DS8494 01

®
RT8494
High Voltage High Current LED Driver Controller for
Buck, Boost or Buck-Boost Topology
General Description
Features
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 builtin clamping comparator and filter allow easy low noise
analog dimming conversion from digital signal with only
one external capacitor.
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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


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General Industrial High Power LED Lighting
Desk Lights and Room Lighting
Building and Street Lighting
Industrial Display Backlight
The RT8494 is available in SOP-14 package.
Ordering Information
Pin Configurations
(TOP VIEW)
RT8494
Package Type
S : SOP-14
RSET
ISW
ISP
ISN
VC
ACTL
DCTL
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :

14
2
13
3
12
4
11
5
10
6
9
7
8
GATE
GBIAS
GND
VCC
OVP
EN
SS
SOP-14
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.

Suitable for use in SnPb or Pb-free soldering processes.
Marking Information
RT8494GS : Product Number
RT8494
GSYMDNN
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS8494-01
November 2015
YMDNN : Date Code
is a registered trademark of Richtek Technology Corporation.
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1
RT8494
Typical Application Circuit
L1
22µH
VIN
4.5V to 36V
CIN
10µF
RT8494
11
9 EN
5V
Analog
Dimming
5 VC
8
SS
13 GBIAS
RVC
10k
CVC
3.3nF
CSS
0.1µF
LEDs
RSW
0.05
ISW 2
3
ISP
4
ISN
10
OVP
1
RSET
6 ACTL
7 DCTL
VOUT
90V (Max.)
COUT
1µF
M1
GATE 14
VCC
RSENSE
0.47
D1
R1
VOUT
R2
GND 12
RRSET
30k
CB
1µF
Figure 1. Analog Dimming in Boost Configuration
D1
COUT
1µF
VIN2
90V (Max.)
CIN2
VIN1
4.5V to 36V C
IN1
10µF
RSENSE
0.47
RT8494
11
9 EN
5V
Analog
Dimming
6 ACTL
7 DCTL
RVC
10k
CVC
3.3nF
VCC
CSS
0.1µF
5 VC
8
SS
13 GBIAS
CB
1µF
LEDs
L1
22µH
ISP 3
ISN 4
GATE
M1
14
RSW
0.05
ISW 2
RSET 1
OVP 10
GND
12
RRSET
Figure 2. Analog Dimming in Buck Configuration
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DS8494-01
November 2015
RT8494
VIN2
90V (max)
VIN1
4.5V to 36V
CIN1
10µF
RT8494
11
VCC
9 EN
5V
Analog
Dimming
6 ACTL
7 DCTL
RVC
10k
CVC
3.3nF
CSS
0.1µF
CIN2
5 VC
8
SS
13 GBIAS
GATE 14
ISW 2
4
ISN
3
ISP
10
OVP
1
RSET
L1
22µH
D1
VOUT
COUT
1µF
M1
RSW
0.05
LEDs
RSENSE
0.47
R1
RRSET
VOUT
R2
GND 12
CB
1µF
Figure 3. Analog Dimming in Buck-Boost Configuration
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
RSET
Switch Frequency Setting. Connect a resistor from RSET to GND. RRSET = 30k will
set f SW = 370kHz.
2
ISW
External MOSFET Switch Current Sense. Connect the current sense resistor
between external N-MOSFET switch and the ground.
3
ISP
LED Current Sense Amplifier Positive Input with Common Mode up to 90V.
4
ISN
LED Current Sense Amplifier Negative Input. Voltage threshold between ISP and
ISN is 190mV with common mode voltage up to 90V.
5
VC
PWM Control Loop Compensation.
6
ACTL
Analog Dimming Control. The effective programming voltage range of the pin is
between 0.2V and 1.2V.
7
DCTL
By adding a 0.47F filtering capacitor on ACTL pin, the PWM dimming signal on
DCTL pin can be averaged and converted into analog dimming signal on the ACTL
pin.
8
SS
Soft-Start Time Setting. A capacitor of at least 10nF is required for proper soft-start.
9
EN
Enable Control Input (Active High). When this pin voltage is low, the chip is in
shutdown mode.
10
OVP
Over-Voltage Protection. The PWM converter turns off when the voltage of the pin
goes to higher than 1.18V.
11
VCC
Power Supply of the Chip. For good bypass, a low ESR capacitor is required.
12
GND
Ground. The Exposed Pad must be Soldered to a Large PCB and Connected to
GND for Maximum Power Dissipation.
13
GBIAS
Internal Gate Driver Bias. A good bypass capacitor is required.
14
GATE
External MOSFET Switch Gate Driver Output.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS8494-01
November 2015
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RT8494
Function Block Diagram
VOC
8.5V
EN
RSET
VCC
+
1.4V
+
-
Shutdown
-
GBIAS
OSC
S
-
4.5V
GATE
Q
+
R
OVP
1.18V
+
R
-
R
+
-
-
110mV
+
VC
ISW
ISN
ISP
GM
+
6µA
SS
DCTL
1.2V
+
1.2V
+
-
-
GND
ACTL
VISP – VISN
(mV)
V
190
0
0.2
1.2
VACTL (V)
Figure 4
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is a registered trademark of Richtek Technology Corporation.
DS8494-01
November 2015
RT8494
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.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS8494-01
November 2015
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).
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RT8494
Absolute Maximum Ratings
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(Note 1)
Supply Input Voltage, VCC ---------------------------------------------------------------------------------------GBIAS, GATE -------------------------------------------------------------------------------------------------------ISW --------------------------------------------------------------------------------------------------------------------ISP, ISN ---------------------------------------------------------------------------------------------------------------DCTL, ACTL, OVP -------------------------------------------------------------------------------------------------EN ----------------------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
SOP-14 ---------------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 3)
SOP-14, θJA ----------------------------------------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------ESD Susceptibility (Note 4)
HBM (Human Body Model) ---------------------------------------------------------------------------------------MM (Machine Model) -----------------------------------------------------------------------------------------------
Recommended Operating Conditions


