ADTECH ADT6780

Thermally enhanced Low VFB Step-Down LED Driver
Thermally enhanced
Low VFB Step-Down LED Driver
ADT6780
ADT6780
General Description
The ADT6780 is a thermally enhanced current mode step down LED driver. That is designed
to deliver constant current to high power LEDs. The device is suitable for various high power
LED application due to the wide operating range(VIN 4.5V~28V) and high output
capability(continuous 2A). with a very low feedback voltage(VFB=0.2V) power dissipation can
be minimized. Fault condition protection includes cycle-by-cycle current limiting and thermal
shutdown.
The package is available in a standard SOP8-PP(with Exposed pad) package.
Features
Applications
• Feedback reference voltage : 0.2V
• Current mode buck LED driver with 925kHz
fixed frequency
• Input voltage range : 4.5V to 28V
• Continuous output current : 2A
• Up to 93% efficiency
• Integrated Power MOSFET switch : 80mΩ
• 10㎂ shutdown mode
• Thermal shutdown & current limit protection
• Under Voltage LOckout
•
•
•
•
High power LED/IR-LED Lighting
Automotive and Marine Lighting
Architecture Lighting
General Lighting Solutions
Typical Application Circuit
C5
1
BST
SW
L1
3
VOUT
C2
D1
2
VIN
VIN
U1
C1
7
EN
FB
EN
D2
5
R1
COMP 6
C3
8
SS
GND
4
R2
C4
C6
OPTION
Figure 1. Typical Application Circuit
* This specifications are subject to be changed without notice
Jul. 12. 2012 / Preliminary
1/13
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Thermally enhanced Low VFB Step-Down LED Driver
ADT6780
Part List
Component
Type
Value (Model)
Manufacturer
U1
IC
ADT6780
ADTech
D1
Schottky Barrier Diode
B230A
DIODES
L1
Chip inductor
6.8uH / 3A
TDK
C1
MLCC
10㎌ / 35V
-
C2
MLCC
10㎌ / 10V
-
C3
MLCC
8.2㎋
-
C4
MLCC
100㎋
-
C5
MLCC
10㎋
-
R1
Chip resistor
0.2Ω / 1%
-
R2
Chip resistor
2㏀
-
Pin Description
Pin No.
Name
Description
1
BST
High-Side Gate Drive Boost Input. This pin acts as the power supply of high-side gate
driving blocks. Connect a 10nF or greater capacitor between SW and BST pin.
2
VIN
Power supply input. Bypass VIN to GND with a suitably large capacitor to eliminate
noise on the input to the IC.
3
SW
Switching node. The free-wheeling diode is connected between SW and GND.
4
GND
5
FB
6
COMP
7
EN
Chip enable input. Also this pin functions UVLO input.
8
SS
Soft start control node. This pin controls the soft start period.
Ground. Connect the exposed pad on backside to GND.
Feedback voltage input. The regulated FB voltage is 0.2V typically.
Compensation node. COMP is used to compensate the regulation control loop.
BST
1
8
SS
VIN
2
7
EN
SW
3
6
COMP
GND
4
5
FB
ADT6780
Package outline
exposed pad
* connect to GND
* This specifications are subject to be changed without notice
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Thermally enhanced Low VFB Step-Down LED Driver
ADT6780
Functional Block Diagram
VIN
2
EN
7
Current
Sense
Amplifier
Shutdown
+
RS
-
Internal
Regulator
Regulator
Voltage
Reference
Σ
OSC
Current
Limit
Control
+
3 SW
Driver
-
Driver
LOGIC
+
FB 5
1 BST
Comparator
Error
Amplifier
6
COMP
8
SS
4
GND
Figure 2. Functional Block Diagram
Absolute Maximum Ratings (Note1)
Parameter
Symbol
Min.
Typ.
Max.
Unit
Power supply voltage
VIN
-0.3
-
30
V
SW pin voltage
VSW
-0.5
-
VIN + 0.3
V
BST pin voltage
VBST
-0.3
-
VSW + 6
V
-
-0.3
-
+6
V
Max. power dissipation (Ta=25℃) (Note2)
PD
-
-
2.08
W
Thermal resistance (Note3)
ΘJA
-
60
-
℃/W
Storage temperature
TSTG
-65
-
+150
℃
Junction temperature
TJ.MAX
-
-
+150
℃
All Other Pins
Note1. Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device.
