RT8458 - Richtek

®
RT8458
High Efficiency PWM Buck LED Driver Controller
General Description
Features
The RT8458 is a PWM controller with an integrated high
side floating gate driver. It is used for step down converters
by well controlling the external MOSFET and regulating a
constant output current. The output duty cycle of the
RT8458 can be up to 100% for wider input voltage
application, such as E27 and PAR30 off-line LED lighting
products.
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Low Cost and Efficient Buck Converter Solution
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Universal Input Voltage Range with Off-Line
Topology
Programmable Constant LED Current
Dimmable LED Current by ACTL
Output LED String Open Protection
Output LED String Short Protection
Output LED String Over Current Protection
Built-in Thermal Protection
TSOT-23-6 Package
RoHS Compliant and Halogen Free
The RT8458 also features a 47kHz fixed frequency
oscillator, an internal −178mV precision reference, and a
PWM comparator with latching logic. The accurate output
LED current is achieved by an averaging current feedback
loop and the LED current dimming can be easily controlled
via the ACTL pin. The RT8458 also has multiple features
to protect the controller from fault conditions, including
Under Voltage Lockout (UVLO), Over Current Protection
(OCP) and Over Voltage Protection (OVP). Additionally,
to ensure the system reliability, the RT8458 is built with
the thermal protection function.
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Applications
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E27, PAR30, Offline LED Lights
Pin Configurations
(TOP VIEW)
SENSE VC ACTL
The RT8458 is housed in a TSOT-23-6 package. Thus,
the components in the whole LED driver system can be
made very compact.
6
5
4
2
3
VCC GND GATE
Ordering Information
TSOT-23-6
RT8458
Package Type
J6 : TSOT-23-6
Lead Plating System
G : Green (Halogen Free and Pb Free)
Marking Information
01=DNN
01= : Product Code
DNN : Date Code
Note :
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.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS8458-03
October 2013
is a registered trademark of Richtek Technology Corporation.
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1
RT8458
Typical Application Circuit
VIN
CIN
10µF/
400V
RVCC1
1M
RVCC2
511k
RB
10
RT8458
1 VCC
ACTL 4
CVCC
4.7µF
CVC2
3.3nF
D2
FR107
RACTL
1M
5 VC SENSE 6
RVC 2
RG 22
GND GATE 3
10k
CVC1
Optional
1nF
VIN : 90 to 264VAC
ZD1 short
Optional
Q1
LED+
ZD2 39V
Optional
LED-
COUT
47µF/50V
D1
ES1J
VOUT : 28V
IOUT : 350mA
L
680µH
RS
0.51
Figure 1. Typical Application for LED Lamp
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
VCC
Power Supply Pin of the Chip. For good bypass, a ceramic capacitor near the VCC
pin is required.
2
GND
Ground of the Chip.
3
GATE
4
ACTL
5
VC
Gate Driver for External MOSFET Switch.
Analog Dimming Control. The typical effective dimming range is between 0V to
1.3V.
PWM Loop Compensation Node.
6
SENSE
LED Current Sense Input Pin. Typical sensing threshold is −178mV.
