The MAX1843 constant-off-time, pulse-width modulated (PWM) step-down DC-DC converter is ideal for use in 5V and 3.3V to low-voltage conversion necessary in notebook and subnotebook computers. This device features an internal PMOS power switch and internal synchronous rectification for high efficiency and reduced component count. An external Schottky diode is not required. The internal 90mΩ power switch and 70mΩ nMOS synchronous-rectifier switch easily deliver continuous load currents up to 2.7A. The MAX1843 produces a preset +2.5V, +1.8V, or +1.5V output voltage or an adjustable output from +1.1V to V_IN. It achieves efficiencies as high as 95%.

The MAX1843 uses a unique current-mode, constant-off-time, PWM control scheme, which includes Idle Mode™ to maintain high efficiency during light-load operation. The programmable constant-off-time architecture sets switching frequencies up to 1MHz, allowing the user to optimize performance trade-offs between efficiency, output switching noise, component size, and cost. The MAX1843 features an adjustable soft-start to limit surge currents during startup, a 100% duty-cycle mode for low dropout operation, and a low-power shutdown mode that disconnects the input from the output and reduces supply current below 1µA. The MAX1843 is available in a 28-pin TQFN package with an exposed backside pad.