Note2. Derate 17mW/℃ above +25℃. This is recommended to operate under this power dissipation specification.
Note3. Measured on JESD51-7, 4-layer PCB
Operating Ratings
Parameter
Symbol
Min.
Typ.
Max.
Unit
VIN
4.5
12.0
28.0
V
Output voltage
VOUT
0.2
-
16
V
Operating temperature
TOPR
-40
-
+85
℃
TJ
-
-
+125
℃
Power supply voltage
Junction temperature
* This specifications are subject to be changed without notice
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Thermally enhanced Low VFB Step-Down LED Driver
ADT6780
Electrical Characteristics (Ta=25℃, VIN=12V, unless otherwise noted)
Parameters
Symbol
Supply current (shutdown)
IOFF
Supply current (quiescent)
IQ
Condition
Min.
Typ.
Max.
Unit
VEN = 0V
-
10
-
㎂
VEN = 3V, VFB = 1.4V
-
0.7
-
㎃
0.184
0.200
0.216
V
4.5V ≤ VIN ≤ 28V,
Feedback voltage
VFB
Error Amplifier Voltage Gain
AEA
-
-
750
-
V/V
Error Amplifier Transconductance
GEA
ΔICOMP = ±10㎂
-
750
-
㎂/V
VCOMP < 2V
High-Side Switch On Resistance (Note4)
RON.H
-
-
80
-
mΩ
Low-Side Switch On Resistance (Note4)
RON.L
-
-
10
-
Ω
High-Side Switch Leakage Current
VEN = 0V , VSW = 0V
-
0.1
10
㎂
Peak Current Limit
Duty=50%
-
3
-
A
Current sense to COMP transconductance
GCS
-
-
11
-
A/V
Oscillator frequency
FSW
-
-
925
-
㎑
VFB = 0V
-
125
-
㎑
76
86
99
%
-
100
-
㎱
2.00
2.35
2.70
V
Fold-back frequency
Maximum Duty cycle
DMAX
Minimum On time
TON
VFB = 0.15V, IO=1A
-
UVLO rising threshold
VEN rising
UVLO threshold hysteresis
-
-
250
-
㎷
EN threshold voltage
-
0.8
1.1
1.4
V
Enable pull-up current
VEN = 0V
-
2.0
-
㎂
Soft-Start Period
C4 = 100㎋
-
3
-
㎳
Thermal shutdown (Note4)
-
-
145
-
℃
Note4. guaranteed by design.
* This specifications are subject to be changed without notice
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4/13
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Thermally enhanced Low VFB Step-Down LED Driver
ADT6780
Typical Operating Characteristics
VIN=12V, Load : 1A / one 4W White LED and Ta=25℃, unless otherwise noted
Efficiency
100%
Steady State Operation
VIN=6V
95%
VSW
10V/div
Efficiency (%)
90%
VIN=12V
85%
VOUT(AC)
20mV/div
VIN=24V
80%
75%
IINDUCTOR
0.5A/div
70%
65%
60%
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1㎲/div
1
LED Current (A)
VFB vs Die Temperature
Switching Frequency vs Die Temperature
1050
0.216
Switching Frequency (kHz)
Feedback Voltage (V)
0.212
0.208
0.204
0.200
0.196
0.192
0.188
1000
950
900
850
800
750
0.184
-40
-20
0
20
40
60
80
100
-40
120
-20
0
20
40
60
80
100
120
Temperature (℃)
Temperature (℃)
Output Short
Peak Current vs Duty
4.0
Inductor Peak Current (A)
VOUT
2V/div
VSW
10V/div
IINDUCTOR
2A/div
3.5
3.0
2.5
2.0
1.5
1.0
4㎲/div
0
20
40
60
80
100
Duty (%)
* This specifications are subject to be changed without notice
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Thermally enhanced Low VFB Step-Down LED Driver
ADT6780
Typical Operating Characteristics
VIN=12V, Load : 1A / one 4W White LED and Ta=25℃, unless otherwise noted
Enable Start-up
Enable Turn-off
(C4=100nF)
(C4=100nF)
VSW
10V/div
VSW
10V/div
VEN
5V/div
VEN
5V/div
VOUT
2V/div
VOUT
2V/div
ILED
0.5A/div
ILED
0.5A/div
1㎳/div
40㎲/div
Enable Start-up
Enable Turn-off
(C4 open)
(C4 open)
VSW
10V/div
VSW
10V/div
VEN
5V/div
VEN
5V/div
VOUT
2V/div
VOUT
2V/div
ILED
0.5A/div
ILED
0.5A/div
40㎲/div
40㎲/div
PWM Dimming Through Enable
LED Current vs PWM Dimming Duty
(C4 open, PWM Frequency=500Hz, Duty=50%)
(C4 open, PWM Frequency=500Hz)
0.9
VEN
5V/div
0.8
LED Current (A)
0.7
VOUT
2V/div
VSW
10V/div
0.6
0.5
0.4
0.3
0.2
ILED
