Modern industrial electronic systems include many of the same components found in consumer electronics, such as microcontrollers, FPGAs, on-chip ASICs and other devices, and therefore require multiple low-voltage rails under a range of load current conditions. Industrial applications also often require a pushbutton interface, a permanently powered supply for an RTC or memory, and the ability to derive input power from a higher-voltage source. Other required features may include a watchdog timer (WDT), a master off or reset pushbutton, software-adjustable voltage levels, and fault reporting for low input/output voltages and elevated chip temperature.
The LTC3375 is a highly configurable multi-output buck power converter that provides features commonly required in industrial electronics and can be flexibly configured to deliver various outputs with maximum currents ranging from 1A to 4A.
Configurable Maximum Output Current
The LTC3375 has eight 1A channels that can be combined in different ways to produce buck regulators of 1A, 2A, 3A and 4A, yielding 15 different output current configurations.
Connecting a given channel's feedback pin to its VIN pin configures that channel as a slave to an adjacent channel. Connecting the switch pins of two channels together lets them share a single inductor and output capacitor. Master/slave channels use the master's enable pin for enabling and regulate to the master's feedback network.
Connecting additional adjacent channels increases output current to 3A or 4A. The circuit in Figure 1 shows an LTC3375 configured to provide one 3A output, one 1A output, two 2A outputs and a permanently powered LDO. The diagram also shows how to wire the LTC3375 so an on-chip pushbutton interface controls the start-up of an upstream external buck controller, which supplies input power to the LTC3375 buck regulators.
Figure 1: Low-voltage rails with pushbutton-controlled upstream HV buck converter and permanently powered LDO
External VCC LDO and Input Power Control
The LTC3375 can control an external LDO pass device to provide its VCC supply and power other low-current components such as an RTC. VCC powers the internal pushbutton circuitry, the WDT, internal registers and drain-open pull-up resistors. In Figure 1 the external LDO is supplied from a 24V rail to produce a 3.3V VCC.
When the pushbutton is pressed, the ON pin is released and the RUN pin of the LTC3891 is pulled high, supplying input power to the LTC3375 buck converters. When the LTC3891 reaches regulation, its PGOOD pin is released, enabling EN1 on the LTC3375 and turning on the 2A regulator. The remaining regulators can be enabled by threshold-accurate enable pins or by software-controlled I2C commands. Pressing and holding the button for 10 seconds or more, or pulling KILL low for 50 ms or longer, pulls the ON pin low and disables all buck regulators.
Power Management Features
The I2C interface provides extensive control over regulator operation. Each regulator can be set to operate in low-power burst mode at light loads or forced continuous mode for lower output ripple. Each regulator's switching phase can be shifted 0°, 90°, 180° or 270° relative to the reference clock to reduce input ripple current when multiple outputs are heavily loaded. Another capability is adjusting the feedback reference voltage from the default 725 mV in 25 mV steps across a range of 425 mV to 800 mV to control the rise and fall of each output voltage. The I2C interface is also used to report regulator fault conditions.
The LTC3375 provides a reset (RST) pin and an interrupt request (IRQ) pin that can be programmed to report when any regulator output falls below 92.5% of its setpoint. The IRQ pin can also be configured to assert when the input voltage drops below the undervoltage lockout (UVLO) threshold or when the chip temperature reaches a configured limit. Regulator PGOOD and UVLO states, a chip temperature alert and the measured chip temperature can be monitored by a microprocessor over the I2C interface.
A common concern with microprocessors is that a software fault can cause a hang. The LTC3375 includes a watchdog input (WDI) that can monitor SCL or another pin to determine whether software is still running. If the software has stopped, the watchdog output (WDO) can reset the microprocessor or disconnect power to the HV buck and the LTC3375 regulators. Connecting WDO to the microprocessor RST pin will reset the processor if WDT conditions are not met. Connecting WDO to the KILL pin pulls ON low and disables the HV buck and all LTC3375 regulators. As a last-resort option, a pushbutton "paperclip" switch can pull KILL low to power down all regulators.
Conclusion
The LTC3375 can be configured to provide multiple stable outputs from 1A to 4A per regulator (up to 8A total) and offers a set of features suitable for modern industrial electronic equipment.