−0.3V to 38V
−0.3V to 10V
−0.3V to 1V
−0.3V to 100V
−0.3V to 8V (Note 2)
−0.3V to 20V
0.87W
113.9°C/W
150°C
260°C
−65°C to 150°C
2kV
200V
(Note 5)
Supply Input Voltage Range, VCC ------------------------------------------------------------------------------- 4.5V to 36V
Junction Temperature Range -------------------------------------------------------------------------------------- −40°C to 125°C
Electrical Characteristics
(VCC = 24V, No Load on any Output, −40°C < TA < 125°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Overall
Supply Current
IVCC
VVC  0.4V (Switching off)
--
6
7.2
mA
Shutdown Current
ISHDN
VEN  0.7V
--
12
--
A
Logic-High VIH
2
--
--
Logic-Low
--
--
0.5
--
--
1.2
182
190
198
--
188
--
EN Threshold
Voltage
VIL
VEN  3V
EN Input Current
V
A
Current Sense Amplifier
VACTL  1.25V,
12V  common mode  90V
1.25V  VACTL  1.2V, (Note 7)
12V  common mode  90V
Input Threshold (VISP  VISN)
mV
ISP Input Current
IISP
4.5V  VISP  90V
--
140
--
A
ISN Input Current
IISN
4.5V  VISN  90V
--
60
--
A
VC Output Current
VC Threshold for PWM
Switch Off
IVC
0.5V  VC  2.4V
--
20
--
A
--
0.7
--
V
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is a registered trademark of Richtek Technology Corporation.
DS8494-01
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RT8494
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
VACTL = 1.2V
--
1
--
VACTL = 0.2V
--
10
--
--
1.3
--
V
--
0.2
--
V
0.3V  VDCTL  5V
--
--
0.5
A
VDCTL_H
(Note 6)
2
--
--
VDCTL_L
(Note 6)
--
--
0.3
f SW
RRSET = 30k
280
370
450
kHz
RRSET = 30k
--
250
--
ns
IGBIAS = 20mA
7.8
8.5
9.2
V
IGATE = 50mA
6
7.2
7.8
IGATE = 100A
7.5
7.8
7.9
IGATE = 10mA
--
0.5
1
IGATE= 100A
--
0.1
0.9
1nF Load at GATE
--
20
100
ns
80
110
145
mV
--
1.18
--
V
LED Dimming
Analog Dimming ACTL Pin
Input Current
IACTL
LED Max Current Threshold at
VACTL_On
ACTL
LED Current Off Threshold at
VACTL_Off
ACTL
DCTL Input Current
IDCTL
DCTL Threshold Voltage
A
V
PWM Control
Switching Frequency
Minimum Off-Time
Switch Gate Driver
GBIAS Voltage
VGBIAS
GATE Voltage High
VGATE_H
GATE Voltage Low
VGATE_L
GATE Drive Rise and Fall Time
PWM Switch Current Limit
ISW_LIM
Threshold
OVP and Soft-Start
V
V
OVP Threshold
VOVP_th
OVP Input Current
IOVP
0.7V  VOVP  1.5V
--
--
0.1
A
Soft-Start Pin Current
ISS
VSS  2V
--
6
--
A
Thermal Shutdown Protection
TSD
--
145
--
Thermal Shutdown Hysteresis
TSD
--
10
--
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. If connected with a 20kΩ serial resistor, ACTL and DCTL can go up to 36V.
Note 3. 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 4. Devices are ESD sensitive. Handling precaution is recommended.
Note 5. The device is not guaranteed to function outside its operating conditions.
Note 6. Guaranteed by design, not subjected to production test.
Note 7. The ACTL dimming curve is saturating when VACTL ≥ 1.2V. Please refer to typical operation characteristics curve of ILED
vs VACTL. This item is not subjected to production test.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
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November 2015
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RT8494
Typical Operating Characteristics
Efficiency vs. Input Voltage
Efficiency vs. Input Voltage
100
Boost
95
95
90
90
Efficiency (%)
Efficiency (%)
100
85
80
Buck − Boost
85
80
75
75
VOUT = 40V, IOUT = 410mA, L = 22μH
70
VOUT = 20V, IOUT = 410mA, L = 22μH
70
12
15
18
21
24
27
30
12
15
18
Switching Frequency (kHz)1
Efficiency (%)
95
90
85
80
75
VOUT = 10V, IOUT = 410mA, L = 22μH
12
15
18
21
24
27
360
340
320
300
4
30
8
12
9
18
Shutdown Current (μA)1
20
8
7
6
5
4
3
2
VIN = 4.5V to 36V
12
16
20
24
28
32
Input Voltage (V)
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24
28
32
36
16
14
12
10
8
6
4
2
VIN = 4.5V to 36V, VEN = 0V
0
0
8
20
Shutdown Current vs. Input Voltage
Supply Current vs. Input Voltage
Supply Current (mA)
16
Input Voltage (V)
10
4
30
380
Input Voltage (V)
1
27
400
Buck
70
24
Switching Frequency vs. Input Voltage
Efficiency vs. Input Voltage
100
21
Input Voltage (V)
