7.1 Overview

The TPS56339 is a 24-V, 3-A, synchronous buck (step-down) converter with two integrated n-channel MOSFETs. The device implements an AECM control which can get fast transient response with fixed frequency. The fast transient response results in low voltage drop and the fixed frequency brings a better jitter permanence and predictable frequency for EMI design. The optimized internal compensation network minimizes the external component counts and simplifies the control loop design.

The TPS56339 is designed for safe monotonic start-up into pre-biased loads. The default start-up is when VIN is typically 4.5 V. The EN pin has an internal pull-up current source that can be used to adjust the input voltage undervoltage lockout (UVLO) with two external resistors. In addition, the EN pin can be floating for the device to operate with the internal pull-up current. The total operating current for the device is approximately 98 μA when not switching and under no load. When the device is disabled, the supply current is approximately 3 μA. The integrated 70-mΩ high-side MOSFET and 35-mΩ allow for high efficiency power supply designs with continuous output currents up to 3 A.

The TPS56339 reduces the external component count by integrating the boot recharge circuit. The bias voltage for the integrated high-side MOSFET is supplied by a capacitor between the BOOT and SW pins. The boot capacitor voltage is monitored by a BOOT to SW UVLO (BOOT-SW UVLO) circuit allowing SW pin to be pulled low to recharge the boot capacitor. The device has a on-time extension function with a maximum on time of 5 μs to keep the boot capacitor voltage higher than the preset BOOT-SW UVLO threshold which is typically 2.2 V. During low dropout operation, large duty cycle is needed. The high-side MOSFET could turn on up to 5 μs. Then the high-side MOSFET turns off and the low-side MOSFET turns on with a minimum off time of 115 ns, supporting a maximum duty cycle of 97%.

The TPS56339 integrates output undervoltage protection. When the regulated output voltage is lower than 62.5% of the nominal voltage due to over current triggered, the undervoltage comparator is activated. 120 μs deglitch timer later, both the high-side and low-side MOSFET turn off, the device steps into hiccup mode.

The TPS56339 has internal 5-ms soft-start time to minimize inrush currents.