ANALOG DEVICES LT8686S Step Down Regulator User Manual

LT8686S Step Down Regulator
User Manual

LT8686S Step Down Regulator

DEMO MANUAL DC3126A
LT8686S
42V Quad, Hangable, Synchronous, Monolithic
Step-Down Regulator

DESCRIPTION

Demonstration circuit 3126A is a quad power supply featuring the LT ® 8686S. The LT8686S is a 42V quad channel current mode monolithic synchronous step-down regulator. The LT8686S combines two 42V capable 2A buck regulators with two 8V capable 2A buck regulators. The demo board is designed for 5V, 3.3V, 1.8V, and 1.2V outputs from a nominal 12V input with switching frequency set at 2MHz. The 5V and 3.3V converters are powered from a wide range of 6V to 42V. The 1.8V and 1.2V converters are powered from the 5V output by default or from 3V to 8V supply alternatively. The current capability is 2A for all 4 outputs when running individually.
The DC3126A provides two 42V regulators that can be combined to deliver up to 4A of output current using a singular inductor. Similarly, the two 8V regulators can be combined to deliver up to 4A of output current using a singular inductor. The allowed channel combinations are given in the data sheet.
The independent track/soft-start and power good for each output simplify the complex design of quad-output power converters. Each output can be independently disabled into low quiescent current shutdown mode with its own TRK/SS pin.
A user-selectable SYNC/MODE pin on the demo board provides two primary modes of operation: pulse-skipping mode and low ripple Burst Mode ® operation, plus the option of selecting frequency spread-spectrum for each to improve the EMI/EMC performance. Burst Mode delivers higher efficiency at light load than pulse-skipping mode. In pulse-skipping mode, full switching frequency is maintained to lower load currents than Burst Mode. The SYNC/MODE pin can also be used to synchronize the switching frequency to an external clock. The switching frequency for all regulators can be programmed either via an oscillator resistor or an external clock over a 350kHz to 3MHz range. At all frequencies, a 180° phase shift is maintained between channel 1 and channel 2, channel 3 and channel 4, reducing the input peak current and voltage ripple.
The demo board has an EMI filter installed on the bottom layer. The conducted and radiated EMI performance of the board is shown on Figure 4. The red line in Figure 4 is CISPR25 Class 5 peak limit. The figure shows that the circuit passes the test with a wide margin. To achieve EMI/ EMC performance as shown in Figure 4, the input EMI filter is required, and the input voltage should be applied at VEMI turret.
The LT8686S data sheet gives a complete description of the part operation and application information. The data sheet must be read in conjunction with this quick start guide for DC3126A. The LT8686S is assembled in a 5mm × 5mm LQFN package with exposed pads for low thermal resistance. Proper board layout is essential for both low EMI operation and best thermal performance.
Design files for this circuit board are available.
All registered trademarks and trademarks are the property of their respective owners.

PERFORMANCE SUMMARY

SYMBOL| PARAMETER| CONDITIONS| MIN| TYP| MAX| UNITS
---|---|---|---|---|---|---
VIN_HV| HV Regulators Input Voltage (VIN1/VIN2) Range| | 6*| 12| 42| V
VIN_LV| LV Regulators Input Voltage (VIN3/VIN4) Range| | 3| | 8**| V
VOUT1| Output1 Voltage| VIN_HV = 12V| 4.8| 5| 5.2| V
VOUT2| Output2 Voltage| VIN_HV = 12V| 3.17| 3.3| 3.43| V
VOUT3| Output3 Voltage| VIN_LV = 5V| 1.73| 1.8| 1.87| V
VOUT4| Output4 Voltage| VIN_LV = 5V| 1.15| 1.2| 1.25| V
IOUT1| Maximum Output1 Current| No Load on Downstream Channel| 2| A
IOUT2| Maximum Output2 Current| | 2| A
IOUT3| Maximum Output3 Current| | 2| A
IOUT4| Maximum Output4 Current| | 2| A
fSW| Switching Frequency| | 1.8| 2| 2.25| MHz
EFF| Efficiency| VIN_HV = 12V, fSW = 2MHz, VOUT1 = 5V, IOUT1 = 1A| 93.3| %
VIN_HV = 12V, fSW = 2MHz, VOUT2 = 3.3V, IOUT2 = 1A| 90.8| %
VIN_LV = 5V, fSW = 2MHz, VOUT3 = 1.8V, IOUT3 = 1A| 92.1| %
VIN_LV = 5V, fSW = 2MHz, VOUT4 = 1.2V, IOUT4 = 1A| 88.6| %

