Linear Technology LTC1877, LTC1878 Demo Manual

Page 1
DEMO MANUAL DC290
NO-DESIGN SWITCHER
LTC1877/LTC1878
Monolithic Synchronous
U
Step-Down Regulator
DESCRIPTIO
Demonstration circuit DC290 is a constant-frequency step-down converter using an LTC®1877 or an LTC1878 monolithic synchronous regulator. The LTC1878 has an input voltage range of 2.65V < VIN < 6V and the LTC1877 has an input voltage range of 2.65V < VIN < 10V. The LTC1878 is optimized for low voltage operation and is ideally suited for single Li-Ion cell or 3-NiCd/NiMH cell applications. The LTC1877, with its higher voltage capa­bility, is ideally suited for two Li-Ion cells or 4- to 6-NiCd/ NiMH cell applications.
The exclusive use of low profile surface mount compo­nents on this demo board results in a highly efficient application in a small volume. The output voltage can be selected from 1.5V, 2.5V, 3.3V or a user programmable
UW
voltage, by means of a jumper. The frequency is internally set at 550kHz or can be synchronized with an external clock. The internal switches allow up to 600mA of output current in an MSO8 package, providing a space-efficient solution for battery-powered applications. The DC supply current is typically only 10µA at no load and less than 1µA in shutdown. In switching-noise sensitive applications, Burst ModeTM operation can be inhibited by grounding the SYNC/MODE pin with a jumper or synchronizing it with an external clock. Gerber files for this circuit board are
available. Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
PERFORmANCE SU ARY
SYMBOL PARAMETER CONDITIONS JUMPER POSITION VALUE
V
IN
V
OUT
Input Voltage Range LTC1877 All (Note 1) 2.65V to 10V
LTC1878 All 2.65V to 6V
Output Voltage V
= 5V, SYNC/MODE = 0V, RUN = 5V, I
IN
V
= 5V, SYNC/MODE = 0V, RUN = 5V, I
IN
= 5V, SYNC/MODE = 0V, RUN = 5V, I
V
IN
V
= 5V, SYNC/MODE = 0V, RUN = 5V, I
IN
= 0mA JP1 = U, JP2 = L, JP3 = “1.5V” 1.51V ± 0.05V
OUT
= 0mA JP1 = U, JP2 = L, JP3 = “2.5V” 2.52V ± 0.08V
OUT
= 0mA JP1 = U, JP2 = L, JP3 = “3.3V” 3.33V ± 0.10V
OUT
= 0mA JP1 = U, JP2 = L, JP3 = “OPEN” (Note 2)
OUT
W
TYPICAL PERFORMANCE CHARACTERISTICS AND BOARD PHOTO
EFFICIENCY CURVE
100
VIN = 3.6V
95 90 85 80 75
VIN = 5V
70
EFFICIENCY (%)
65 60 55 50
0.1
1.0 100 OUTPUT CURRENT (mA)
VIN = 10V
VIN = 7.2V
V
OUT
L = 10µH Burst Mode OPERATION
10
= 3.3V
U
U
DEMO BOARD
1000
290A • TA01
1
Page 2
DEMO MANUAL DC290
NO-DESIGN SWITCHER
UWWW
PERFOR A CE SU ARY
SYMBOL PARAMETER CONDITIONS JUMPER POSITION VALUE
I
Q
I
OUT
f
OSC
V
RIPPLE
V
OUT
V
SYNC
V
RUN
NOTE 1: JP1 and JP2 connect RUN (E1) and SYNC/MODE (E7) to either GND (E3) or V RUN or SYNC/MODE is connected to V the upper position (U), then RUN or SYNC/MODE is connected to GND. If external sources drive RUN or SYNC/MODE, then JP1, JP2 or both must be removed.
