RT8475 - Richtek

®
RT8475
High Voltage High Current LED Driver Controller for
Buck, Boost or Buck-Boost Topology
General Description
Features
The RT8475 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 RT8475 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.
The RT8475 is available in SOP-14 and WQFN-16L 3x3
packages.
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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
RoHS Compliant and Halogen Free
Applications




General Industrial High Power LED Lighting
Desk Lights and Room Lighting
Building and Street Lighting
Industrial Display Backlight
Pin Configurations
(TOP VIEW)
RT8475
Package Type
S : SOP-14
QW : WQFN-16L 3x3 (W-Type)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Z : ECO (Ecological Element with
Halogen Free and Pb free)
RSET
ISW
ISP
ISN
VC
ACTL
DCTL
Richtek products are :
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.

Suitable for use in SnPb or Pb-free soldering processes.
13
3
12
4
11
5
10
6
9
7
8
SOP-14
Note :

GATE
GBIAS
GND
VCC
OVP
EN
SS
14
2
NC
NC
GATE
GBIAS
Ordering Information
16 15 14 13
RSET
ISW
ISP
ISN
1
12
2
11
GND
3
10
17
4
6
7
9
8
VC
ACTL
DCTL
SS
5
GND
VCC
OVP
EN
WQFN-16L 3x3
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DS8475-06
December 2014
is a registered trademark of Richtek Technology Corporation.
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RT8475
Marking Information
RT8475GS
RT8475ZQW
RT8475GS : Product Number
RT8475
GSYMDNN
5A : Product Code
YMDNN : Date Code
YMDNN : Date Code
5A YM
DNN
RT8475ZS
RT8475
ZSYMDNN
RT8475ZS : Product Number
YMDNN : Date Code
Typical Application Circuit
L1
22µH
VIN
4.5V to 36V
CIN
10µF
RT8475
11
9 EN
5V
Analog
Dimming
5 VC
8
SS
13 GBIAS
RVC
10k
CSS
0.1µF
CVC
3.3nF
CB
1µF
RSW
0.05
ISW 2
3
ISP
4
ISN
10
OVP
6 ACTL
7 DCTL
GND
12
RSET
1
VOUT
90V (Max.)
COUT
1µF
M1
GATE 14
VCC
RSENSE
0.47
D1
LEDs
R1
RRSET
30k
VOUT
R2
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
LEDs
L1
22µH
RT8475
11
9 EN
5V
Analog
Dimming
ISN 4
6 ACTL
7 DCTL
RVC
10k
CVC
3.3nF
ISP 3
VCC
CSS
0.1µF
5 VC
8
SS
13 GBIAS
CB
1µF
GATE
M1
14
ISW 2
RSET 1
RSW
0.05
RRSET
OVP 10
GND
12
Figure 2. Analog Dimming in Buck Configuration
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DS8475-06
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RT8475
VIN2
90V (max)
VIN1
4.5V to 36V
CIN1
10µF
RT8475
11
VCC
9 EN
5V
Analog
Dimming
6 ACTL
7 DCTL
5
RVC
10k
CVC
3.3nF
VC
8
CSS
0.1µF
CIN2
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
SOP-14
WQFN-16L 3x3
1
1
RSET
2
2
ISW
3
3
ISP
4
4
ISN
5
5
VC
6
6
ACTL
Analog Dimming Control. The effective programming voltage range of
the pin is between 0.2V and 1.2V.
7
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
8
SS
9
9
EN
10
10
OVP
11
11
VCC
12
12,
GND
17 (Exposed Pad)
Switch Frequency Set Pin. Connect a resistor from RSET to GND.
RRSET = 30k will set f SW = 350kHz.
External MOSFET Switch Current Sense. Connect the current sense
resistor between external N-MOSFET switch and the ground.
LED Current Sense Amplifier Positive Input with Common Mode Up to
90V.
LED Current Sense Amplifier Negative Input. Voltage threshold
between ISP and ISN is 190mV with common mode voltage up to 90V.
PWM Control Loop Compensation.
Soft-Start. A capacitor of at least 10nF is required for proper soft-start.
Chip Enable (Active High). When this pin voltage is low, the chip is in
shutdown mode.
Over Voltage Protection. The PWM converter turns off when the
voltage of the pin goes to higher than 1.18V.
Power Supply Pin of the Chip. For good bypass, a low ESR capacitor
is required.
Ground. The Exposed Pad must be Soldered to a Large PCB and
Connected to GND for Maximum Power Dissipation.
13
13
GBIAS
Internal Gate Driver Bias. A good bypass capacitor is required.
14
14
GATE
External MOSFET Switch Gate Driver Output.
--
15, 16
NC
No Internal Connection.
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RT8475
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
1.2V
DCTL
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.
DS8475-06
December 2014
RT8475
Absolute Maximum Ratings