Function Block Diagram
+
VCC
+
-
Chip Enable
12V
17V/7.5V
OVP
+
35.5V
-
OSC
GATE
S
200k
R
R
+
-
Control
Circuit
VC
Dimming
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2
ACTL
-
SENSE
+
GND
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DS8458-03
October 2013
RT8458
Absolute Maximum Ratings
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(Note 1)
Supply Input Voltage, VCC ----------------------------------------------------------------------------------------------GATE Voltage -------------------------------------------------------------------------------------------------------------ACTL Voltage (Note 5) ------------------------------------------------------------------------------------------------VC Voltage -----------------------------------------------------------------------------------------------------------------SENSE Voltage -----------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
TSOT-23-6 ------------------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2)
TSOT-23-6, θJA ------------------------------------------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Model) ---------------------------------------------------------------------------------------------MM (Machine Model) -----------------------------------------------------------------------------------------------------
Recommended Operating Conditions
z
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−0.3V to 40V
−0.3V to 14V
−0.3V to 8V
−0.3V to 6V
−1V to 0.3V
0.392W
255°C/W
150°C
260°C
−65°C to 150°C
2kV
200V
(Note 4)
Supply Input Voltage, VCC ----------------------------------------------------------------------------------------------- 17V to 32V
Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C
Electrical Characteristics
(VCC = 24VDC, TA = 25°C, unless otherwise specified)
Parameter
Input Start-Up Voltage
Symbol
Test Conditions
Min
Typ
Max
Unit
VST
15
17
19
V
VIN(MIN)
6
7.5
9
V
Minimum Operation Voltage After
Start-Up
Maximum Startup Current in VCC
Hiccup Operation
Input Quiescent Current
IST(MAX)
Maximum ICC at low end of VCC
--
250
300
μA
IQC
After Start-Up, VCC = 24V
--
1.65
5
mA
Input Shutdown Current
ISHDN
Before Start-Up, VCC = 15V
--
0.1
5
μA
Over Voltage Protection
VOVP
VCC Pin
32.5
35.5
36.5
V
Current Sense Voltage
VSENSE
−169
−178
−187
mV
Switching Frequency
f SW
38
47
55
kHz
Oscillator Maximum Duty Cycle
DMAX
--
--
100
%
Minimum Turn-On Time
tON(MIN)
300
--
--
ns
GATE Pin Maxim um Voltage
VGATE
No Load at GATE Pin
11.5
12.5
13.5
V
GATE Voltage High
VGATE_H
IGATE = −20mA
11.4
12.4
13.4
IGATE = −100μA
VGATE_L
IGATE = 20mA
12.5
0.75
13.5
GATE Voltage Low
11.5
0.55
0.95
IGATE = 100μA
0.3
0.5
0.7
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS8458-03
October 2013
VC = 3V
V
V
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RT8458
Parameter
Min
Typ
Max
Unit
1nF Load at GATE
--
40
60
ns
1nF Load at GATE
--
0.5
--
A
--
1
5
μA
0
--
1.3
V
High Level
--
1.2
1.3
Low Level
0
0.1
--
1.1
1.25
1.4
V
150
--
--
°C
GATE Drive Rise and Fall Time
GATE Drive Source and Sink
Peak Current
ACTL LED Dimming
Analog Dimming ACTL Pin Input
Current
Analog Dimming Range
Analog Dimming
Threshold Voltage
Symbol
Test Conditions
IACTL
VC Threshold for PWM Switch Off VVC
V
Thermal Protection
Thermal Shutdown Temperature
TSD
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 low effective thermal conductivity single-layer test board per JEDEC 51-3.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Note 5. If the ACTL pin is connected with a serial 1MΩ resistor, the maximum voltage can go up to 36V.
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is a registered trademark of Richtek Technology Corporation.
DS8458-03
October 2013
RT8458
Typical Operating Characteristics
Efficiency vs. Input Voltage
100%
100
VIN = 85V to 264VAC, IOUT = 350mA,
LED 3 to 10 pcs
10LEDs
9LEDs
8LEDs
7LEDs
95%
95
Efficiency vs. Number of LEDs
85
85%
6LEDs
5LEDs
4LEDs
3LEDs
75
75%
VIN = 85V to 264VAC, IOUT = 350mA
95%
95
Efficiency (%)
Efficiency (%)
90
90%
80
80%
100%
100
90
90%
85
85%
85VAC
110VAC
180VAC
150VAC
220VAC
264VAC
80%
80
75
75%
70
70%
70
70%
85
105
125
145
165
185
205
225
245
3
265
4
Input Voltage (V)
Switching Frequency vs. Supply Voltage
8
9
10
Switching Frequency vs. Temperature
Switching Frequency (kHz)1
Switching Frequency (kHz)1
7
50
51
47
43
39
48
46
44
42
40
35
0
5
10
15
20
25
30
-50
35