1A/div
0.1
0.0
1㎳/div
0
20
40
60
80
100
PWM Dimming Duty (%)
* This specifications are subject to be changed without notice
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Thermally enhanced Low VFB Step-Down LED Driver
ADT6780
OVERVIEW
The ADT6780 is a current mode step-down converter with integrated high side NMOS power switch. It
operates from a 4.5V to 28V input voltage range and supplies up to 2A of load current. Features include
enable control, under voltage lockout, programmable soft start, current limit ,thermal shutdown and PWM
dimming of LEDs. The ADT6780 uses current mode control to regulate the LED current. The LED current
is measured at FB pin voltage and amplified through the internal error amplifier. The error amplifier output
voltage is used to control the high side NMOS power switch and consequently LED current is regulated.
DETAILED DESCRIPTION
Enable and Soft Start
EN pin of the ADT6780 operates both chip enable and UVLO function. EN pin voltage under 800mV shuts
down all the chip function except for pulling up EN pin. When the EN pin voltage exceeds 1.1V, the internal
regulator will be enabled. A EN pin voltage over 2.7V, the soft start capacitor will begin to charge and
enables all the operations including switching function. When the EN pin is floating, EN voltage is high for
its pull-up function.
The soft start function is adjustable. When the EN pin becomes high, a tens of ㎂ current begins charging the
capacitor which is connected from the SS pin to GND. Smooth control of the output voltage is maintained
during start up. The soft start time is adjusted by changing capacitance of C4 and the typical soft start time is
3msec at 100nF of C4.
Switching Frequency
The ADT6780 switching frequency is fixed and set by an internal oscillator. The practical switching
frequency could range from 777kHz to 1050kHz due to device variation. If the FB voltage is under 80mV,
the switching frequency is changed to 125kHz for reducing abrupt inrush current.
Power Boosting
The ADT6780 uses an internal NMOS power switch to step-down the input voltage to the regulated output
current. Since the NMOS power switch requires a gate voltage greater than the input voltage, a boost
capacitor connected between SW and BST drives the gate. The capacitor is internally charged when SW is
low.
Error Amplifier
The high gain error amplifier extracts the difference between the reference voltage and the feedback voltage.
This extracted difference, called error signal, amplified and fed into the COMP, which is for compensation.
The feedback voltage is regulated to the reference voltage, typical 0.2V for the ADT6780.
Current Sensing
The current sensing output is proportional to the current flowing into the inductor, This output goes to the
comparator to make a proper PWM control signal. This output waveform resembles normally ramp shape.
Current Limit Protection
The output over-current protection (OCP) is implemented using a cycle-by-cycle peak detect control circuit.
The switch current is monitored by measuring the high side NMOS switch current. The measured switch
current is compared against a preset voltage which represents the current limit, between 2.2A and 4A. When
the output current is more than current limit, the high side switch will be turned off and PWM duty is
reduced. The output current is monitored in the same manner at each cycle and finally the power switch
almost turned off not to be damaged under fault conditions.