Burst Mode Operation Enabled V
= 5V, SYNC/MODE = RUN = 5V, I
IN
= 0mA JP1 = JP2 = L, JP3 = “1.5V” 10µA (Note 3)
OUT
Supply Current Pulse Skipping Mode Supply Current V
Shutdown Current V Maximum Output Current V
= 5V, SYNC/MODE = 0V, RUN = 5V, I
IN
= 5V, RUN = 0V JP2 = U <1µA
IN
IN
V
IN
= 5V, V = 4V, V
= 2.5V (LTC1877) JP2 = L, JP3 = “1.5V” 600mA
OUT
= 2.5V (LTC1878) JP2 = L, JP3 = “1.5V” 600mA
OUT
= 0mA JP1 = U, JP2 = L 230µA
OUT
Operating Frequency Unsynchronized JP2 = L 550kHz
Synchronized JP1 = OPEN, JP2 = L 400kHz to 700kHz Typical Output Ripple I Typical Load Regulation 0mA < I
= 600mA, VIN = 5V, V
OUT
< 0.6A, V
OUT
= 2.5V JP2 = L, JP3 = “2.5V” 20mV
OUT
= 5V, SYNC/MODE = 0V JP1 = U, JP2 = L 0.3%
IN
P-P
Synchronize Threshold Voltage All 1.2V Minimum Shutdown Threshold Voltage All 0.4V
(E2). When JP1 or JP2 is in the lower position (L), then
IN
. Alternatively, if JP1 or JP2 is in
IN
NOTE 2: Programmable via optional R7. V NOTE 3: With V
= 5V, the external feedback resistor contributes 0.24µA
IN
with JP3 = “1.5V” selected, 0.95µA with JP3 = “2.5V” selected and 1.85µA
= 0.8V(1+ 887kΩ/R7)
OUT
with JP3 = “3.3V” selected.
WUW
PACKAGE A D SCHE ATIC DIAGRA SM
JP2 JP1
RUN
GND
E1
E3
OPTIONAL
C1
R1
0.01µF
10k
1
RUN
2
I
SYNC/MODE
R2
C2
C
ITH
220pF
TH
LTC187X
3
V
FB
4
GND
R4 1M
1.5V 2.5V 3.3V OPEN
PLL LPF
V
IN
SW
R5 412kR6280k
8
7
6
5
L1
10µH
C 20pF
R7* OPTIONAL
JP3
CIN*** CER
R3
FW
887k
E2
10V (LTC1877)
V
IN
6V (LTC1878)
+
C
OUT
47µF
6.3V
C 1µF CER
OUT1
E7 SYNC/MODE
E6 V
OUT
**
E5 GND
E4 GND
TOP VIEW
RUN
1
I
2
TH
3
V
FB
4
GND
MS8 PACKAGE
8-LEAD PLASTIC MSOP
LTC187XEMS8
8
PPL LPF
7
SYNC/MODE
6
V
IN
SW
5
*SPACE PROVIDED FOR AN OPTIONAL RESISTOR TO PROGRAM A CUSTOM OUTPUT VOLTAGE. THE OUTPUT VOLTAGE MUST NOT EXCEED 3.3V.
IS AN OPTIONAL CAPACITOR TO FILTER OUT VERY HIGH FREQUENCY SWITCHING NOISE.