-25
25
50
75
100
125
LED Current vs. Input Voltage
LED Current vs. VACTL
500
0
Temperature (°C)
Supply Voltage (V)
380
VIN = 110VAC, IOUT = 350mA, LED 10 pcs,
L = 0.68mH, VACTL = 0 to 1.8V
LED Current (mA)
400
LED Current (mA)
6
Number of LED (pcs)
55
300
200
100
0
VIN = 85V to 264VAC, IOUT = 350mA,
LED 3 to 10 pcs
6LEDs
5LEDs
4LEDs
3LEDs
370
10LEDs
9LEDs
8LEDs
7LEDs
360
350
340
0
0.3
0.6
0.9
1.2
1.5
VACTL (V)
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DS8458-03
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October 2013
1.8
85
115
145
175
205
235
265
Input Voltage (V)
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RT8458
LED Current vs. Output Voltage
Input Voltage and Input Current
400
VIN
(400V/Div)
LED Current (mA)
380
360
I IN
(500mA/Div)
340
320
VIN = 110VAC IOUT = 350mA,
LED 3 to 10 pcs, L = 0.68mH
VIN = 264VAC, IOUT = 350mA,
LED 10 pcs, L = 0.68mH
300
10
13
16
19
22
25
28
31
34
Time (25ms/Div)
37
Output Voltage (V)
Output Current and Output Voltage Ripple
VGATE Voltage and Inductor Current
VOUT
(1V/Div)
VGATE
(10V/Div)
IOUT
(10mA/Div)
IL
(500mA/Div)
VIN = 264VAC, IOUT = 350mA,
LED10 pcs, L = 0.68mH
VIN = 264VAC, IOUT = 350mA,
LED 10 pcs, L = 0.68mH
Time (10μs/Div)
Time (10μs/Div)
Power On
Power Off
VIN
(400V/Div)
VIN
(400V/Div)
VOUT
(20V/Div)
VOUT
(20V/Div)
IOUT
(200mA/Div)
VIN = 264VAC,
IOUT = 350mA, LED 10 pcs, L = 0.68mH
Time (25ms/Div)
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IOUT
(200mA/Div)
VIN = 264VAC
IOUT = 350mA, LED 10 pcs, L = 0.68mH
Time (25ms/Div)
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DS8458-03
October 2013
RT8458
Application Information
The RT8458 is a high efficiency PWM buck LED driver
controller for high brightness LED application. Its high side
floating gate driver is used to control the buck converter
via an external MOSFET and regulate the constant output
current.
The RT8458 can achieve high accuracy LED output current
via the average current feedback loop control. The internal
sense voltage (−178mV typ.) is used to set the average
output current. The oscillator’s frequency is fixed at
47kHz to get better switching performance. Once the
average current is set by the external resistor, RS, the
output LED current can be dimmed by varying the ACTL
voltage.
Under Voltage Lockout (UVLO)
The RT8458 includes a UVLO feature with 9.5V hysteresis.
The GATE terminal turns on when VCC rises over 17V
(typ.). The GATE terminal turns off when VCC falls below
7.5V (typ.)
Setting Average Output Current
The output current that flows through the LED string is
set by an external resistor, RS, which is connected between
the GND and SENSE terminal. The relationship between
output current, IOUT, and RS is shown below :
0.178
IOUT =
(A)
RS
Analog Dimming Control
The ACTL terminal is driven by an external voltage, VACTL,
to adjust the output current to an average value set by RS.
The voltage range for VACTL to adjust the output current is
from 0V to 1.3V. If VACTL becomes larger than 1.3V, the
output current value will just be determined by the external
resistor, RS.
V
IOUTavg = (0.178V/RS ) × ACTL
1.3
Component Selection
For component selection, an example is shown below for
a typical RT8458 application, where VIN = 110 to 90VAC/
60Hz, LED output voltage = 30V, and output current =
200mA. The user can follow this procedure to design
applications with wider AC voltage input and DC output
voltage as well.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS8458-03
October 2013
Start-up Resistor
Start-up resistor should be chosen not to exceed the
maximum start-up current. Otherwise, the RT8458 may
latch low and will never start. Start-up current = 130V/R1
for 110VAC regions, 260V/R1 for 220VAC regions. The
typical start-up current is 250μA.
Input Diode Bridge Rectifier Selection
The current rating of the input bridge rectifier is dependent
on the VOUT /VIN transformation ratio. The voltage rating of
the input bridge rectifier, VBR, on the other hand, is only
dependent on the input voltage. Thus, the VBR rating is
calculated as below :
VBR = 1.2 × ( 2 × VAC(MAX) )
where VAC,Max is the maximum input voltage (RMS) and
the parameter 1.2 is used for safety margin.
For this example :
VBR = 1.2 × ( 2 × VAC(MAX) ) = (1.2 × 2 × 110) = 187V
If the input source was universal, VBR will reach 466V. In
this case, a 600V, 0.5A bridge rectifier can be chosen.