* This specifications are subject to be changed without notice
Jul. 12. 2012 / Preliminary
7/13
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Thermally enhanced Low VFB Step-Down LED Driver
ADT6780
LED PWM Dimming
The LED brightness can be controlled by applying a pulse-width modulation(PWM) signal to the EN pin.
PWM frequency is limited by turn-on and turn-off time of the LED current. So, Using a PWM dimming
application, soft-start time control capacitor, C4 is not used for higher PWM dimming frequency. PWM
frequency is recommended in range of 100Hz to 1kHz to get a good dimming linearity.
APPLICATION INFORMATION
Figure 1 is the typical ADT6780 application circuit. And Figure 2 is the functional block diagram of the
ADT6780. For the application information, refer to the Figure 1 & 2 unless otherwise noted.
LED Current Resistor Selection
The LED current is set with a current sense resistor R1 between FB and GND. It is recommended to use 1%
tolerance or better resistor. LED current is calculated by the below equation.
I LED =
0.2V
R1
For 1A LED current, choose R1 = 0.2Ω
Inductor
The inductor required to supply constant current to the output load when it is driven by a switching voltage.
For given input and output voltage, inductance and switching frequency together decide the inductor ripple
current, that is:
ΔI L =
The peak inductor current is:
VOUT ⎛ VOUT ⎞
⎟
× ⎜1 −
FSW × L ⎜⎝
VIN ⎟⎠
I L.peak = I OUT +
ΔI L
2
Higher inductance gives low inductor ripple current but requires larger size inductor to avoid saturation.
Low ripple current reduces inductor core losses. Also it reduces RMS current through inductor and switches,
which results in less conduction loss. Usually, peak to peak ripple current on inductor is designed to be 20%
to 30% of the output current limit. Make sure it is capable to handle the peak current without saturation.
Surface mount inductors in different shape and styles are available from TDK, TOKO and Murata. Shielded
inductors are small and radiate less EMI noise. But they cost more than unshielded inductors. The choice
depends on EMI requirement, price and size.
Output Freewheeling Diode
When the high side switch is off, freewheeling diode supplies the current to the inductor. The forward
voltage and reverse recovery times of the freewheeling diode are the key loss factors, so schottky diode is
mostly used for the freewheeling diode. Choose a diode whose maximum reverse voltage rating is greater
than the maximum input voltage, and whose current rating is greater than the maximum load current.
* This specifications are subject to be changed without notice
Jul. 12. 2012 / Preliminary
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Thermally enhanced Low VFB Step-Down LED Driver
ADT6780
APPLICATION INFORMATION (continued)
Input Capacitor
The input capacitor is used to filter out discontinuous, pulsed input current and to maintain input voltage
stable. Therefore input capacitor should be able to supply the AC current to the step-down converter. Its
input ripple voltage can be estimated by:
ΔVIN =
I OUT
V
× OUT
FSW × C IN VIN
⎛ V ⎞
× ⎜⎜1 − OUT ⎟⎟
VIN ⎠
⎝
where, CIN is input capacitor value.
The voltage rating of input capacitor must be greater than the maximum input voltage plus ripple voltage.
Since the input capacitor absorbs the input switching current, it requires an proper ripple current rating. The
RMS current in the input capacitor can be approximated by:
I CIN_RMS = I OUT ×
VOUT
VIN
⎛ V ⎞
× ⎜⎜1 − OUT ⎟⎟
VIN ⎠
⎝
The worst-case condition occurs at VIN=2×VOUT (50% duty condition), and its worst RMS current is
approximately half of the IOUT. For reliable operation and best performance, the input capacitors must have
current rating higher than ICIN_RMS at worst operating conditions. Ceramic capacitors are preferred for input
capacitors because of their low ESR and high current rating. When selecting ceramic capacitors, X5R or
X7R type dielectric ceramic capacitors should be used for their better temperature and voltage
characteristics. For most applications, a 10㎌ ceramic capacitor is sufficient.
