LMZ31707RVQR

The LMZ31707RVQR from Texas Instruments is a SIMPLE SWITCHER power module that integrates a 7-A DC-DC buck converter with power MOSFETs, a shielded inductor, and passive components into a compact 10 x 10 x 4.3 mm QFN package. It requires as few as three external components (input capacitor, output capacitor, and voltage set resistor) to implement a complete point-of-load (POL) power supply, eliminating the loop compensation and magnetics selection process that is typically the most time-consuming part of power supply design.

The LMZ31707 is part of a pin-compatible family that includes the LMZ31704 (4 A) and LMZ31710 (10 A), allowing designers to scale current capacity without changing the PCB layout. This scalability is particularly valuable in product families where different variants require different power levels.

The input voltage range of 2.95 V to 17 V (with split rail) or 4.5 V to 17 V (single rail) covers the most common intermediate bus voltages: 3.3 V, 5 V, 12 V, and 15 V. The split rail option allows PVIN to be connected to a lower voltage (e.g., 3.3 V or 5 V) while VIN powers the control circuitry from a higher voltage, improving efficiency in applications where a low-voltage high-current rail is available.

The adjustable output voltage range of 0.6 V to 5.5 V covers all common digital and analog supply voltages: 0.9 V (FPGA core), 1.0 V, 1.2 V, 1.5 V, 1.8 V, 2.5 V, 3.3 V, 5.0 V. The 1 percent reference accuracy ensures tight voltage regulation for sensitive loads such as FPGAs, DSPs, and processors.

The switching frequency is adjustable from 200 kHz to 1.2 MHz via a single external resistor, allowing the designer to trade off efficiency (lower frequency) against solution size (higher frequency). At 1.2 MHz, the output filter uses smaller capacitors and the inductor is integrated, resulting in a very compact overall solution.

The parallel operation capability allows two or more LMZ31707 modules to share the load current through the ISHARE pin. This enables 14 A, 21 A, or higher output current configurations using the same device, with the ISHARE pin automatically balancing the current between modules.

The integrated shielded inductor eliminates the EMI concerns associated with external inductors and reduces the radiated emissions to levels that meet EN55022 Class B without additional shielding. The thermal impedance of 13.3 degrees C/W (junction to ambient) is achieved through the exposed thermal pad on the bottom of the QFN package, which must be soldered to the PCB ground plane for heat dissipation.

The LMZ31707 includes comprehensive protection features: overcurrent protection (OCP) with hiccup mode, overtemperature protection (OTP) at 175 degrees C with 10 degree hysteresis, programmable UVLO, and pre-bias startup capability that prevents the output from discharging a pre-existing voltage during startup.

At $7.21 per unit in volume (1000+), the LMZ31707RVQR is a premium power module. However, when considering the total cost of ownership (eliminating the inductor, reducing design time, smaller PCB area, fewer components, and guaranteed EMI compliance), it can be more cost-effective than a discrete implementation for many applications.

The LMZ31707RVQR operates as a synchronous buck (step-down) DC-DC converter with integrated power MOSFETs and shielded inductor, implementing current-mode control for fast transient response and easy loop compensation.

Synchronous Buck Topology: The converter uses a high-side N-channel MOSFET (control FET) and a low-side N-channel MOSFET (synchronous FET) in a synchronous buck configuration. The high-side MOSFET connects the input voltage (PVIN) to the inductor, and the low-side MOSFET provides a low-resistance current path during the off-time (replacing the Schottky diode in an asynchronous buck). This synchronous rectification eliminates the diode forward voltage drop during the off-time, significantly improving efficiency at low output voltages.

Integrated Shielded Inductor: The power inductor is integrated into the QFN package, connecting the PH (phase) node to the VOUT pin internally. The inductor is magnetically shielded to minimize radiated EMI and prevent coupling with nearby components. The inductor value is optimized for the 7 A current rating and the adjustable switching frequency range. The integration eliminates the need for the designer to select and qualify an external inductor.

Current-Mode Control: The LMZ31707 uses peak current-mode control, where the error amplifier output (VADJ voltage) sets the peak inductor current threshold. The control loop regulates the output voltage by adjusting the peak current on a cycle-by-cycle basis. Current-mode control provides inherent cycle-by-cycle current limiting, faster transient response than voltage-mode control, and easier loop compensation (single-pole response). The internal loop compensation is optimized for the integrated inductor and typical output capacitor values.

Voltage Setting: The output voltage is set by a single external resistor (RADJ) connected between the VADJ pin and AGND. The internal reference voltage is 0.6 V, and the output voltage is determined by the formula VOUT = 0.6 V x (1 + RADJ/RINT), where RINT is an internal resistor. Alternatively, the output voltage can be calculated from the lookup table in the datasheet that maps RADJ values to output voltages.

Switching Frequency and Synchronization: The switching frequency is set by an external resistor (RSET) connected from the RT/CLK pin to AGND. The frequency range is 200 kHz to 1.2 MHz. An external clock can be applied to the RT/CLK pin to synchronize the switching frequency, which is useful for avoiding beat frequencies in multi-converter systems. The SYNC_OUT pin provides a 180-degree out-of-phase clock signal that can drive another LMZ31707, reducing input ripple current in parallel or multi-rail configurations.

Parallel Current Sharing: Multiple LMZ31707 modules can be connected in parallel through the ISHARE pin. The ISHARE pin carries a current proportional to the load current of each module. When modules are connected in parallel, the ISHARE pins are tied together, and the internal control loop adjusts each module’s output voltage slightly to equalize the load current. The current sharing accuracy is typically within 10 percent between modules.

Split Rail Operation: The LMZ31707 supports separate VIN and PVIN connections. VIN powers the internal control circuitry (4.5 V minimum), while PVIN powers the output stage (2.95 V minimum). This allows the output stage to operate from a lower voltage rail (e.g., 3.3 V or 5 V) for higher efficiency at low output voltages, while the control circuitry is powered from a higher voltage (e.g., 12 V) for reliable operation.

Protection Mechanisms: Overcurrent protection (OCP) limits the peak inductor current on a cycle-by-cycle basis. If the current exceeds the OCP threshold for multiple consecutive cycles, the device enters hiccup mode (turns off and retries periodically) to prevent overheating. Overtemperature protection (OTP) shuts down the device at 175 degrees C junction temperature and restarts at 165 degrees C. The UVLO function holds the device off until the input voltage exceeds the programmed threshold.