Input Capacitor Selection
The input capacitor supplies the peak current to the
inductor and flattens 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. For
maximum stability over the entire operating temperature
range, capacitors with better dielectric are suggested. The
minimum capacitor is given by :
VOUT(MAX) × IOUT(MAX)
CIN ≥
⎡( 2 × VAC(MIN) )2 − V 2DC(MIN) ⎤ ×η × fAC
⎣
⎦
where fAC is the AC input source frequency and η is the
efficiency of whole system.
Notice that VDC(MIN) is the minimum voltage at bridge
rectifier, output and VDC(MIN) should be larger than 2 x
VOUT(MAX).
For a 90 to 264VAC universal input range, the VDC(MIN) is
90V, therefore the LED string voltage VOUT(MAX) should be
less than 45V.
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RT8458
For this particular example :
30 × 0.2
CIN ≥
= 13.7μF
2
⎡( 2 × 90) − 902 ⎤ × 0.9 × 60
⎣
⎦
In addition, the voltage rating of the input filter capacitor,
VCIN, should be large enough to handle the input
voltage.
VCIN ≥ (1.2 × 2 × VAC(MAX) ) = (1.2 × 2 × 110) = 187V
Thus, a 22μF / 250V electrolytic capacitor can be chosen
in this case. Due to its large ESR, the electrolytic capacitor
is not suggested for high current ripple applications.
For DC applications, an input capacitor, CIN, is needed to
filter out the trapezoid current on the high side MOSFET.
To prevent large ripple voltage, a low ESR input capacitor
sized for the maximum RMS current should be used.
Choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet the size or height requirements of the design.
Generally, one 10μF low ESR ceramic capacitor is
recommended for the input capacitor. Ceramic capacitors
have high ripple current, high voltage rating and low ESR,
which makes them ideal for switching regulator
applications.
Forward Diode Selection
When the power switch turns off, the path for the current
is through the diode connected between the switch output
and ground. This forward biased diode must have minimum
voltage drop and recovery time. The reverse voltage rating
of the diode should be greater than the maximum input
voltage and the current rating should be greater than the
maximum load current.
In reality, the peak current through the diode is more than
the maximum output current. This component current
rating should be greater than 1.2 times the maximum load
current and the diode reverse voltage rating should be
greater than 1.2 times the maximum input voltage,
assuming a ± 20% output current ripple.
The peak voltage stress of diode is :
VD = 1.2 × ( 2 × VAC(MAX) ) = 1.2 × ( 2 × 110) = 187V
The current rating of diode is :
ID = 1.2 × IOUT,PK = 1.2 × 1.2 × 0.2 = 0.288A
If the input source is universal (VIN = 90V to 264V), VD will
reach 466V. A 600V, 2A ultra-fast diode can be used in
this example.
MOSFET Selection
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current, ΔIL, increases with higher VIN
and decreases with higher inductance, as shown in
equation below :
⎤
⎡V
⎤ ⎡ V
ΔIL = ⎢ OUT ⎥ × ⎢1− OUT ⎥
VIN ⎦
⎣ fxL ⎦ ⎣
To optimize the ripple current, the RT8458 operates the
buck converter in BCM (Boundary-Condition Mode). The
largest ripple current will occur at the highest VIN. To
guarantee that the ripple current stays below the specified
value, the inductor value should be chosen according to
the following equation :
L=
VOUT × TS × (1− D)
2 × IOUT
30 × 20.83μs × (1− 0.333)
=
= 1.04mH
2 × 0.2
where D is the duty cycle and TS is the switching period.
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The peak current through this MOSFET will be over the
maximum output current. This component current rating
should be greater than 1.2 times the maximum load
current and the reverse voltage rating of the MOSFET
should be greater than 1.2 times the maximum input
voltage, assuming a ± 20% output current ripple.
The peak voltage rating of the MOSFET is :
VQ = 1.2 × ( 2 × VAC(MAX) ) = 1.2 × ( 2 × 110) = 187V
The current rating of MOSFET is :
IQ = 1.2 × IOUT,PK = 1.2 × 1.2 × 0.2 = 0.288A
If the input source was universal (VIN = 90V to 264V), VQ
will reach 466V. A 600V, 2A N-MOSFET can be chosen
for this example.
