Design of low voltage and high current programmable output power supply for new CPU and microprocessor

As modern electronic products, especially portable electronic products, have increasingly higher requirements for noise, interference and power consumption, the power supply voltage requirements of many new microprocessors, CPUs, MCUs and DSPs and other system core devices are getting lower and lower, but in order to process more interfaces and peripheral data, their current requirements are relatively increased. By 2007, PCs will require DC/DC converters to provide up to 200A of current at 0.95V. To this end, Fairchild Semiconductor has developed a distributed voltage regulation module (VRM) composed of multiple modules, each of which can provide up to 40A of current per phase with an efficiency of more than 80%. The FAN5240 introduced in this article is a phase-synchronous compensation single-output PWM controller integrated circuit designed by Fairchild Semiconductor for the main power supply of CPUs. The chip can program the output voltage of the circuit through its own 5-bit digital-to-analog converter. The setting range of its main PWM output voltage is 0.925~2.0V, and it can be set during the operation of the circuit. At the same time, FAN5240 has high efficiency, which can achieve more than 90% conversion efficiency under wide load conditions, and even under light load, it can achieve more than 80% conversion efficiency. Therefore, FAN5240 is suitable for designing low voltage, high current, programmable output power supply for CPU and microprocessor.

1 Main features and functional parameters of FAN5240

1.1 Main features

The FAN5240 chip has a precision voltage reference and a special integrated compensation circuit inside, which can provide excellent static noise and main voltage dynamic regulation characteristics. In addition, the chip's internal regulator adopts a special circuit design, which can balance the two-phase circuit to achieve the best efficiency.

FAN5240 can monitor the output voltage of the circuit during the startup process. The chip has a PGOOD pin. When the circuit soft-start is completed and the output voltage is normal, the PGOOD pin will send a power ready signal to indicate that the entire circuit is working normally. The soft-start delay time can be determined by an external capacitor outside the PG00D pin.

The overvoltage protection (OVP) circuit inside the FAN5240 can effectively prevent the output voltage of the low-end MOSFET in the circuit from exceeding the set value, while the overcurrent protection circuit in the PWM controller can monitor the load condition of the converter by alternately detecting the voltage drop on the low-end MOSFET. The overcurrent threshold can be set by an external resistor. If more precise overcurrent monitoring is required, it can be achieved by selecting a high-precision external current detection resistor.

The main features of FAN5240 are as follows.

The CPU main power supply voltage output range is 0.925～2.000V;

With ±1% precision superheat reference;

The output voltage can be set dynamically through a 5-bit DAC;

The input voltage range is as wide as 6~24V;

The two channels in the circuit can be switched alternately to optimize efficiency;

Can effectively reduce the size of output capacitor;

Adopt remote differential voltage detection technology;

Adopting voltage feed-forward and average current mode control technology, it can obtain excellent dynamic response;

With dynamic duty cycle clamping function, it can effectively reduce the increase of inductor current;

The current detection resistor can be used to accurately detect the current on the low-end MOSFET;

With failure protection functions such as overvoltage protection, overcurrent protection and overheating shutdown;

With functions such as device enable, power ready, power ready delay and forced PWM control;

1.2 Pin Function

The FAN5240 controller has 28 pins, as shown in Figure 1. It has two package types: QSOP28 and TSSP28. The pin functions of FAN5240 are as follows.

Pins 1 and 27 (LDRYV2, LDRY1) are the low-side MOSFET drive inputs of the two channels respectively, and should be connected to the gates of the two external low-side N-channel MOSFETs for synchronous operation. At the same time, the connection between this terminal and the MOSFET gate should be as short as possible.

Pins 2, 26 (PGND2, PGND1) are power grounds, connected to the sources of the two low-side MOSFETs respectively.

Pins 3 and 25 (BOOT2 and BOOT1) are the on-chip bootstrap for the high-side MOSFETs of the two channels respectively.

Pins 4 and 24 (HDRV2 and HDRV1) are the high-side MOSFET drive inputs of the two channels respectively, and should be connected to the gates of the high-side N-channel MOSFETs of the two external channels respectively. At the same time, the connection between this terminal and the MOSFET gate should be as short as possible.