*The operating input voltage range for VIN1 and VIN2 is 3V to 42V. The 6V minimum input voltage spec is limited by the 5V output voltage.
**The operating input voltage range for VIN3 and VIN4 is 3V to 8V. The absolute maximum input voltage for VIN3 and VIN4 is 10V.

QUICK START PROCEDURE

Demonstration circuit 3126A is easy to set up to evaluate the performance of the LT8686S. Refer to Figure 1 for proper measurement equipment setup and use the following the procedure.

1. With power off, place the jumpers in the following positions:
   JP1| JP2| JP3| JP4| JP5| JP6
   ---|---|---|---|---|---
   EN/UVLO| EN2| EN3| EN4| BIAS| SYNC/MODE
   ON| ON| ON| ON| VOUT1| BURST

2. With the power off, connect the input power supply to VEMI+ and VEMI–.

3. With the power off, connect the loads from VOUT1, VOUT2, VOUT3, and VOUT4 to GND.

4. Voltmeters can be placed across the output terminals to get accurate output voltage measurements.

5. Turn on the power at the input.
   NOTE: Make sure that the input voltage does not exceed 42V.

6. Check for proper output voltages. The output should be regulated at 5V (±4%), 3.3V (±4%), 1.8V (±4%), and 1.2V (±4%).
   NOTE: If there is no output, temporarily disconnect the load to ensure that the load is not set too high.

7. Once the proper output voltage is established, adjust the load within the operating ranges and observe the output voltage regulation, ripple voltage, efficiency, and other parameters. NOTE: When measuring the input or output voltage ripple, take care to avoid a long ground lead on the oscilloscope probe. Measure the output voltage ripple by touching the probe tip directly across the output capacitor. See Figure 2 for the proper scope technique.

8. An external clock can be added to the SYNC terminal when SYNC function is used (JP6 on the SYNC posi- tion). Use resistor R25 to set the LT8686S switching frequency close to the synchronization frequency.

9. (Option) Operation with Different Input Voltages The low voltage channels, channel 3 and channel 4, can operate with different input voltages instead of 5V VOUT1. The DC3126A provides and onboard 0Ω jumper (R29) to connect VIN_LV to VOUT1 by default. The 0Ω jumper can be removed to disconnect VIN_LV from VOUT1. Apply different input voltages for channel 3 and channel 4 between VIN_LV and GND. NOTE: Make sure that the VIN_LV input voltage does not exceed 8V.

10. (Option) Combining Channels (CH1 + CH2, CH3 + CH4) Configuration The DC3126A can combine two regulators to create channels with higher current ratings using a single inductor. Channel 1 and channel 2 can be combined to deliver up to 4A of output current. Similarly, channel 3 and channel 4 can be combined to deliver up to 4A of output current. Channel 1 and channel 3 are main channels, channel 2 and channel 4 are subordinates.

