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Download DN520 - Versatile Industrial Power Supply Takes High Voltage Input
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Versatile Industrial Power Supply Takes High Voltage Input and Yields from Eight 1A to Two 4A Outputs – Design Note 520 Martin Merchant Introduction Today’s industrial electronic systems contain many of the same components as consumer electronics— microcontrollers, FPGAs, system-on-chip ASICs and other electronics—requiring multiple low voltage rails at widely varied load currents. Industrial applications can also demand a pushbutton interface, an alwayson supply for a real-time clock (RTC) or memory and the ability to take input power from a high voltage supply. Other required features may be a watchdog timer (WDT), a kill or reset button, software adjustable voltage levels and error reporting of low input/output voltages and high die temperature. The LTC ®3375 is a highly configurable multioutput step-down power converter that offers the features often required by industrial electronics while providing the flexibility to configure various outputs with maximum currents ranging from 1A to 4A. Configurable Maximum Output Current The LTC3375’s eight 1A channels can be combined to produce various combinations of 1A, 2A, 3A and 4A buck regulators, as shown by the 15 different output current configurations in Table 1. Connecting the feedback pin of a given channel to its VIN pin configures that channel as a slave to the adjacent channel. The switch pins of the two channels are connected together to share a single inductor and output capacitor. Master/slave channels are enabled via the master’s enable pin and regulate to the master’s feedback network. Output current can be increased to 3A or 4A by connecting additional adjacent channels. The circuit in Figure 1 shows the LTC3375 configured with a 3A output, a 1A output, two 2A outputs and an always-on LDO. It also illustrates how the LTC3375 can be connected to control the start-up of an upstream external buck controller via the on-chip pushbutton interface to supply input power to the LTC3375 buck regulators. 11/13/520 Table 1. LTC3375 Maximum Current Configurations NUMBER OF BUCKS OUTPUT CONFIGURATION 8 1A, 1A, 1A, 1A, 1A, 1A, 1A, 1A 7 1A, 1A, 1A, 1A, 1A, 1A, 2A 6 1A, 1A, 1A, 1A, 1A, 3A 6 1A, 1A, 1A, 1A, 2A, 2A 5 1A, 1A, 1A, 1A, 4A 5 1A, 1A, 1A, 2A, 3A 5 1A, 1A, 2A, 2A, 2A 4 1A, 1A, 2A, 4A 4 1A, 1A, 3A, 3A 4 1A, 2A, 2A, 3A 4 2A, 2A, 2A, 2A 3 1A, 3A, 4A 3 2A, 2A, 4A 3 2A, 3A, 3A 2 4A, 4A External VCC LDO and External Input Power Supply Start-Up Control The LTC3375 can control an external LDO pass device to supply its VCC power and any other low current electronics such as an RTC. The VCC powers the internal pushbutton circuitry, WDT, internal registers and open-drain pull-ups. The external LDO in Figure 1 creates a 3.3V supply from the 24V rail. When the pushbutton is pressed, the ON pin is released and the RUN pin is pulled high on the LTC3891, supplying input power to the buck regulators of the LTC3375. When the LTC3891 achieves regulation, the PGOOD pin is released, enabling EN1 of the LTC3375 and turning on the 2A regulator. The remaining regulators can be enabled with the precision threshold enable pins or via software-controlled I2C commands. Pressing the pushbutton again for 10 seconds or more, or pulling KILL low for 50ms or more, causes the ON pin to be pulled low, disabling all of the buck regulators. L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Unique Power Control and Features The I2C interface allows extensive control of regulator operation. Each regulator may be set to a high efficiency Burst Mode® operation to save power at light loads or set to forced continuous mode for lower output ripple voltage. Each regulator can also have the switching cycle phase shifted by 0°, 90°, 180°, or 270° with respect to the reference clock to allow a lower input ripple current when multiple outputs are supplying large loads. Another feature is the ability to manipulate each output voltage up or down by adjusting the feedback reference voltage from the default 725mV setting in 25mV steps (ranging from 425mV to 800mV). The I2C interface is also used for reporting error conditions for each regulator. The LTC3375 has a reset (RST) pin and an interrupt request (IRQ) pin, which can be programmed to report when any regulator’s output voltage has dropped below 92.5% of the regulation point. The IRQ pin can also be programmed to report when the input voltage drops below the undervoltage lockout (UVLO) threshold or when the die temperature has reached a set temperature threshold. The regulator’s PGOOD and UVLO status, 24V TG LTC3891 RUN BOOST SENSE+ SGND 0.1µF SENSE– 1µF MTOP L1 8µH RSENSE 7mΩ VCC COUT 3.3V 330µF 15mA SW3 SW4 VSHNT 22µF 1.02M SGND 19.1k LTC3891 PGOOD MTOP, MBOT: Si7850DP L1 COILCRAFT SER1360-802KL COUT: SANYO 10TPE330M D1: DFLS1100 2.5V 2A 2.2µH 1.02M 100µF 3.3V 2A 287k LTC3375 SW5 FBVCC 2.2µH 22µF 649k 576k 1.8V 1A FB5 100k EXTVCC VFB 100µF FB3 VCC 1nF 1.02M 412k 107Ω 1Ω 2.2µH FB1 47k FZT629B 5V 6A MBOT BG ITH TRACK/SS D1 SW1 SW2 FB2 FB4 FB7 FB8 24V SW FREQ 0.1µF VIN1-VIN8 2.2µF PGND ILIM 34.8k Conclusion The LTC3375 can be configured with multiple regulated 1A to 4A outputs totaling up to 8A, and includes many features required by today’s industrial electronics. INTVCC PLLIN/MODE 470pF One problem with microprocessors is that a software bug can cause the program to hang. The LTC3375 includes a watchdog timer input (WDI) pin to monitor the SCL pin or some other pin to determine if the software is still running. If the software has stopped running, the watchdog timer output (WDO) pin can be used to reset the microprocessor or power down the HV buck and the LTC3375 buck regulators. Connecting the WDO pin to the RST pin of a microprocessor causes the microprocessor to reset when the WDT is not satisfied. Connecting the WDO pin to the KILL pin causes the ON pin to go low, disabling the HV buck and all LTC3375 regulators. The KILL pin can be pulled low by a pushbutton “paper clip” switch to power down all the regulators as a last resort. 5V 1.5A CIN 22µF VIN LT3375 EN1 PGOOD LTC3375 ON the die temperature warning and the measured die temperature can be monitored by the microprocessor via the I2C interface. 432k 1µF I2C CONTROL SW6 SW7 SW8 SCL SDA WDI VCC 0.01µF PB PUSHBUTTON 1.2V 3A 649k RT CT 402k 68µF 422k FB6 EN1 SYNC EN3 EN5 EN6 EN2 EN4 EN7 EN8 MICROPROCESSOR CONTROL 2.2µH EXPOSED PAD IRQ RST WDO TEMP ON KILL MICROPROCESSOR CONTROL LTC3891 RUN PUSHBUTTON DN520 F01 Figure 1. Low Voltage Power Supply with Pushbutton Control of Upstream HV Buck and Always-On LDO Data Sheet Download www.linear.com/LTC3375 Linear Technology Corporation For applications help, call (978) 656-4778 dn520f LT/AP 1113 111K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2013