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(Note 1)
Supply Input Voltage, VCC ---------------------------------------------------------------------------------------------GBIAS, GATE -------------------------------------------------------------------------------------------------------------ISW --------------------------------------------------------------------------------------------------------------------------ISP, ISN ---------------------------------------------------------------------------------------------------------------------DCTL, ACTL, OVP (Note 2) ------------------------------------------------------------------------------------------EN ----------------------------------------------------------------------------------------------------------------------------SS, RSET, VC -------------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
SOP-14 ---------------------------------------------------------------------------------------------------------------------WQFN-16L 3x3 -----------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 3)
SOP-14, θJA ----------------------------------------------------------------------------------------------------------------WQFN-16L 3x3, θJA ------------------------------------------------------------------------------------------------------WQFN-16L 3x3, θJC -----------------------------------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------------ESD Susceptibility (Note 4)
HBM (Human Body Model) ---------------------------------------------------------------------------------------------MM (Machine Model) -----------------------------------------------------------------------------------------------------
Recommended Operating Conditions


−0.3 to 38V
−0.3 to 10V
−0.3 to 1V
−0.3 to 100V
−0.3 to 8V
−0.3 to 20V
−0.3 to 5V
1.0W
1.471W
100°C/W
68°C/W
7.5°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, TA = 25°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
EN Threshold
Voltage
Logic-High
VIH
2
--
--
Logic-Low
VIL
--
--
0.5
--
--
1.2
182
190
198
--
188
--
--
140
--
A
VEN  3V
EN Input Current
V
A
Current Sense Amplifier
ISP Input Current
IISP
VACTL  1.25V,
12V  common mode  90V
1.25V  VACTL  1.2V, (Note 7)
12V  common mode  90V
4.5V  VISP  90V
ISN Input Current
IISN
4.5V  VISN  90V
--
60
--
A
VC Output Current
IVC
0.5V  VC  2.4V
--
20
--
A
--
0.7
--
V
Input Threshold (VISP  VISN)
VC Threshold for PWM Switch Off
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December 2014
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RT8475
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
VACTL = 1.2V
--
1
--
VACTL = 0.2V
--
10
--
VACTL_On
--
1.3
--
V
VACTL_Off
--
0.2
--
V
A
LED Dimming
Analog Dimming ACTL Pin
Input Current
LED Current On Threshold at
ACTL
LED Current Off Threshold at
ACTL
DCTL Input Current
DCTL Threshold Voltage
IACTL
A
IDCTL
0.3V  VDCTL  5V
--
--
0.5
VDCTL_H
(Note 6)
2
--
--
VDCTL_L
(Note 6)
--
--
0.3
f SW
RRSET = 30k
280
350
420
kHz
RRSET = 30k
--
250
--
ns
IGBIAS = 20mA
6
8.5
7.8
V
IGATE = 50mA
6.6
7.2
7.4
IGATE = 100A
7.5
7.8
7.9
IGATE = 50mA
--
0.5
1
IGATE= 100A
--
0.1
0.9
1nF Load at GATE
--
20
100
ns
80
110
145
mV
--
1.18
--
V
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. For WQFN-16L 3x3, θJA is measured in natural convection at TA = 25°C on a high-effective thermal conductivity test
board of JEDEC 51-7 thermal measurement standard. The measurement case position of θJC is on the exposed pad
of the package. For SOP-14, θJA is measured in natural convection at TA = 25°C on a low-effective thermal conductivity
test board of JEDEC 51-3 thermal measurement standard.
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.
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is a registered trademark of Richtek Technology Corporation.
