feature

Programmable output, from 1.10V to 1.85V, 25mV step using integrated 5-bit DAC Two interleaved sync slices, maximum performance 100nsec response time slice Built-in current sharing between slices Remote sensing Programmable active droop (voltage positioning) 200kHz to 2mHz Programmable frequency adaptive delay gate switch Integrated high current gate driver Integrated power good, ov, uv, enable/soft start

Function

Drive N-channel mosfet operation Optimized for 5V operation Light load high efficiency mode (E*) Overcurrent protection using mosfet sensing 24-pin TSSOP package

application

Pentium IV Power Supply Athlon Powered VRM Programmable Buck Power Supply for Pentium IV Processors

describe

The FAN5091 is a synchronous multi-chip DC-DC controller that provides high-precision programmable output voltages for all high-performance processors. Two interleaved synchronous buck regulator chips with built-in current sharing operate 180° out of phase to provide the fast transient response required for high current applications with minimal external components. The FAN5091 features remote voltage sensing and programmable active droop for 100nsec converter transient response with minimal output capacitance. It integrates high-current gate drivers with adaptive delay gate switching and eliminates the need for external drive devices. The FAN5091 uses a 5-bit D/A converter to program the output voltage from 1.10V to 1.85V in 25 mV steps with 1% accuracy. FAN5091 uses high integration to provide over 50A, 5V power supply with minimal external circuitry. The FAN5091 also offers integrated features including power good, output enable/soft start, under voltage lockout, over voltage protection and adjustable current limit with independent current sensing on each slice. It is packaged in a 24-pin TSSOP.

Application Information Operation: FAN5091 Controller The FAN5091 is a programmable synchronous multi-chip DC-DC controller IC. When surrounded by appropriate external components, the FAN5091 can be configured to output current greater than 50A next-generation high-current processors. The FAN5091 acts as a fixed frequency PWM buck regulator with high efficiency mode (E*) at light loads. The main control loop of this FAN5091 consists of two interleaved synchronous buck converters, implemented by summing mode control. Each slice has its own current feedback and voltage feedback. The two buck converters controlled by the FAN5091 are interleaved, that is, they are interleaved with each other. This will minimize the rms input ripple current and minimize the number of input capacitors required. It also doubles the effective switching frequency, improving transient response. FAN5091 implements "summation mode control", which is different from traditional voltage mode and current mode control. It outperforms allowing over a wide range of output loads and external components. The control loop of the regulator consists of two main parts: the analog control block and the digital control block. This analog part consists of a signal conditioning amplifier to form an input comparator, which is a digital control block. The signal conditioning part accepts the input of the current sensor and the voltage sensor. The voltage sensor is shared by the two pieces, and the current sensors are separated. The voltage sensor amplifies the difference between the VFB signal and the reference voltage from the DAC and presents the output separately to the comparator. The current control path of each slice uses the difference between its pgnd and sw pins when the low-side mosfet is turned on, the signal to the voltage amplifier through the mosfet and the resulting input current signal to the same input of its summing amplifier is certain gain. These, therefore, add up the two signals. This sum is then presented to a comparator that observes the oscillator ramp, which provides the main PWM control signal block to the digital control. The oscillator is ramped with each other so that the two slices are turned on alternately. The digital control block accepts the analog comparator input to the HDRV and LDRV output pins for each slice to provide the appropriate pulses. These outputs control external power mosfets.

Remote Voltage Sensing The FAN5091 has true remote voltage sensing capability, eliminating errors due to tracking resistors. With remote sensing, the VFB and AGND pins should be connected as Kelvin tracking pairs to adjustment points such as processor pins. The converter keeps the voltage at that point. The layout of these fields requires care; see layout guidelines in this data sheet. High Current Output Driver FAN5091 contains four high current output drivers using mosfet in push-pull configuration. Drivers for high-side MOSFETs use boot pins for input power and switch pins for return. The low side driver mosfet uses the vcc pin for input power and the pgnd return pin. Typically, the boot pins will use 12V directly. Note that the Boot and VCC pins are tied to the chip's internal power and ground, bypass and agnd for switching noise immunity. The adaptive delay gate driver Fan5091 uses an advanced design that ensures minimal mosfet transition time when eliminating the water flow. It senses the state of the mosfet and adaptively adjusts the door drivers to ensure they never open at the same time. When the high-side mosfet turns off, the voltage on the power supply starts to drop. When the voltage here reaches about 2.5 volts, the low-side mosfet gate drive uses a delay of about 50 nsec. when? The low side mosfet is off and the voltage at the ldrv pin is sensed. When the voltage drops below about 2V, the gate drive of the high-side mosfet is used.