**C
OUT1
= 10µF FOR LTC1877 AND CIN = 22µF FOR LTC1878
***C
IN
Figure 1. LTC1877/LTC1878 Constant Frequency, High Efficiency Converter
2
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DEMO MANUAL DC290
NO-DESIGN SWITCHER
PARTS LIST
REFERENCE DESIGNATOR QUANTITY PART NUMBER DESCRIPTION VENDOR TELEPHONE
C
FW
C
ITH
C1 1 06035C103MAT 0.01µF 50V Chip X7R Capacitor AVX (843) 946-0362 CIN (LTC1877) 1 LMK325BJ106MN 10µF 10V Chip X7R Capacitor TAIYO-YUDEN (408) 573-4150 CIN (LTC1878) 1 JMK325BJ226MM 22µF 6.3V Chip X5R Capacitor TAIYO-YUDEN (408) 573-4150 C
OUT
C
OUT1
E1 to E7 7 2501-2 Turret, Testpoint Mill-Max (516) 922-6000 JP1, JP2 2 2802S-03-G1 0.079” Single Row Header COMM CON (626) 301-4200 JP3 1 2202S-08-G1 0.079” Double Row Header COMM CON (626) 301-4200 JP1 to JP3 3 CCIJ2MM-138G 0.079” Center Shunt COMM CON (626) 301-4200 L1 1 A920CY-100M 10µH 20% Inductor TOKO (847) 699-3430 R1 1 CR16-103JM 10K 5% 1/16W Chip Resistor TAD (714) 255-9123 R3 1 CR16-8873FM 887K 1% 1/16W Chip Resistor TAD (714) 255-9123 R4 1 CR16-1004FM 1M 1% 1/16W Chip Resistor TAD (714) 255-9123 R5 1 CR16-4123FM 412K 1% 1/16W Chip Resistor TAD (800) 508-1521 R6 1 CR16-2803FM 280K 1% 1/16W Chip Resistor TAD (800) 508-1521 U1 1 LTC1877/8EMS8 Monolithic Synchronous Step-Down Regulator LTC (408) 432-1900 C2 Optional R2, R7 Optional
1 06035A200JAT 20pF 50V Chip NPO Capacitor AVX (843) 946-0362 1 06035A221JAT 220pF 50V Chip NPO Capacitor AVX (843) 946-0362
1 6TPA47M 47µF 6V POSCAP Capacitor SANYO (619) 661-6835 1 LMK107F105ZA 1µF 10V Chip Y5V Capacitor TAIYO-YUDEN (408) 573-4150
QUICK START GUIDE
This demonstration board is easily set up to evaluate the performance of the LTC1877 or LTC1878 IC. Please follow the procedure outlined below for proper opera­tion.
• Refer to Figure 5 for proper connection of monitoring equipment to ensure correct measurement.
• Connect the input power supply to the VIN and GND terminals on the left-hand side of the board. Do not increase VIN over its rated maximum supply voltage or the part will be damaged. For the LTC1877 the maxi­mum VIN is 10V and for the LTC1878 the maximum VIN is 6V.
• Connect the load between the V on the right side of the board.
• Select the desired operating mode using JP1 and JP2, as shown in Table 1. JP1 connects SYNC/MODE to GND in the upper position, and connects it to VIN in the lower position. JP2 connects RUN to GND in the upper
and GND terminals
OUT
position, and connects it to VIN in the lower position. If a signal is applied at RUN (E1) or SYNC/MODE (E7), then jumper JP1 or JP2, respectively, must be re­moved.
• Set the desired output voltage with jumper JP3 as shown in Figure 2.
Table 1. Operating Mode Selection With Jumpers JP1 and JP2
JP2 JP1 OPERATING MODE
UPPER X SHUTDOWN LOWER UPPER PULSE SKIPPING LOWER LOWER BURST MODE LOWER OPEN EXTERNAL CLOCK AT SYNC/MODE
OPEN X EXTERNAL SIGNAL AT RUN
JP3
OPEN
2.5V
3.3V
1.5V
Figure 2. Output Voltage Selection (JP3)
(3.3V Position Shown)
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DEMO MANUAL DC290
NO-DESIGN SWITCHER
U
OPERATIO
INTRODUCTION
The circuit in Figure 1 highlights the capabilities of the LTC1877 and the LTC1878. The LTC1877 and the LTC1878 are high efficiency monolithic synchronous step-down regulators using a fixed-frequency architecture.
This demo board is set up for a variety of output voltages. Output voltages including 1.5V, 2.5V and 3.3V or user programmable voltages can be obtained by selecting the appropriate jumper position. For other output voltages, select the “OPEN” position and add the appropriate resis­tor value in the space provided. The output voltage must never exceed 3.3V because the output capacitor may be damaged. The input supply can range from 2.65V to 10V for the LTC1877 and 2.65V to 6V for the LTC1878.
The operating frequency of this demo circuit is 550kHz. For other frequencies, JP1 must be removed and SYNC/ MODE (E7) synchronized with an external clock. Burst Mode operation is automatically disabled when SYNC/ MODE is externally driven. Grounding SYNC/MODE also disables Burst Mode operation, potentially reducing noise and RF interference.