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RT8475
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
VOUT = 20V, IOUT = 410mA, L = 22μH
70
70
12
15
18
21
24
27
30
12
15
18
Switching Frequency (kHz)1
Buck
Efficiency (%)
90
85
80
75
VOUT = 10V, IOUT = 410mA, L = 22μH
70
15
18
21
24
27
380
360
340
320
300
30
4
8
12
Supply Current vs. Input Voltage
9
18
Shutdown Current (μA)1
20
8
7
6
5
4
3
2
VIN = 4.5V to 36V
0
20
24
28
32
36
Shutdown Current vs. Input Voltage
10
1
16
Input Voltage (V)
Input Voltage (V)
Supply Current (mA)
30
400
95
16
14
12
10
8
6
4
2
VIN = 4.5V to 36V, VEN = 0V
0
4
8
12
16
20
24
28
32
Input Voltage (V)
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Switching Frequency vs. Input Voltage
Efficiency vs. Input Voltage
12
24
Input Voltage (V)
Input Voltage (V)
100
21
December 2014
36
4
8
12
16
20
24
28
32
36
Input Voltage (V)
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RT8475
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)
50
75
100
125
LED Current vs. ACTL Voltage
450
0.27
400
0.26
350
LED Current (mA)
ACTL Off Threshold (V)
ACTL Off Threshold vs. Input Voltage
0.28
0.25
0.24
0.23
0.22
300
250
200
150
100
0.21
50
0.20
0
4
8
12
16
20
24
28
32
36
0.2
0.4
0.6
Input Voltage (V)
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)
LED Current vs. DCTL PWM Duty
LED Current (mA)
25
Temperature (°C)
120
110
100
90
80
70
60
50
0
20
40
60
80
DCTL PWM Duty (%)
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100
4
8
12
16
20
24
28
32
36
Input Voltage (V)
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RT8475
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
GATE_Lo
1.14
0
4
8
12
16
20
24
28
32
36
4
8
12
16
20
24
28
Input Voltage (V)
Input Voltage (V)
Power On from EN
Power Off from EN
VEN
(5V/Div)
VEN
(5V/Div)
VOUT
(20V/Div)
GATE
(10V/Div)
VOUT
(20V/Div)
GATE
(10V/Div)
IOUT
(500mA/Div)
IOUT
(500mA/Div)
Time (2.5ms/Div)
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No Load
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36
Time (100μs/Div)
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RT8475
Applications Information
The RT8475 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 RT8475
can be up to 36V and the output voltage can be up to 90V.
The RT8475 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
RT8475 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 RT8475, typically the extra GBIAS load
should be limited to less than 10mA.
Loop Compensation
The RT8475 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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from the VC pin to GND to provide a pole and a zero for
proper loop compensation. The typical compensation for
the RT8475 is 10kΩ and 3.3nF.
Soft-Start
The soft-start of the RT8475 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 :
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
is a registered trademark of Richtek Technology Corporation.
DS8475-06
December 2014
RT8475
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
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-do not leave this pin open.
Table 1. Switching Frequency vs. RREST Value (1%
Resistor)
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 RT8475 is equipped with 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,
fOSC (kHZ)
RRSET (k)
1000
8.34
800
11.41
600
16.68
500
20.9
Over Temperature Protection
300
38.04
200
60.35
100
130
The RT8475 has 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.
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS8475-06
0
December 2014
R1 and R2 are the voltage dividers from VOUT to GND with
the divider center node connected to OVP pin.
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RT8475
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 :
LBCM 
 VOUT  VIN 
VIN2
 