Input Voltage (V)
36
4
8
12
16
20
24
28
32
36
Input Voltage (V)
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RT8494
VISP – VISN Threshold vs. Temperature
VISP – VISN Threshold vs. Input Voltage
200
VISP – VISN Threshold (mV)
VISP – VISN Threshold (mV)
200
195
190
185
195
190
185
180
180
4
8
12
16
20
24
28
32
-50
36
-25
0
Input Voltage (V)
100
125
450
400
0.27
0.26
LED Current (mA)
ACTL Off Threshold (V)
75
LED Current vs. ACTL Voltage
ACTL Off Threshold vs. Input Voltage
0.25
0.24
0.23
0.22
350
300
250
200
150
100
0.21
50
0.20
0
4
8
12
16
20
24
28
32
0.2
36
0.4
0.6
LED Current vs. DCTL PWM Duty
400
140
350
130
ISW Threshold (mV)
150
300
250
200
150
100
f = 10kHz
0
1
1.2
1.4
ISW Threshold vs. Input Voltage
450
50
0.8
ACTL Voltage (V)
Input Voltage (V)
LED Current (mA)
50
Temperature (°C)
0.28
120
110
100
90
80
70
60
50
0
20
40
60
80
DCTL PWM Duty (%)
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November 2015
100
4
8
12
16
20
24
28
32
36
Input Voltage (V)
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RT8494
OVP vs. Input Voltage
GATE Voltage vs. Input Voltage
1.20
8
1.19
OVP_H
1.18
OVP (V)
GATE Voltage (V)
GATE_Hi
1.17
OVP_L
1.16
6
4
2
1.15
No Load
GATE_Lo
1.14
0
4
8
12
16
20
24
28
32
4
36
12
16
20
24
28
Input Voltage (V)
Power On from EN
Power Off from EN
VEN
(5V/Div)
VEN
(5V/Div)
VOUT
(20V/Div)
VOUT
(20V/Div)
GATE
(10V/Div)
GATE
(10V/Div)
IOUT
(500mA/Div)
IOUT
(500mA/Div)
Time (2.5ms/Div)
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8
Input Voltage (V)
32
36
Time (100μs/Div)
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RT8494
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
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DS8494-01
November 2015
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 :
2.4V
tSS  CSS 
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 :
(VACTL  0.2)  0.19
ILED 
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
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RT8494
The amplitude of this signal is increased by high LED load
current, low switching frequency and/or a smaller value
output filter capacitor. The compensation capacitor on the
VC pin filters the signal so the average difference between
ISP and ISN is regulated on the user-programmed value.
Frequency vs. RRSET
1000
900
800
Frequency (kHz)1
The ACTL pin can also be used in conjunction with a
thermistor to provide over temperature protection for the
LED load, or with a voltage divider to VIN to reduce output
power and switching current when VIN is low. The presence
of a time varying differential voltage signal (ripple) across
ISP and ISN at the switching frequency is expected.
700
600
500
400
300
200
100
0
0
Programmable Switching Frequency
The RSET frequency adjust pin allows the user to program
the switching frequency from 100kHz to 1MHz for optimized
efficiency and performance or external component size.
Higher frequency operation allows for smaller component
size but increases switching losses and gate driving
current, and may not allow sufficiently high or low duty
cycle operation. Lower frequency operation gives better
performance but with larger external component size. For
an appropriate RRSET resistor value see Table 1 or Figure
5. An external resistor from the RSET pin to GND is
required and do not leave this pin open.
Table 1. Switching Frequency vs. RRSET Value (1%
Resistors)
fOSC (kHZ)
RRSET (k)
1000
8.34
800
11.41
600
16.68
500
20.9
300
38.04
200
60.35
100
130
15
30
45
60
75
90
105
120
135
R
RRSET
(kΩ)
RSET (kΩ)
Figure 5. Switching Frequency vs. RRSET
Output Over-Voltage Setting
The RT8494 is equipped with an Over-Voltage Protection
(OVP) function. When the voltage at OVP pin exceeds a
threshold of approximately 1.18V, the power switch is
turned off. The power switch can be turned on again once
the voltage at OVP pin drops below 1.18V. For the Boost
and Buck-Boost application, the output voltage could be
clamped at a certain voltage level. The OVP voltage can
be set by the following equation :
R1 

