The processor is key to computer operation where power specifications continue to place more stringent requirements on their dc source. This applies to desktop and embedded computer systems employed in industrial applications. Processor voltages are decreasing, current is increasing, voltage regulation is getting tighter, and transient response is becoming more critical. Stepping in to provide the necessary dc power are a combination of controller ICs and power MOSFETs.
Among the new controller ICs are International Rectifier's IRU3004 and IRU3005, 5-bit programmable synchronous buck types. IRU3004 (Fig. 1) provides adjustable output for the 2.5V output, whereas the IRU3005 supplies a fixed 2.5V. Both parts are housed in 20-pin SOIC plastic packages and are rated at 0°C to 70°C ambient.
Both devices have on-board dual LDO regulators, which provide 1.5V GTL+ and 2.5V clock supplies for Pentium III™ microprocessors as well as the next generation of P6 family processors. These devices feature a patented topology, which in combination with a few external components, provide an excess of 20A output. They automatically provide the right output voltage via the five-bit internal DAC that meets the VRM 8.4 specification. These products also feature loss-less current sensing by using the RDS(on) of the high side power MOSFET as the sensing resistor. The associated Power Good window comparator switches its open collector output low when the output is outside a ±10% window. Other features include undervoltage lockout for 5V and 12V supplies, an external programmable soft start function, and a programmable oscillator frequency using an external capacitor.
Selecting the appropriate output capacitor involves finding the maximum ESR that meets the transient voltage budget of the total DVo specification. Assuming the regulator's dc initial accuracy, plus the output ripple, is 2% of the output voltage, or 100mV, and the output current is 14.2A, then the maximum ESR of the output capacitor is 100×10-3÷14.2 = 7mW. You can achieve this by paralleling several ceramic or film capacitors.
Select the output inductor so under low line and maximum output voltage, the inductor current slope times the output capacitor ESR ramps up faster than the capacitor voltage droops during a load current step. If the inductor is too small, the output ripple current and ripple voltage become too large. One solution is to reduce the ripple current to increase the switching frequency. You can do this only at the expense of reduced efficiency and higher system cost.
The IR3004 and IR3005 employ a soft start capacitor that you must select, so when the output capacitors charge at start-up, the peak inductor current does not reach the current limit threshold. A minimum of 1 μF capacitor is suitable for most applications. An internal 10μA current source charges the soft start capacitor, which slowly ramps up the inverting input of the PWM comparator. This ensures the output voltage ramps up at the same rate as the soft start capacitor, thereby limiting the input current.
It's recommended to place an inductor between the system 5V supply and the input capacitors of the switching regulator (Fig. 1). This isolates the 5V supply from the switching noise occurring during the turn on and off of the switching components. Typically, an inductor in the 1 μH to 3 μH range will be sufficient in this type of application.
The best way to shutdown the switcher is to pull down on the soft start pin using an external small signal transistor, such as a 2N3904 or 2N7002 small signal MOSFET. This allows slow output ramp up — same as the power up.
Intersil's HIP630X/HIP660X multiphase product family consists of five multiphase PWM controllers and three MOSFET drivers. The family uses a patented circuit topology to obtain a high performance, multiphase dc-dc converter — while requiring fewer and smaller passive devices. It employs a unique current balance approach between the dc-dc converter phases that guarantees accurate phase-to-phase current balance for microprocessor power supplies.
Included in this product family are the HIP6301 multiphase PWM control IC and its companion gate driver, the HIP6602 which provide precision voltage regulation for advanced microprocessors (Fig. 2). Multiphase converters distribute the power and load current, which results in smaller and lower cost transistors with fewer input and output capacitors. These reductions accrue from the higher effective conversion frequency with higher frequency ripple current due to the phase interleaving process of this topology. For example, a 4-phase converter operating at 350 kHz has a 1.4 MHz ripple frequency. Moreover, compared with single-phase topologies, the metaphase converter's wider bandwidth provides faster response to load transients.
The HIP6301 controller IC includes programmable VID codes from the microprocessor that range from 1.1V to 1.85V with a system accuracy of ±1%. Pull-up currents on these VID pins eliminate the need for external pull up resistors. In addition, “droop” compensation, used to reduce the overshoot or undershoot of the core voltage, is easily programmed with a single resistor.
Another feature of the HIP6301 is the PGOOD monitor circuit that is held low until the core voltage increases, during its soft-start sequence, to within 10% of the programmed voltage. An overvoltage, 15% above programmed core voltage, results in the converter shutting down and turning the lower MOSFETs ON to clamp and protect the microprocessor. Under voltage is also detected and results in PGOOD low if the core voltage falls 10% below the programmed level. Overcurrent protection reduces the regulator current to less than 25% of the programmed trip value. These features provide monitoring and protection for the microprocessor and power system.
The HIP6602 is a high frequency, two-power channel MOSFET driver intended to drive four power N-Channel MOSFETs in a synchronous-rectified buck converter topology (Fig. 2). It drives upper and lower gates over a range of 5V to 12V. This drive-voltage flexibility provides the advantage of optimizing applications involving tradeoffs between switching losses and conduction losses.
Output drivers in the HIP6602 have the capacity to efficiently switch power MOSFETs at high frequencies. Each driver is capable of driving a 3000 pF load with a 30 ns propagation delay and 50 ns transition time. This device implements bootstrapping on the upper gates with only a single external capacitor required for each power channel. This reduces implementation complexity and allows the use of higher performance, cost-effective, N-Channel MOSFETs. Adaptive shoot-through protection is integrated to prevent both MOSFETs from conducting simultaneously.
The HIP630X family also includes the HIP6303 and HIP6311, which are 2- to 4-phase PWM controllers that regulate output current above 100A, whereas the HIP6302 and HIP6304 are dedicated 2-phase devices that control microprocessor loads in the 50A range. Also in the product family are the HIP6601A and HIP6603A that are single-channel MOSFET drivers, while the HIP6602A drives four n-channel MOSFETs in a multiphase buck dc-dc converter, as shown in Fig. 2. The manufacturer offers HIP6301, HIP6303, and HIP6311 in 20-pin narrow-body SOIC packages. It also offers HIP6302 and HIP6304 in 16-pin narrow-body SOIC packages, while offering HIP6601 and HIP6603 in 8-pin narrow-body SOIC, 8-pin TSSOP, and the new, thermally enhanced 8-pin Exposed-Paddle (E-Pad) SOIC and 16-pin, 4×4 Micro Lead Frame (MLF) packages. HIP6602 comes in 14-pin narrow-body SOIC with the E-PAD.
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