-Start
The soft-start of the RT8494 can be achieved by connecting
a capacitor from SS pin to GND. The built-in soft-start
circuit reduces the start-up current spike and output
voltage overshoot. The soft-start time is determined by
the external capacitor charged by an internal 6μA constant
charging current. The SS pin directly limits the rate of
voltage rise on the VC pin, which in turn limits the peak
switch current.
The soft-start interval is set by the soft-start capacitor
selection according to the equation :
tSS
CSS
2.4V
6μA
A typical value for the soft-start capacitor is 0.1μF. The
soft-start capacitor is discharged when EN/UVLO falls
below its threshold, during an over temperature event or
during an GBIAS under voltage event.
LED Current Setting
The LED current is programmed by placing an appropriate
value current sense resistor between the ISP and ISN pins.
Typically, sensing of the current should be done at the
top of the LED string. The ACTL pin should be tied to a
voltage higher than 1.2V to get the full-scale 190mV
(typical) threshold across the sense resistor. The ACTL
pin can also be used to dim the LED current to zero,
although relative accuracy decreases with the decreasing
voltage sense threshold. When the ACTL pin voltage is
less than 1.2V, the LED current is :
ILED
(VACTL 0.2) 0.19
RSENSE
Where,
RSENSE is the resistor between ISP and ISN.
When the voltage of ACTL is higher than 1.2V, the LED
current is regulated to :
ILED(MAX)
190mV
RSENSE
Copyright ©2015 Richtek Technology Corporation. All rights reserved.
DS8494-00 January 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
11 Page | ||
| Páginas | Total 17 Páginas | |
| PDF Descargar | [ Datasheet RT8494.PDF ] | |
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