VOUT, OVP  1.18   1 

 R2 
Where,
R1 and R2 are the voltage divider from VOUT to GND with
the divider center node connected to OVP pin.
For Buck-Boost application, select a resistor according
to :


VIN  0.08V
RSW, Buck-Boost  



(V
V
)
I
IN
OUT
OUT


For Boost application, select a resistor according to :
 VIN  0.08V 
RSW, Boost  

 VOUT  IOUT 
The placement of RSW should be close to the source of
the N-MOSFET and GND of the RT8494. The ISW pin
input to RT8494 should be a Kelvin connection to the
positive terminal of RSW.
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is a registered trademark of Richtek Technology Corporation.
DS8494-01
November 2015
RT8494
Over-Temperature Protection
The RT8494 provides an Over-Temperature Protection
(OTP) function to prevent the excessive power dissipation
from overheating. The OTP function will shut down
switching operation when the die junction temperature
exceeds 145°C. The chip will automatically start to switch
again when the die junction temperature cools off.
Inductor Selection
The converter operates in discontinuous conduction mode
when the inductance value is less than the value LBCM.
With an inductance greater than LBCM, the converter
operates in Continuous Conduction Mode (CCM). The
inductance LBCM is determined by the following equations.
For Buck application :
LBCM 
VOUT
 VIN  VOUT 


2  IOUT  f 
VIN

For Boost application :
 VOUT  VIN 
VIN2
LBCM 
 

2  IOUT  f  VOUT 2 
For Buck-Boost application :
VIN2
VOUT
LBCM 

2  IOUT  f  VIN  VOUT 2
For Buck application :
VOUT
 VIN  VOUT 
L=


2  0.3  IOUT  f 
VIN

For Boost application :
L=
 VOUT  VIN 
VIN2
 

2  0.3  IOUT  f  VOUT 2 
For Buck-Boost application :
VIN2
VOUT
L=

2  0.3  IOUT  f  VIN  VOUT 2
The inductor must also be selected with a saturation
current rating greater than the maximum inductor current
during normal operation. The maximum inductor current
can be calculated by the following equations.
For Buck application :
VOUT  VIN  VOUT 