This demonstration board is intended for the evaluation of the LTC1877 and the LTC1878 switching regulator ICs and was not designed for any other purpose.
Main Control Loop (Refer to Functional Diagram)
The LTC1877 and the LTC1878 use a constant frequency, current mode step-down architecture. Their main and synchronous switches, consisting of a top (main) P-channel and a bottom (synchronous) N-channel power MOSFET, are internal. During normal operation, the inter­nal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current comparator, I inductor current at which I controlled by the voltage on the ITH pin, which is the output of error amplifier EA. The VFB pin allows EA to receive an output feedback voltage from an external resistive divider. When the load current increases, it causes a slight de­crease in the feedback voltage relative to the 0.8V refer­ence, which, in turn, causes the I
, resets the RS latch. The peak
COMP
resets the RS latch is
COMP
voltage to increase
TH
until the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current reversal comparator, I
, or until the beginning of the next clock cycle.
RCMP
Comparator OVDET guards against transient overshoots as well as other more serious conditions that may cause an overvoltage condition on the output (> 6.25%). When this condition is sensed, both MOSFETs are turned off until the fault is removed.
Burst Mode Operation
The LTC1877 and the LTC1878 are capable of Burst Mode operation, in which the internal power MOSFETs operate intermittently based on load demand. To enable Burst Mode operation, simply position jumper JP1 in the lower position to connect SYNC/MODE to V Mode operation and enable PWM pulse skipping mode, position JP1 in the upper position to connect SYNC/MODE to GND (see Figure 5). In this mode, the efficiency is lower at light loads, but becomes comparable to Burst Mode operation when the output load exceeds 50mA. The advantage of pulse skipping mode is lower output ripple and less interference to audio circuitry.
When the converter is in Burst Mode operation, the peak current of the inductor is set to approximately 250mA, even though the voltage at the I value. The voltage at the I average current is greater than the load requirement. As the I BURST comparator trips, causing the internal sleep line to go high and turn off both power MOSFETs. The I then disconnected from the output of the EA amplifier and parked a diode above ground.
In sleep mode, both power MOSFETs are held off and the internal circuitry is partially turned off, reducing the quies­cent current to 10µA. The load current is now being supplied from the output capacitor. When the output voltage drops, the I EA amplifier and the top MOSFET is again turned on and this process repeats.
voltage drops below approximately 0.45V, the
TH
TH
pin drops when the inductor’s
TH
pin reconnects to the output of the
. To disable Burst
IN
pin indicates a lower
TH
TH
pin is
4
Page 5
OPERATIO
BURST
DEFEAT
PLL LPF
8
U
Y = “0” ONLY WHEN X IS A CONSTANT “1”
Y
X
DEMO MANUAL DC290
NO-DESIGN SWITCHER
V
IN
SYNC/MODE
7
0.6V
3
V
FB
RUN
1
– +
V
0.8V REF
IN
SHUTDOWN
VCO
FREQ
SHIFT
0.85V
0.8V
OVDET
+
SLOPE
COMP
OSC
+
EA
+
0.45V
SLEEP
V
IN
V
IN
I
2
TH
EN
+
BURST
Q
S
R
Q
RS LATCH
SLEEP
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
0.8V
I
COMP
ANTI
SHOOT-
THRU
I
RCMP
V
6
IN
+
+ –
6
5
4
dc290A BD
SW
GND
Figure 3. Functional Block Diagram
Short-Circuit Protection
When the output is shorted to ground, the frequency of the oscillator is reduced to about 80kHz, one-seventh of the nominal frequency. This frequency foldback ensures that the inductor current has more time to decay, thereby preventing runaway. The oscillator’s frequency will progressively increase to 550kHz (or the synchronized frequency) when V
rises above 0.3V.