Output Capacitor
The output capacitor is required to maintain the DC output voltage. In a step-down converter circuit, output
ripple voltage is determined by the inductor value, switching frequency, output capacitor value and ESR.
That is:
where,
CO is output capacitor value,
⎛
⎞
1
⎟
ΔVOUT = ΔI L × ⎜⎜ ESR +
8 × FSW × C O ⎟⎠
⎝
ESR is the equivalent series resistance of the output capacitor.
Low ESR capacitors are preferred to keep the output voltage ripple low. When low ESR ceramic capacitor is
used as output capacitor, its ESR value can be waived. So, the impedance at the switching frequency is
dominated by the capacitance. Therefore the output voltage ripple is:
⎛
⎞
1
⎟⎟
ΔVOUT = ΔI L × ⎜⎜
⎝ 8 × FSW × C O ⎠
On the other hand, in the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at the
switching frequency. In this case, the output voltage ripple is:
ΔVOUT = ΔI L × (ESR )
In a step-down converter, output capacitor current is continuous. Usually, the ripple current rating of the
output capacitor is not concerned because of its low ripple current. For most applications, a 10㎌ ceramic
capacitor is sufficient.
* This specifications are subject to be changed without notice
Jul. 12. 2012 / Preliminary
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Thermally enhanced Low VFB Step-Down LED Driver
ADT6780
APPLICATION INFORMATION (continued)
Loop Compensation
The ADT6780 uses a fixed frequency, peak current mode control scheme to provide easy compensation and
fast transient response. Peak current mode control eliminate the double pole effect of the output LC filter.
Therefore, the step-down converter can be simplified to be a one-pole system in frequency domain.
The goal of compensation design is to shape the converter transfer function to get the desired gain and
phase. System stability is provided with the addition of a simple series capacitor-resistor from COMP to
GND. This pole-zero combination serves to adjust the desired response of the closed-loop system.
The DC gain of the voltage feedback loop is given by:
A VDC = R 1 × A EA × G CS
Where AEA is the error amplifier voltage gain. GCS is the current sense transconductance and R1 is the current sense
resistor value.
The system has two dominant poles. One is made by the combination of both the output resistor of the error
amplifier and the compensation capacitor (C3). And the other is due to the output capacitor and the LED’s
AC resistor(RLED=△VOUT/△ILED) . These poles are expressed as:
f P1 =
G EA
2π × C3 × A EA
f P2 =
1
2π × C O × R LED
where, GEA is the error amplifier transconductance.
For a stable one-pole converter system, one of two dominant poles needs to be eliminated by one zero. One
zero made by the series capacitor-resistor (R2-C3) cancels fP2 out. This zero is:
f Z1 =
1
2π × C3 × R2
If the output capacitor has a large capacitance and/or a high ESR value, unwanted zero is generated to the
location of:
f Z2 =
1
2π × C O × ESR
In this case, third pole is needed to compensate fZ2. This pole, fP3, is made by the R2 and the selectively
added optional capacitor (C6) between COMP to GND. fP3 is expressed to:
f P3 =
1
2π × C6 × R2
The system crossover frequency (Fc), where the feedback loop has the unity gain, is important. The system
crossover frequency is called the converter bandwidth. Generally higher Fc means faster transient response
and load regulation. However, higher Fc could cause system unstable. A standard rule of thumb sets the
crossover frequency to be equal or less than 1/10 of switching frequency.
* This specifications are subject to be changed without notice
Jul. 12. 2012 / Preliminary
10/13
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Thermally enhanced Low VFB Step-Down LED Driver
ADT6780
APPLICATION INFORMATION (continued)
Table1 and Table2 list the typical values of compensation components and external components for general
applications.
Table 1. Components values for IRLED application (Refer to the Typical Application Circuit, for other components.)
VIN (V)
# of series IRLEDs
VOUTMAX (V)
R2 (㏀)
C3 (㎋)
C6 (㎊)
L1 (uH)
C2 (㎌)
12
1~5
10
2
8.2
None
6.8 ~ 10
10
24
1~8
16
2
8.2
None
15 ~ 22
10
Table 2. Components values for WLED application (Refer to the Typical Application Circuit, for other components.)