The following simple modification is required:

1. Tie the SW1 and SW2 pins together with a low impedance connection. Tie SW3 and SW4 together with a low impedance connection. Since SW1 and SW2, SW3 and SW4 are connected, there is only one inductor needed for each output rail. Calculate and insert the inductors needed for L1 and L3, and remove L2 and L4.
2. Tie the BST1 and BST2 pins together with 0Ω resistors (R9, R12). Tie the BST3 and BST4 pins together with 0Ω resistors (R15, R18).
3. Tie the FB2 and FB4 pins to INTVCC with 0Ω resistors (R31, R32). Keep the resistor divider networks on the main channels and remove the resistor divider networks on subordinate channels.
4. Float (do not use) PG2 and PG4. Only PG1 and PG3 are active.
5. Float (do not use) TRK/SS2 and TRK/SS4. Only TRK/SS1 and TRK/SS3 are active.
6. Tie EN/UVLO and EN2 together with a 0Ω resistor (R27), EN3 and EN4 together with a 0Ω resistor (R30).

TYPICAL PERFORMANCE CHARACTERISTICS

TYPICAL PERFORMANCE CHARACTERISTICS

PARTS LIST

ITEM| QTY| REFERENCE| PART DESCRIPTION| MANUFACTURER/PART NUMBER
---|---|---|---|---

Required Circuit Components

GCM155R71H104KE02D
3| 2| C4, C5| CAP., 10µF, X7R, 50V, 10%, 1210| MURATA, GRM32ER71H106KA12L
4| 1| C8| CAP., 10µF, X7R, 50V, 10%, 1206| SAMSUNG, CL31B106KBHNNNE
5| 1| C10| CAP., 4.7µF, X5R, 10V, 10%, 0603| AVX, 0603ZD475KAT2A
6| 1| C12| CAP., 22µF, ALUM ELECT, 63V, 20%, SMD, RADIAL, CE-BS SERIES| SUN ELECTRONIC INDUSTRIES CORP, 63CE22BS
7| 4| C14, C15, C16, C17| CAP., 0.01µF, X7R, 25V, 10%, 0603| MURATA, GRM188R71E103KA01D
8| 2| C18, C36| CAP., 1µF, X7R, 6.3V, 10%, 0402| MURATA, GRM155R70J105KA12D
9| 2| C19, C33| CAP., 100µF, X7S, 6.3V, 20%, 1210| MURATA, GRM32EC70J107ME15L
10| 2| C21, C23| CAP., 0.1µF, X7R, 25V, 10%, 0402| MURATA, GCM155R71E104KE02D
11| 1| C22| CAP., 4.7µF, X7R, 16V, 20%, 0805| TAIYO YUDEN, MCASE21GAB7475MTNA01
12| 2| C24, C31| CAP., 4.7pF, X7R, 10V, 10%, 0603| AVX, 06033A4R7KAT2A
13| 4| C25, C26, C34, C35| CAP., 0.1µF, X7R, 16V, 10%, 0402| MURATA, GCM155R71C104KA55D
14| 2| C27, C30| CAP., 1µF, X5R, 10V, 10%, 0402| MURATA, GRM155R61A105KE15D
15| 2| C28, C29| CAP., 47µF, X7R, 10V, 10%, 1210| MURATA, GRM32ER71A476KE15L
16| 2| C32, C37| CAP., 47pF, C0G, 25V, 10%, 0603| AVX, 06035A4R7BAT2A
17| 1| FB1| IND., 100Ω AT 100MHz, FERRITE BEAD, 25%, 4A, 20mΩ, 0805| TDK, MPZ2012S101ATD25
18| 1| L1| IND., 2.2µH, PWR, SHIELDED, 20%, 8A, 23.5mΩ| COILCRAFT, XFL4020-222MEB
19| 1| L2| IND., 1.5µH, PWR, 20%, 9.1A, 15.8mΩ, SMD, SHIELDED| COILCRAFT, XFL4020-152MEB
20| 1| L3| IND., 1µH, PWR, SHIELDED, 20%, 11A, 11.9mΩ, SMD| COILCRAFT, XFL4020-102MEB
21| 1| L4| IND., 0.56µH, PWR, 20%, 6A, 5.53mΩ, SMD| XFL4020-561MEB
22| 1| L5| IND., 0.22µH, PWR, SHIELDED, 30%, 9.5A, 7.3mΩ, 4020| WURTH ELEKTRONIK, 744373240022
23| 6| R1, R3, R5, R7, R10, R16| RES., 1M, 1%, 1/10W, 0603| VISHAY, CRCW06031M00FKEA
24| 1| R11| RES., 191k, 1%, 1/10W, 0603| PANASONIC, RK73H1JTTD1913F
25| 1| R13| RES., 124k, 1%, 1/10W, 0603| PANASONIC, ERJ3EKF1243V
26| 6| R14, R19, R21, R22, R23, R24| RES., 100k, 1%, 1/10W, 0603| PANASONIC, ERJ3EKF1003V
27| 1| R17| RES., 316k, 1%, 1/10W, 0603| VISHAY, CRCW0603316KFKEA
28| 1| R20| RES., 200k, 1%, 1/10W, 0603| VISHAY, CRCW0603200KFKEA
29| 1| R25| RES., 22.6k, 1%, 1/10W, 0603| NIC, NRC06F2262TRF
30| 1| R26| RES., 26.1k, 1%, 1/10W, 0603| PANASONIC, ERJ3EKF2612V
31| 1| R29| RES., 0Ω, JUMPER, 45A, 0603, COPPER, SENSE| VISHAY, WSL060300000ZEA9
32| 1| U1| IC, 42V QUAD SYNC. MONOLITHIC STEP-DOWN REGULATOR, LQFN-32| ANALOG DEVICES, LT8686SJV#PBF
ITEM| QTY| REFERENCE| PART DESCRIPTION| MANUFACTURER/PART NUMBER
---|---|---|---|---