2  L  f 
VIN

For Boost application :
IPEAK = IOUT +
IPEAK =
VOUT  IOUT
VIN
 VOUT  VIN 
+


2  L  f  VOUT
  VIN

For Buck-Boost application :
IPEAK =
 VIN  VOUT   IOUT
  VIN
+
VIN
 VOUT


2  L  f  VIN  VOUT 
where
where
VOUT = output voltage.
η is the efficiency of the power converter.
VIN = input voltage.
f = operating frequency.
IOUT = LED current.
Choose an inductance based on the operating frequency,
input voltage and output voltage to provide a current mode
ramp signal during the MOSFET on period for PWM control
loop regulation. The inductance also determines the
inductor ripple current. Operating the converter in CCM is
recommended, which will have the smaller inductor ripple
current and hence the less conduction losses from all
converter components.
As a design example, to design the peak to peak inductor
ripple to be ±30% of the output current, the following
equations can be used to estimate the size of the needed
inductance :
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS8494-01
November 2015
Power MOSFET Selection
For applications operating at high input or output voltages,
the power N-MOS FET switch is typically chosen for drain
voltage VDS rating and low gate charge. Consideration of
switch on-resistance, R DS(ON), is usually secondary
because switching losses dominate power loss. The
GBIAS regulator on the RT8494 has a fixed current limit
to protect the IC from excessive power dissipation at high
VIN, so the N-MOSFET should be chosen so that the
product of Qg at 5V and switching frequency does not
exceed the GBIAS current limit.
ISW Sense Resistor Selection
The resistor, RSW, between the Source of the external NMOSFET and GND should be selected to provide adequate
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RT8494
switch current to drive the application without exceeding
the current limit threshold set by the ISW pin sense
threshold of RT8494. The ISW sense resistor value can
be calculated according to the formula below :
RSW 
Current Limlit Threshold Minimum Value
IOCP
where IOCP is about 1.33 to 1.5 times of inductor peak
current IPEAK.
The placement of RSW should be close to the source of
the N-MOSFET and the IC GND of the RT8494. The ISW
pin input to RT8494 should be a Kelvin sense connection
to the positive terminal of RSW.
Schottky Diode Selection
The Schottky diode, with their low forward voltage drop
and fast switching speed, is necessary for the RT8494
applications. In addition, power dissipation, reverse voltage
rating and pulsating peak current are the important
parameters for the Schottky diode selection. Choose a
suitable Schottky diode whose reverse voltage rating is
greater than maximum output voltage. The diode’s
average current rating must exceed the average output
current. The diode conducts current only when the power
switch is turned off (typically less than 50% duty cycle).
If using the PWM feature for dimming, it is important to
consider diode leakage, which increases with the
temperature, from the output during the PWM low interval.
Therefore, choose the Schottky diode with sufficiently low
leakage current.
Capacitor Selection
The input capacitor reduces current spikes from the input
supply and minimizes noise injection to the converter. For
most the RT8494 applications, a 10μF ceramic capacitor
is sufficient. A value higher or lower may be used
depending on the noise level from the input supply and
the input current to the converter.
For LED applications, the equivalent resistance of the LED
