Main Power- Supply Controllers for Notebook Computers MAX8744A/MAX8745A

General Description

The MAX8744A/MAX8745A are dual step-down, switchmode, power-supply (SMPS) controllers with synchronous rectification, intended for main 5V/3.3V power generation in battery-powered systems. Fixed-frequency operation with optimal interleaving minimizes input ripple current from the lowest input voltages up to the 26V maximum input. Optimal 40/60 interleaving allows the

input voltage to go down to 8.3V before duty-cycle overlap occurs, compared to 180° out-of-phase regulators where the duty-cycle overlap occurs when the

input drops below 10V.

Output current sensing provides peak current-limit protection, using either an accurate sense resistor or using lossless inductor DCR current sensing. A low-noise mode maintains high light-load efficiency while keeping the switching frequency out of the audible range.

An internal, fixed 5V, 100mA linear regulator powers up the MAX8744A/MAX8745A and their gate drivers, as well as external keep-alive loads. When the main PWM regulator is in regulation, an automatic bootstrap switch bypasses the internal linear regulator, providing current up to 200mA. An additional adjustable linear-regulator driver with an external pnp transistor can be used with a secondary winding to provide a 12V supply, or powered directly from the main outputs to generate lowvoltage outputs as low as 1V.

Independent enable controls and power-good signals allow flexible power sequencing. Voltage soft-start gradually ramps up the output voltage and reduces inrush current, while soft-shutdown gradually ramps the output voltage down, preventing negative voltage dips.

The MAX8744A/MAX8745A feature output undervoltage and thermal-fault protection. The MAX8744A also includes output overvoltage-fault protection. The MAX8744A/MAX8745A are available in a 32-pin, 5mm x 5mm, thin QFN package. The exposed backside pad improves thermal characteristics for demanding linear keep-alive applications.

Features

Fixed-Frequency, Current-Mode Control

40/60 Optimal Interleaving

Internal BST Switches

Internal 5V, 100mA Linear Regulator

Auxiliary Linear-Regulator Driver (12V or

Adjustable Down to 1V)

Dual Mode™ Feedback—3.3V/5V Fixed or

Adjustable Output Voltages

200kHz/300kHz/500kHz Switching Frequency

Undervoltage and Thermal-Fault Protection

Overvoltage-Fault Protection (MAX8744A Only)

6V to 26V Input Range

2V Å}0.75% Reference Output

Independent Enable Inputs and Power-Good

Outputs

Soft-Start and Soft-Shutdown (Voltage Ramp)

 8μA (typ) Shutdown Current.