2  IOUT  f  VOUT 2 
For Buck-Boost application :
VIN2
VOUT
LBCM 

2  IOUT  f  VIN  VOUT 2
where
For Boost application :
L=
 VOUT  VIN 
VIN2
 

2  0.3  IOUT  f  VOUT 2 
For Buck-Boost application :
L=
VIN2
VOUT

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 :
IPEAK  IOUT 
For Boost application :
IPEAK 
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 :
VOUT  IOUT
VIN
 VOUT  VIN 



2  L  f  VOUT
  VIN

For Buck-Boost application :
VOUT = output voltage.
VIN = input voltage.
VOUT  VIN  VOUT 


2  L  f 
VIN

IPEAK 
 VIN  VOUT   IOUT
  VIN

VIN
2L  f
 VOUT



V
V

OUT 
 IN
where
η is the efficiency of the power converter.
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 RT8475 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.
For Buck application :
L
VOUT
 VIN  VOUT 


2  0.3  IOUT  f 
VIN

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is a registered trademark of Richtek Technology Corporation.
DS8475-06
December 2014
RT8475
ISW Sense Resistor Selection
The resistor, RSW, between the Source of the external
N-MOSFET and GND should be selected to provide
adequate switch current to drive the application without
exceeding the current limit threshold set by the ISW pin
sense threshold of RT8475. The ISW sense resistor value
can be calculated according to the formula below :
RSW 
current limit 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 RT8475. The ISW
pin input to RT8475 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 RT8475
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 of the RT8475 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.
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS8475-06
December 2014
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 :
COUT 
IOUT  VOUT
VIN  VRIPPLE  fSW
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
WQFN-16L 3x3 package, the thermal resistance, θJA, is
68°C/W on a standard JEDEC 51-7 four-layer thermal test
board. For SOP-14 package, the thermal resistance, θJA,
is 100°C/W on a standard JEDEC 51-3 single-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) / (68°C/W) = 1.471W for
WQFN-16L 3x3 package
P D(MAX) = (125°C − 25°C) / (100°C/W) = 1.0W for
SOP-14 package
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RT8475
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.
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.
Maximum Power Dissipation (W)1
1.60
1.40
WQFN-16L 3x3 (Four-Layer PCB)
1.20
1.00
0.80
0.60
SOP-14 (Single-Layer PCB)
0.40
0.20
0.00
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 6. Derating Curve of Maximum Power Dissipation
Place these components
as close as possible
D1
Power trace must be wide
and short when compared
to the normal trace.
VIN power trace to L1
must be wide and short.
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
RVC
CIN
CVCC
The input capacitor as
close VCC pin as possible.
Normal trace.
CSS
CVC
GND
Locate The compensation
components to VC pin as
close as possible.
Figure 7. PCB Layout Guide
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is a registered trademark of Richtek Technology Corporation.
DS8475-06
December 2014
RT8475
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
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS8475-06
December 2014
is a registered trademark of Richtek Technology Corporation.
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15
RT8475
D
SEE DETAIL A
D2
L
1
E
E2
e
b
A
A1
1
1
2
2
DETAIL A
Pin #1 ID and Tie Bar Mark Options
A3
Note : The configuration of the Pin #1 identifier is optional,
but must be located within the zone indicated.
Dimensions In Millimeters
Dimensions In Inches
Symbol
Min
Max
Min
Max
A
0.700
0.800
0.028
0.031
A1
0.000
0.050
0.000
0.002
A3
0.175
0.250
0.007
0.010
b
0.180
0.300
0.007
0.012
D
2.950
3.050
0.116
0.120
D2
1.300
1.750
0.051
0.069
E
2.950
3.050
0.116
0.120
E2
1.300
1.750
0.051
0.069
e
L
0.500
0.350
0.020
0.450
0.014
0.018
W-Type 16L QFN 3x3 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.
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DS8475-06
December 2014