is typically low and the output filter capacitor should be
sized to attenuate the current ripple. Use of X7R type
ceramic capacitors is recommended. Lower operating
frequencies will require proportionately higher capacitor
values.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
PD(MAX) = (TJ(MAX) − TA) / θJA
where TJ(MAX) is the maximum junction temperature, TA is
the ambient temperature, and θJA is the junction to ambient
thermal resistance.
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
SOP-14 packages, the thermal resistance, θ JA , is
113.9°C/W on a standard JEDEC 51-7 four-layer thermal
test board. The maximum power dissipation at TA = 25°C
can be calculated by the following formula :
PD(MAX) = (125°C − 25°C) / (113.9°C/W) = 0.87W for
SOP-14 package
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. The derating curves in Figure 6 allow the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
In Boost application, the output capacitor is typically a
ceramic capacitor and is selected based on the output
voltage ripple requirements. The minimum value of the
output capacitor COUT is approximately given by the
following equation :
IOUT  VOUT
COUT 
VIN  VRIPPLE  fSW
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is a registered trademark of Richtek Technology Corporation.
DS8494-01
November 2015
RT8494
Maximum Power Dissipation (W)1
1.0
Four-Layer PCB
0.8
0.6
0.4
0.2
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 6. Derating Curve of Maximum Power Dissipation
Layout Consideration
PCB layout is very important to design power switching
converter circuits. The layout guidelines are suggested
as follows :

The power components L1, D1, CIN, M1 and COUT must
be placed as close to each other as possible to reduce
the ac current loop area. The PCB trace between power
components must be as short and wide as possible
due to large current flow through these traces during
operation.

The input capacitor CVCC must be placed as close to
VCC pin as possible.

Place the compensation components to VC pin as close
as possible to avoid noise pick up.

Connect GND pin and Exposed Pad to a large ground
plane for maximum power dissipation and noise
reduction.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS8494-01
November 2015
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15
RT8494
Place these components as close as possible
D1
L1
VIN
M1
COUT
GND
RSW
GND
RSENSE
RSET
ISW
ISP
ISN
VC
ACTL
DCTL
14
2
13
3
12
4
11
5
10
6
9
7
8
GATE
GBIAS
GND
VCC
OVP
EN
SS
CIN
CVCC
RVC
CSS
CVC
GND
Figure 7. PCB Layout Guide
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is a registered trademark of Richtek Technology Corporation.
DS8494-01
November 2015
RT8494
Outline Dimension
H
A
M
J
B
F
C
I
D
Dimensions In Millimeters
Dimensions In Inches
Symbol
Min
Max
Min
Max
A
8.534
8.738
0.336
0.344
B
3.810
3.988
0.150
0.157
C
1.346
1.753
0.053
0.069
D
0.330
0.508
0.013
0.020
F
1.194
1.346
0.047
0.053
H
0.178
0.254
0.007
0.010
I
0.102
0.254
0.004
0.010
J
5.791
6.198
0.228
0.244
M
0.406
1.270
0.016
0.050
14–Lead SOP Plastic Package
Richtek Technology Corporation
14F, No. 8, Tai Yuen 1st Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789
Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should
obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot
assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be
accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third
parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.
DS8494-01
November 2015
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