7.3.1 Step-Down Converter

The step-down converter is intended to allow maximum flexibility in the end equipment. Figure 16 shows the necessary connections.

Figure 16. DC-DC Converter Block Diagram

Externally adjustable output voltages and additional current-limit options are also possible. Contact TI for further information.

The step-down converter has two modes of operation to maximize efficiency at different load conditions. At moderate to heavy load currents, the device operates in a fixed-frequency pulse-width modulation (PWM) mode that results in small output ripple and high efficiency. Pulling the MODE pin to a DC-high level results in PWM mode over the entire load range.

At light load currents, the device operates in a pulsed frequency-modulation (PFM) mode to improve efficiency. The transition to this mode occurs when the inductor current through the low-side FET becomes zero, indicating discontinuous conduction. PFM mode also results in the output voltage increasing by 1% from the PWM mode value. This voltage positioning is intended to minimize both the voltage undershoot of a load step from light to heavy loads, as when a processor moves from sleep to active modes, and the voltage overshoot at load removal. shows the voltage positioning behavior for a light-to-heavy load step.

Figure 17. PFM Voltage Positioning

Pulling the MODE pin to DC ground results in an automatic transition between PFM and PWM modes to maximize efficiency.

The DC-DC converter output automatically discharges to ground through an internal 450-Ω load when EN_DCDC goes low or when the UVLO condition is met.