Output Capacitor Selection
The selection of COUT is determined by the required ESR
to minimize output voltage ripple. Moreover, the amount
of bulk capacitance is also a key for COUT selection to
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RT8458
where fOSC is the switching frequency and ΔIL is the
inductor ripple current. The output voltage ripple will be
the highest at the maximum input voltage since ΔIL
increases with input voltage. Multiple capacitors placed in
parallel may be needed to meet the ESR and RMS current
handling requirement. Dry tantalum, special polymer,
aluminum electrolytic and ceramic capacitors are all
common selections and available in surface mount
packages. Tantalum capacitors have the highest
capacitance density, but it is important to only use ones
that pass the surge test for use in switching power
supplies. Special polymer capacitors offer very low ESR
value, but with the trade-off of lower capacitance density.
Aluminum electrolytic capacitors have significantly higher
ESR, but still can be used in cost-sensitive applications
for ripple current rating and long term reliability
considerations.
Thermal Protection
A thermal protection feature is included to protect the
RT8458 from excessive heat damage. When the junction
temperature exceeds a threshold of 150°C, the thermal
protection will turn off the GATE terminal.
Soldering Process of Pb-free Package Plating
To meet the current RoHS requirements, pure tin is
selected to provide forward and backward compatibility
with both the current industry standard SnPb-based
soldering processes and higher temperature Pb-free
processes. In the whole Pb-free soldering processes pure
tin is required with a maximum 260°C (<10s) for proper
soldering on board, referring to J-STD-020 for more
information.
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
TSOT-23-6 package, the thermal resistance, θJA, is 255°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) / (255°C/W) = 0.392W for
TSOT-23-6 package
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. The derating curve in Figure 2 allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
Maximum Power Dissipation (W)1
ensure that the control loop is stable. Loop stability can
be checked by viewing the load transient response. The
output voltage ripple, ΔVOUT, is determined by :
⎡
⎤
1
ΔVOUT ≤ ΔIL ⎢ESR +
⎥
8fOSCCOUT ⎦
⎣
0.45
Single-Layer PCB
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 2. Derating Curve of Maximum Power Dissipation
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
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DS8458-03
October 2013
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RT8458
Layout Considerations
`
Keep the main current traces as short and wide (2 to
3mm) as possible. Lay out the traces straight without
any via.
For best performance of the RT8458, the following layout
guidelines should be strictly followed.
`
The hold up capacitor, CVCC, must be placed as close
as possible to the VCC pin.
`
Place L, Q1, RS, and D1 as close to each other as
possible.
`
The output capacitor, COUT, must be placed as close as
possible to the LED terminal.
`
`
The power GND should be connected to a strong ground
plane.
The components Q1, D1, D2, AC Line L / N terminal and
CIN could take very high voltage. Please keep the gaps
between them to be larger than 3mm to meet the
requirements of safety standards.
`
RS should be connected between the GND pin and
SENSE pin.
`
The trace from the GATE pin to Q1 should be short and
has no vias.
`
AC Line L / N layout traces should not cross and overlap
LED+ and LED− traces to prevent the noise interference
between each other.
RVCC1
RACTL
RT8458
L
VIN
CIN
N
RVC
CVCC
D2
RB
1
VCC ACTL
4
5
VC SENSE
6
2
GND GATE
3
RG
Q1
CVC
RS
CS
Analog GND
L
LED+
COUT
D1
Power GND
LED-
Place the capacitor CVCC as
close as possible to the VCC.
Place the output capacitor COUT as
close as possible to LED terminal.
Figure 3. PCB Layout Guide
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is a registered trademark of Richtek Technology Corporation.
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October 2013
RT8458
Outline Dimension
H
D
L
C
B
b
A
A1
e
Dimensions In Millimeters
Dimensions In Inches
Symbol
Min
Max
Min
Max
A
0.700
1.000
0.028
0.039
A1
0.000
0.100
0.000
0.004
B
1.397
1.803
0.055
0.071
b
0.300
0.559
0.012
0.022
C
2.591
3.000
0.102
0.118
D
2.692
3.099
0.106
0.122
e
0.838
1.041
0.033
0.041
H
0.080
0.254
0.003
0.010
L
0.300
0.610
0.012
0.024
TSOT-23-6 Surface Mount 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.
DS8458-03
October 2013
www.richtek.com
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