Additional Demo Board Circuit Components

1| 0| C20| CAP., 22µF, 35V, 20%, SMD, RADIAL, CE-BSS SERIES| SUN ELECTRONIC INDUSTRIES CORP, 35CE22BSS
---|---|---|---|---
2| 0| R9, R12, R15, R18| RES., OPTION, 0402|
3| 0| R2, R4, R6, R8, R27, R30, R31, R32| RES., OPTION, 0603|
4| 0| E17, E18| TEST POINT, TURRET, 0.094″ MTG. HOLE, PCB 0.062″ THK| MILL- MAX, 2501-2-00-80-00-00-07-0

Hardware: For Demo Board Only

1| 26| E1, E2, E3, E4, E5, E6, E7, E8, E9, E10, E11, E12, E13, E14, E15, E16, E19, E20, E21, E22, E23, E24, E25, E26, E27, E28| TEST POINT, TURRET, 0.094″ MTG. HOLE, PCB 0.062″ THK| MILL-MAX, 2501-2-00-80-00-00-07-0
---|---|---|---|---
2| 4| JP1, JP2, JP3, JP4| CONN., HDR, MALE, 1×3, 2mm, VERT, ST, THT| WURTH ELEKTRONIK, 62000311121
3| 1| JP5| CONN., HDR, MALE, 2×3, 2mm, VERT, ST, THT| WURTH ELEKTRONIK, 62000621121
4| 1| JP6| CONN., HDR, MALE, 2×4, 2mm, VERT, ST, THT| WURTH ELEKTRONIK, 62000821121
5| 4| MP1, MP2, MP3, MP4| STANDOFF, NYLON, SNAP-ON, 0.50″| KEYSTONE, 8833
6| 6| XJP1, XJP2, XJP3, XJP4, XJP5, XJP6| CONN., SHUNT, FEMALE, 2-POS, 2mm| WURTH ELEKTRONIK, 60800213421

SCHEMATIC DIAGRAM

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DEMO MANUAL DC3126A
ESD Caution ESD (electrostatic discharge) sensitive device. Charged devices and circuit boards can discharge without detection. Although this product features patented or proprietary protection circuitry, damage may occur on devices subjected to high energy ESD. Therefore, proper ESD precautions should be taken to avoid performance degradation or loss of functionality.
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