VIN (V)
# of series WLEDs
VOUTMAX (V)
R2 (㏀)
C3 (㎋)
C6 (㎊)
L1 (uH)
C2 (㎌)
12
1~2
8
2
8.2
None
6.8 ~ 10
10
24
1~4
16
2
8.2
None
15 ~ 22
10
The output voltage is calculated by the below equation.
VOUT = n × VF + VFB
Where, n is the number of LEDs connected in series, VF is the forward voltage of the LED and VFB is the
voltage drop across the current sense resistor.
A general procedure to choose the compensation components for conditions is following:
1. Select the desired crossover frequency. Set the crossover frequency to be equal or less than 1/10 of
switching frequency.
2. Select R2 (compensation resistor) to operate the desired crossover frequency in a given condition. R2
value is calculated by the following equation:
R2 =
2π × FC × C2 × R LED
G EA × G CS × R1
3. Select C3 (compensation capacitor) to achieve the desired loop phase margin. C3 determines the desired
first system zero, fZ1. Typically, set fZ1 below 1/4 of the Fc to provides sufficient phase margin. C3 value is
calculated by:
C3 ≥
4
2π × FC × R2
4. If the ESR output zero (fZ2) is located at less than one-half the switching frequency, use the (optional)
secondary compensation capacitor (C6) to cancel it. As fP3=fZ2, then:
C6 =
C2 × ESR
R2
* This specifications are subject to be changed without notice
Jul. 12. 2012 / Preliminary
11/13
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Thermally enhanced Low VFB Step-Down LED Driver
ADT6780
APPLICATION INFORMATION (continued)
Thermal Management
The ADT6780 contains an internal thermal sensor that limits the total power dissipation in the device and
protects it in the event of an extended thermal fault condition. When the die temperature exceeds +145°C
typically, the thermal sensor shuts down the device, turning off the DC-DC converter to allow the die to
cool. After the die temperature falls by 10°C typically, the device automatically restarts, using the soft-start
sequence.
The ADT6780 is available in a thermally enhanced SOP package and can dissipate up to 1.25W at Ta=50°C
(TJ=125°C). The exposed pad should be connected to GND externally, preferably soldered to a large ground
plane to maximize thermal performance. Maximum available power dissipation should be de-rated by
17mW/℃ above Ta=25℃ not to damage the device.
PCB Layout Consideration
PCB layout is very important to achieve clean and stable operation. It is highly recommended to follow
below guidelines for good PCB layout.
1. Input capacitor (C1) should be placed as near as possible to the IC and connected with direct traces.
2. Keep the high current paths as short and wide as possible.
3. Keep the switching current path short and minimize the loop area, formed by SW, the output capacitors
and the input capacitors.
4. Route high-speed switching nodes (such as SW and BST) away from sensitive analog areas (such as FB
and COMP).
5. Ensure all feedback connections are short and direct. Place the current sense resistor and compensation
components as close as possible to the IC.
6. Exposed pad of device must be connected to GND with solder. For single layer, do not solder exposed
pad of the IC.
* This specifications are subject to be changed without notice
Jul. 12. 2012 / Preliminary
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Thermally enhanced Low VFB Step-Down LED Driver
ADT6780
Package ; SOP8-PP(E-pad), 4.9mm x 3.94mm body (units : mm)
Symbol
Dimensions In Millimeters
Dimensions In Inches
Min
Max
Min
Max
A
1.350
1.750
0.053
0.069
A1
0.050
0.150
0.004
0.010
A2
1.350
1.550
0.053
0.061
b
0.330
0.510
0.013
0.020
c
0.170
0.250
0.006
0.010
D
4.700
5.100
0.185
0.200
D1
3.202
3.402
0.126
0.134
E
3.800
4.000
0.150
0.157
E1
5.800
6.200
0.228
0.244
E2
2.313
2.513
0.091
e
1.270 (BSC)
L
0.400
θ
0
o
0.099
0.050 (BSC)
1.270
8
o
0.016
0
o
0.050
8
o
* This specifications are subject to be changed without notice
Jul. 12. 2012 / Preliminary
13/13
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