ISL6236A

High-Efficiency, Quad-Output, Main Power Supply Controllers for Notebook Computers

The ISL6236A dual step-down, switch-mode power-supply (SMPS) controller generates logic-supply voltages in battery-powered systems. The ISL6236A includes two pulse-width modulation (PWM) controllers, 5V/3. 3V and 1. 5V/1. 05V. The output of SMPS1 can also be adjusted from 0. 7V to 5. 5V. The SMPS2 output can be adjusted from 0V to 2. 5V by setting REFIN2 voltage. An optional external charge pump can be monitored through SECFB.

This device features a linear regulator providing 3. 3V/5V, or adjustable from 0. 7V to 4. 5V output via LDOREFIN. The linear regulator provides up to 100mA output current with automatic linear-regulator bootstrapping to the BYP input. When in switchover, the LDO output can source up to 200mA. The ISL6236A includes on-board power-up sequencing, the power-good (POK) outputs, digital soft-start, and internal soft-stop output discharge that prevents negative voltages on shutdown.

A constant on-time PWM control scheme operates without sense resistors and provides 100ns response to load transients while maintaining a relatively constant switching frequency. The unique ultrasonic pulse-skipping mode maintains the switching frequency above 25kHz, which eliminates noise in audio applications. Other features include pulse skipping, which maximizes efficiency in light-load applications, and fixed-frequency PWM mode, which reduces RF interference in sensitive applications.

A power supply unit (PSU) converts mains AC to low-voltage regulated DC power for the internal components of a computer. Modern personal computers universally use switched-mode power supplies. Some power supplies have a manual switch for selecting input voltage, while others automatically adapt to the mains voltage.

Most modern desktop personal computer power supplies conform to the ATX specification, which includes form factor and voltage tolerances. While an ATX power supply is connected to the mains supply, it always provides a 5-volt standby (5VSB) power so that the standby functions on the computer and certain peripherals are powered. ATX power supplies are turned on and off by a signal from the motherboard. They also provide a signal to the motherboard to indicate when the DC voltages are in spec, so that the computer is able to safely power up and boot. The most recent ATX PSU standard is version 2.31 as of mid-2008.

Functions

Simplified circuit diagram of a typical PSU

Diagram of a typical XT and AT voltage regulator circuit

Internals of a PSU with passive PFC (left) and active PFC (right)

The desktop computer power supply converts the alternating current (AC) from a wall socket of mains electricity to a low-voltage direct current (DC) to operate the motherboard, processor and peripheral devices. Several direct-current voltages are required, and they must be regulated with some accuracy to provide stable operation of the computer. A power supply rail or voltage rail refers to a single voltage provided by a PSU.

Some PSUs also supply a standby voltage, so that most of the computer system can be powered off after preparing for hibernation or shutdown, and powered back on by an event. Standby power allows a computer to be started remotely via wake-on-LAN and Wake-on-ring or locally via Keyboard Power ON (KBPO) if the motherboard supports it. This standby voltage may be generated by a small linear power supply inside the unit or a switching power supply, sharing some components with the main unit to save cost and energy.

Input voltage switch

Power supplies designed for worldwide use were once equipped with an input voltage selector switch that allowed the user to configure the unit for use on local power grid. In the lower voltage range, around 115 V, this switch is turned on changing the power grid voltage rectifier into a voltage doubler in delon circuit design. As a result, the large primary filter capacitor behind that rectifier was split up into two capacitors wired in series, balanced with bleeder resistors and varistors that were necessary in the upper input voltage range, around 230 V.

Connecting the unit configured for the lower range to a higher-voltage grid usually resulted in an immediate permanent damage. When the power-factor correction (PFC) was required, those filter capacitors were replaced with higher-capacity ones, together with a coil installed in series to delay the inrush current. This is the simple design of a passive PFC.

Active PFC is more complex and can achieve higher PF, up to 99%. The first active PFC circuits just delayed the inrush. Newer ones are working as an input and output condition-controlled step-up converter, supplying a single 400 V filter capacitor from a wide-range input source, usually between 80 and 240 V. Newer PFC circuits also replace the NTC-based inrush current limiter, which is an expensive part previously located next to the fuse.

What is a power supply do?

A power supply takes the AC from the wall outlet, converts it to unregulated DC, and reduces the voltage using an input power transformer, typically stepping it down to the voltage required by the load. For safety reasons, the transformer also separates the output power supply from the mains input.

What is at in power supply?

AT power scheme is the original power scheme that most of the older computers used. AT-style computer cases had a power button that is directly connected to the system computer power supply. Pushing the power button kills all the power instantly (whether the Operating System is ready for it or not!)