To ensure current sensing and charge pump operation, the fan 5091 guarantees that the low side mosfet will be on a specific part of each cycle. For low frequencies, the maximum duty cycle is approximately 90%. Therefore, at 500kHz, with a period of 2 microseconds, the low side is at least 2 microseconds•10%=200nsec. At higher frequencies, this time may drop so low as to be ineffective. The FAN5091 guarantees a minimum low-side turn-on time of about 330nsec, no matter what duty cycle this corresponds to. Current Sensing The FAN5091 has two independent current sensors, one for each chip which is punctual. Each slice has its own power ground pin, allowing slices to be placed in different positions without compromising measurement accuracy. For best results, the pgnd and sw pins of each slice must be connected as a Kelvin trace pair directly to the source and drain, respectively, appropriate low-side MOSFETs. Care needs to be taken in the layout of these venues; see this data sheet. The two independent current sensors of the current sharing fan 5091 work. Independent current control loops ensure that the two circuits each provide half of the total output current. The only mismatch between the two slices occurs between the RDS of the low-side MOSFET. Short-Circuit Current Characteristics The FAN5091 short-circuit current characteristics include the event of a short circuit in the function that protects the DC-DC converter from damage. The short circuit limit is set to the RS resistance, as shown in the formula when the isc reaches the desired current limit, the rt oscillator resistance and the rds, the on resistance of the low-side mosfet of the slice.

Remember to make the RS large enough to include the initial tolerance and the rds of the temperature variation to the mosfets, on. It is recommended to set the ISC above the maximum operating current to avoid disturbing travel. important hint! The oscillator frequency must be selected before the current limit resistor is selected because rt is used in the calculation of rs. When overcurrent is detected, the high-side mosfet turns off and the low-side mosfet turns on, and they remain in this state until the measured current through the low-side mosfet has returned to zero amps. After reaching zero, the fan 5091 soft-starts again, making sure it can also safely turn into a short. One limitation of the current sense circuit is iscrds, on must be less than 375MV. To ensure proper operation, use an ISC radio data system with an opening ≤ 300 mV; between 300 mV and 375 mV there will be some non-linearity in the short circuit current as specified in the equation. For example, consider dual FDP6670AL low-side DC-DC converter circuit mosfets (rds=6.5mΩ at 25°C, max; rds=1.2 at 75°C) = 7.8MΩ per slice, or 3.9MΩ total), RT=42.1 KΩ (600kHz oscillator) and 50KΩ RS. The converter has normal load regulation characteristics until the voltage on the mosfet exceeds the internal short-circuit threshold of 50kΩ/(3.9mΩ41.2kΩ6.66). =47A. [Note that this current limit level can be as high as 50kΩ / (3.5mΩ 41.2kΩ 6.66) = 52A if the mosfet has typical rds, on instead of max, and at 25°C. ] At this point, the internal comparator trips and leaves the low-side mosfets to the controller and keeps the high-side mosfets off. The inductor current decreases until the inductor current reaches 0A and the converter attempts to soft-start again. Precision Current Sensing Tolerances Associated with Using mosfet Current Using current sensing avoids sensing the resistors provided by the FAN5092. Light Load Efficiency Under light load conditions, the FAN5091 employs various techniques to improve efficiency. Because the synchronous buck converter is two-quadrant, capable of supplying both source and sink currents, at light loads the inductor current will flow from the output to the input during the switching cycle. The fan 5091 detects the reverse current as a positive voltage appearing on the low side mosfet on its timing. When reverse current is detected, the low-side mosfet turns off for the remainder of the cycle, and conversely, current flows through the high-side mosfet, returning power to the mains. This technology greatly improves light-load efficiency. E*-Mode In addition, efficiency can be further improved by placing the fan 5091 in E* mode. When the droop pin is pulled to the 5V bypass voltage, the fan 5091 turns off completely, reducing the gate charge power consumed by half. E*-Mode CAN

PCB layout guidelines are critical relative to the placement of the FAN5091's mosfet. Placing the mosfet on the FAN5091 gate to the FET's HIDRV and LODRV pins is minimized. The long lead lengths on these pins will be due to the gate capacitance of the FET. This noise radiates to the entire board because it is switching at such high voltages and frequencies that it is difficult to suppress. In general, keep all noisy switch lines away from the quiet analog section of the fan 5091. That connected to pins 8-17 (Lodrv, Hidrv, pgnd and boot) should be away from the traces connected to pins 1 to 7 and pins 18-24. Place 0.1µf decoupling capacitors as close to the fan 5091 pins as possible. The extra lead length reduces their ability to suppress noise. Each power and ground pin should have its own to the appropriate plane. This helps in the pins. Place the given mosfet, inductor and Schottky as close as possible for the same reasons as in the first bullet above. Place the input bulk capacitor as close as possible to the drain of the high-side mosfet. Also, placing a 0.1µf decoupling cap right at the drain of each high-side MOSFET helps suppress some of the high frequency switching noise at the input of the DC-DC converter. Placing the output bulk capacitors as close to the CPU as possible optimizes their immediate supply capability during current transients in the load current. The output capacitor and CPU will allow the parasitic resistance of the board traces to degrade the performance of the dc-dc converter under severe load transient conditions, resulting in higher voltage excursions. For capacitor placement, refer to Application Bulletin AB-5. Fairchild offers PC board layout checklist applications. Application Bulletin AB-11. PC Board Example Layout and Gerber File A reference design for a board implementing the FAN5091 and PCAD layout Gerber files and silk screens are available through your local Fairchild representative. FAN5091 Evaluation Board Fairchild provides an evaluation board to verify the horizontal performance of the system fan 5091. It is using externally provided components and PCB layout.