FB
Frequency Synchronization
A phase-locked loop (PLL) is available on the LTC1877 and the LTC1878 to allow the oscillator to be synchronized to an external source connected to the SYNC/MODE pin. The output of the phase detector at the PLL LPF pin operates over a 0V to 2.4V range, corresponding to 400kHz to 700kHz. When locked, the PLL aligns the
turn-on of the MOSFETs to the rising edge of the synchro­nizing signal.
When the LTC1877 or the LTC1878 is clocked by an external source, Burst Mode operation is disabled; the LTC1877 or the LTC1878 then operates in PWM pulse skipping mode. In this mode, when the output load is very low, the current comparator, I
, may remain tripped
COMP
for several cycles and force the main switch to stay off for the same number of cycles. Increasing the output load slightly allows constant frequency PWM operation to resume.
Frequency synchronization is inhibited when the feedback voltage, VFB, is below 0.6V. This prevents the external clock from interfering with the frequency foldback for short-circuit protection.
5
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DEMO MANUAL DC290
NO-DESIGN SWITCHER
U
OPERATIO
Dropout Operation
When the input supply voltage decreases toward the output voltage, the duty cycle increases toward the maxi­mum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor.
Low Supply Operation
The LTC1877 and the LTC1878 can function on an input supply voltage as low as 2.65V. The maximum allowable output current is reduced at this low voltage because the R user should calculate the power dissipation when the LTC1877 or the LTC1878 is used at 100% duty cycle with low VIN. See the LTC1877 or the LTC1878 data sheet for additional information.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre­quency architectures by preventing subharmonic oscilla­tions at high duty cycles. It is accomplished by internally adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. As a result, the maximum inductor peak current is reduced for duty cycles >40%. See the inductor peak current as a function of duty cycle graph in Figure 4.
of the P-channel switch increases. Therefore, the
DS(ON)
1100
1000
900
800
700
MAXIMUM INDUCTOR PEAK CURRENT (mA)
600
20
0
LTC1877 @ VIN = 5V LTC1878 @ V
60
40
DUTY CYCLE (%)
IN
80
= 3.3V
100
dc290A F04
Figure 4. Maximum Inductor Peak Current vs Duty Cycle
How To Measure Voltage Regulation
When trying to measure voltage regulation, remember that all measurements must be taken at the point of regulation. This point is where the LTC1877’s and LTC1878’s control loop looks for the information to keep the output voltage constant. In this demonstration board, this point occurs between V
(E6) and GND (E4).
OUT
Measurements should be taken at these points and not at the end of test leads at the load. Refer to Figure 5 for the proper monitoring equipment configuration.
This applies to line regulation (input-to-output voltage regulation) as well as load regulation tests. In doing the line regulation tests, always look at the input voltage across the input terminals, VIN (E2) and GND (E3) .
UPPER POSITION (U)
I
IN
LOWER POSITION (L)
+
A
+
V
IN
6
LTC1877/LTC1878
High Efficiency Monolithic Synchronous
E1
E2
IN
E3
Step-Down Regulator
JP1JP2
JP3
JP3JP3
2.5V
3.3V
0PEN
1.5V
DEMO CIRCUIT DC290A
(408) 432-1900
RUN
V
+
V
GND
Figure 5. Proper Measurement Setup
SYNC/MODE
E7
V
OUT
E6
GND
E5
GND
E4
I
OUT
+
A
+
V
LOAD
V
OUT
Page 7
DEMO MANUAL DC290
NO-DESIGN SWITCHER
For the purposes of these tests, the demonstration circuit should be powered from a regulated DC bench supply, so that variations on the DC input do not add errors to the regulation measurements.
Another source of error may be the use of small spring­clip leads when testing this circuit. Small spring-clip leads are very convenient for small-signal bench testing and voltage measurements, but should not be used with this circuit. Soldered wire connections are required to prop­erly ascertain the performance of the PC board.
Checking Transient Response
Switching regulators take several cycles to respond to a step in DC (resistive) load current. When a load step occurs, V where ESR is the effective series resistance of C I
LOAD
shifts by an amount equal to (∆I
OUT
also begins to charge or discharge C
OUT
LOAD
until the
)(ESR),
OUT
.
regulator loop adapts to the current change and returns V
to its steady-state value. During this recovery time,
OUT
V
can be monitored for overshoot or ringing, which
OUT
would indicate a stability problem. The external compo­nents shown in Figure 1 will prove adequate for most applications.
