Eight-Channel Programmable 1A Buck DC/DCs for Multi-Rail Systems

Linear Technology’s LTC3375 is a highly-integrated, mixed-signal power manager intended for systems requiring multiple low voltage power supplies (Fig. 1). The IC features eight independent 1A output channels, with flexible sequencing and fault monitoring in a compact QFN package. The IC can operate as standalone buck converters or under microprocessor control via its I²C interface.

Each of the eight channels operates as a synchronous step-down regulator with its own independent 2.25V to 5.5V input and an adjustable output from 0.425V to V_IN. Device grades E and I have an operating junction temperature range of -40°C to +125°C; the H grade features operation from -40°C to +150°C.

All eight switching regulators include:

Internal compensation: need only external feedback resistors to set output voltage
Forward and reverse-current limiting
Soft-start to limit inrush current during start-up
Short-circuit protection

There are two operating modes: burst mode (power-up default) for higher efficiency at light loads and forced continuous PWM mode for lower noise at light loads. In burst mode, the regulator bursts at light loads, whereas at higher loads it operates in the constant frequency PWM mode. In forced continuous mode (selectable via I²C command), the oscillator runs continuously and the buck switch currents are allowed to reverse under very light load conditions to maintain regulation.

The LTC3375 communicates with a microprocessor using the standard I²C two-wire interface. Two bus lines, SDA and SCL, must be high when the bus is not in use. These lines require external pull-up resistors or current sources, such as the LTC1694 SMBus accelerator. The LTC3375 is both a slave receiver and slave transmitter. The I²C port operates at speeds of up to 400kHz. It has built-in timing delays to ensure correct operation when addressed from an I²C compatible master device.

The I²C interface can be used to select the mode of operation, phasing, feedback regulation voltage and switch slew rate. In shutdown, an I²C control bit keeps all the SW (regulator output) nodes in a high impedance state (default) or forces all the SW nodes to decay to GND through 1kΩ (typical) resistors. Also, the slew rate of the SW nodes may be switched from the default value to a lower value for reduced radiated EMI at the expense of a small decrease in efficiency.

Combined Power Stages

Up to four adjacent buck regulators may be combined in a master-slave configuration by connecting their SW pins together, connecting their V_IN pins together, and connecting the higher numbered bucks’ FB pin(s) to the input supply. The lowest numbered buck is always the master. Fig. 2 shows a typical configuration.

Any combination of 2, 3, or 4 adjacent buck regulators may be combined to provide either 2 A, 3 A, or 4 A of average output load current. For example, buck regulator 1 and buck regulator 2 may run independently, while buck regulators 3 and 4 may be combined to provide 2 A, while buck regulators 5 through 8 may be combined to provide 4 A. Buck regulator 1 is never a slave, and buck regulator 8 is never a master. 15 unique output power stage configurations are possible, which maximizes application flexibility.

Standalone or Microprocessor Control

As mentioned earlier, the LTC3375 programmable IC can operate under microprocessor control via its I²C interface, or as standalone buck converter using the part’s EN pin in default mode. Fig. 3 shows a diagram of the IC’s various functions. Among the LTC3375’s features is a pushbutton interface that allows power-up or power-down control for either the IC or the application using the PB, KILL, and ON pins. I²C control has a similar capability.

You can enable any of the buck switching regulators by asserting a HIGH signal on any of the eight EN (enable) pins or by writing a buck switching regulator EN command via I²C. If no I²C enable has been written, the buck switching regulator may be powered down, simply by pulling the EN function LOW. You can use I²C to power down the buck converters regardless of the status of their associated EN pin.

When using microprocessor control, a watchdog circuit monitors its activity. The microprocessor must change the logic state of the WDI (watchdog input) pin at least once every 1.5 seconds (typical) in order to clear the watchdog timer and prevent the WDO (watchdog output) pin from signaling a timeout.

Operating Frequency

The LTC3375 features a programmable and synchronizable oscillator. Selection of the operating frequency is a trade-off between efficiency and component size. High frequency operation allows the use of smaller inductor and capacitor values. Operation at lower frequencies improves efficiency by reducing internal gate charge losses but requires larger inductance values and/or capacitance to maintain low output voltage ripple.