Long supply leads connected to V ringing at VIN and V
resembling loop instability. This is
OUT
and GND may induce
IN
actually caused by the inductance of the long wires resonating with the input ceramic capacitor. This phe­nomenon is particularly pronounced when a ceramic
output capacitor is used. When using short leads to connect VIN and GND is impractical, a 100µF electrolytic bulk capacitor can be soldered onto the board between V
IN
and GND. This should eliminate all ringing associated with long V
and GND leads. Space is provided on the PC board
IN
for this purpose, as shown in Figure 6.
LTC1877/LTC1878
High Efficiency Monolithic Synchronous
E1
E2
IN
E3
Step-Down Regulator
JP1JP2
2.5V
3.3V
OPEN
1.5V
DEMO CIRCUIT DC290A
RUN
V
SOLDER BULK
CAPACITOR HERE
Figure 6. Space Provided for a Bulk Capacitor
GND
SYNC/MODE
JP3
JP3JP3
(408) 432-1900
E7
V
OUT
E6
GND
E5
GND
E4
Component Manufacturers
Table 2 is a partial list of manufacturers of components that can be used in LTC1877 and LTC1878 applications. Using components other than the ones supplied on the demonstration board will require careful analysis to verify that all component specifications are not exceeded. Finally, recharacterizing the circuit for efficiency is necessary.
Table 2. List of Alternative Component Manufacturers MANUFACTURER DEVICE PHONE FAX
Central Semiconductor Diodes (516) 435-1110 (516) 435-1824 Coilcraft Inductors (847) 639-6400 (847) 639-1469 Coiltronics Inductors (561) 241-7876 (561) 241-9339 Dale Inductors (605) 665-9301 (605) 665-0817 International Rectifier Diodes (310) 322-3331 (310) 322-3332 ON Semiconductor Diodes (602) 244-6600 (602) 244-4015 Zetex Diodes (631) 543-7100 (631) 864-7630 Murata-Erie Capacitors (770) 436-1300 (770) 436-3030 Sprague Capacitors (207) 324-4140 (603) 244-1430 Sumida Inductors (847) 956-0667 (847) 956-0702
TDK Inductors (847) 803-6100 [81] 03-3278-5358
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen­tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
[81] 03-3607-5111 [81] 03-3607-5114
7
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DEMO MANUAL DC290
NO-DESIGN SWITCHER
UW
PCB LAYOUT A D FIL
Silkscreen Top Component Side Solder Mask Top
Paste Mask Top Solder Side Solder Mask Bottom
U
PC FAB DRAWI G
2.000
C
B
D
A
B
NOTES: UNLESS OTHERWISE SPECIFIED
1. MATERIAL; FR4 OR EQUIVALENT EPOXY, 2 OZ COPPER CLAD
THICKNESS 0.031 ± 0.006 TOTAL OF 2 LAYERS
2. FINISH: ALL PLATED HOLES 0.001 MIN/0.0015 MAX COPPER PLATE ELECTRODEPOSITED TIN-LEAD COMPOSITION BEFORE REFLOW, SOLDER MASK OVER BARE COPPER (SMOBC)
3. SOLDER MASK; BOTH SIDES USING GREEN PC-401 OR EQUIVALENT
4. SILKSCREEN; USING WHITE NONCONDUCTIVE EPOXY INK
5. ALL DIMENSIONS ARE IN INCHES
6. SCORING:
D
2.000
C
0.017
SYMBOL
A B C
D
DIAMETER
0.020
0.040
0.072
0.095
NUMBER
OF HOLES
10 14
2 7
PLATED
YES YES
NO
YES
Linear Technology Corporation
8
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
www.linear-tech.com
dc290 LT/TP 0500 500 • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2000
Page 9
Mouser Electronics
Authorized Distributor
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