An external resistor connected between the RT pin and ground determines the operating frequency for all eight LTC3375 regulators. The LTC3375 can operate between 1 MHz and 3 MHz with a default switching frequency of 2 MHz. Safety clamps prevent the oscillator from running faster than 4 MHz (typical) or slower than 250 kHz (typical).

You can synchronize the LTC3375’s internal oscillator to an external frequency by applying a square wave clock signal to the SYNC pin. The synchronization frequency range is 1 MHz to 3 MHz. DriV_INg SYNC with an external clock synchronizes all switchers to the applied frequency. Slope compensation is automatically adapted to the external clock frequency. The absence of an external clock signal enables the frequency programmed by the RT pin.

Shunt Regulator

The IC also contains a high voltage input shunt regulator controller, whose quiescent current is only 11µA with all DC/DC regulators off. The V_CC pin is for an always-on LDO output voltage/internal bias supply. When used as a regulator, V_CC should be connected to the emitter/source of an external LDO NPN/NFET transistor (as shown in Fig. 3). V_CC serves as a low voltage rail that may be used to provide power to external circuitry, and is also used to power the LTC3375’s internal top level circuitry. The FBV_CC pin receives feedback from a resistor divider connected across V_CC. The V_SHNT pin is a shunt regulator base control voltage. V_SHNT should be connected to the base/gate of an external high voltage NPN/NFET transistor and to its collector/drain through a resistor. Alternatively, you can connect the V_CC pin to a 2.7 V to 5.5 V external power supply and connect FBV_CC and V_SHNT to ground.

All eight buck regulators are designed to be used with inductors ranging from 1 μH to 3 μH, depending on the lowest switching frequency of buck regulator operation. To operate at 1 MHz use a 3 μH inductor, at 3 MHz use a 1μH inductor.

To prevent thermal damage to the LTC3375 and its surrounding components, the IC incorporates a user-programmable overtemperature (OT) threshold. A die temperature monitor output (readable via I²C or the analog voltage on the TEMP pin) indicates the internal die temperature. The TEMP pin outputs 220 mV (typical) at room temperature and changes by 7 mV/°C (typical). An overtemperature warning indicates the die temperature is approaching the overtemperature threshold. The IC will enter thermal shutdown at 1.20 V (165°C typical), with enabled buck switching regulators then shutting down. The buck regulators remain shut down until the die temperature falls to 155°C (typical), at which point it is safe for them to return to normal operation.

PCB Layout

Electrical and thermal considerations should be taken into account when implementing the LTC3375 in a circuit. To ensure proper operation of the IC, the designer should lay out the printed circuit board (PCB) as follows:

Minimize thermal and electrical impedance by connecting the exposed pad of the package (pin 49) directly to a large ground plane.
Decouple each buck regulator’s input supply pins with at least a 10μF low ESR capacitor to GND. Place these capacitors as close to the pins as possible. Ceramic dielectric capacitors are a good compromise between high dielectric constant and stability versus temperature and DC bias. The X5R/X7R dielectric capacitors offer good overall performance.
Keep connections to the switching regulator input supply pins and their respective decoupling capacitors as short as possible. The GND side of these capacitors should connect directly to the PCB ground plane.
Input capacitors provide AC current for internal power MOSFETs and their drivers. Therefore, minimize inductance from these capacitors to the LTC3375’s V_IN pins.
Reduce radiated EMI and parasitic coupling by minimizing the switching power trace lengths connecting SW outputs to their respective inductors. Due to the large voltage swing of the switching nodes, high input impedance sensitive nodes, such as the feedback nodes, should be kept far away or shielded from the switching nodes.
Decouple all outputs with at least a 22μF capacitor. The GND side of the switching regulator output capacitors should connect directly to the PCB’s ground plane. Minimize the trace length from the output capacitor to the inductor(s)/pin(s).
To ensure proper operation in a combined buck regulator application keep all trace lengths equal from switch nodes to the associated inductor.

Following these steps will help the user gain the maximum benefit of the LTC3375’s flexible power sequencing and fault monitoring functions in systems requiring multiple low voltage power supplies.