Pins 5 and 23 (SW2 and SW1) are the switch voltage output ports of the two channels, respectively. They should be connected to the source of the high-side MOSFET and the drain of the low-side MOSFET of the two channels.

Pin 6, 22 (ISNS2, ISNS1) Current sense input, used to sense the feedback current on the low-side MOSFET or current sense resistor in the circuit.

Pins 7~11 (VID4~VID0) are voltage identification code input pins. The output voltage can be programmed by TTL level or open collector input signal.

Pin 12 (FPWM) Forced PWM mode. When this pin is high, the chip will be forced into synchronous operation mode; when this pin is low, the device will enter delay mode.

Pin 13 (ILIM) is the current limit threshold setting terminal. When designing, the overcurrent threshold can be set by a resistor between this terminal and the ground terminal.

Pin 14 (EN) is the chip enable terminal. Leaving this pin open or pulling it up to VCC can enable the chip. In addition, when the circuit lock fails, the device can be reset by triggering the EN terminal.

Pin 15 (AGND) is the analog ground. This terminal is the signal reference ground terminal of the entire device. All voltages in the circuit are referenced to the zero level of this pin.

Pin 16 (DELAY) is the power ready or over-current protection delay setting terminal. Connecting a capacitor between this pin and ground can be used to set the power ready or over-current shutdown delay time.

Pin 17, 18 (VCORED.VCORE+) Main voltage output detection terminal, which can realize the monitoring and adjustment of power good, low voltage and overvoltage protection by differentially detecting the output voltage of these two ports. When designing, a resistor can be connected in series to the VCORE+ terminal to set the detection voltage drop of the output voltage.

Pin 19 (PGOOD) is the power good flag output. When the output main voltage is below 825mV, this pin outputs a low level. When the output main voltage rises above 875mV, the delay time from the low level to the high level of the PGOOD terminal is completely determined by the capacitance between DELAY and ground.

Pin 20 (SS) Soft-start pin. The soft-start rate can be programmed by connecting an external capacitor between this pin and ground.

Pin 21 (VIN) Battery voltage input, used for fast compensation of on-chip oscillator input voltage transients.

Pin 28 (VCC) is the power supply terminal of the device. When the voltage of this pin rises above 4.6V, the device starts to operate; when the voltage of this pin drops below 4.3V, the device shuts down.

FAN5240 can work in two working modes, the first mode is fixed frequency PWM mode, the second mode is variable frequency delay mode depending on the load. When the load current is lower than the peak current of the filter inductor in the circuit, the circuit will work in variable frequency delay mode; and when the current of the filter inductor is restored, the circuit will return to PWM working mode. The conversion from PWM mode to delay mode can improve the switching frequency of the circuit under light load and extend the battery life.

1.3 Main parameters

Power supply operating voltage range (Vcc) 4.75～5.25V;

Input voltage range (VIN) 6～24V;

VID high level input voltage>2.0V;

VID low level input voltage

Programmable output voltage range 0.925～2.000V;

DC output voltage accuracy ±1%

Working frequency 300kHz;

Overvoltage protection delay time 2μs;

Power ready delay time 12ms;

Ambient operating temperature -20℃～+85℃

Storage temperature range: -65℃～+150℃;

10-second welding limit temperature 300°C;

Thermal shutdown temperature 150°C.

1.4 Output voltage programming

FAN5240 is a 2-phase single-output power management chip that can provide low voltage and high current output for new processors in notebook computers. With few external connections, FAN5240 can control the precision programmable synchronous converter to drive the external N-channel power MOSFET. Its output voltage can be set between 0.925 and 2.000V through different logic combinations of 5 pins such as VID0~VID4. When the output voltage is set between 0.925 and 1.300V, the setting step is 25mV; when the output voltage is set between 1.300 and 2.000V, the setting step is 50mV. The specific setting method is listed in Table 1.

2 CPU power supply circuit based on FAN5240

A CPU main power circuit composed of FAN5240. In Figure 2, when the capacitor C11 outside the SS pin is 0.1μF, the soft start delay time of the circuit is 5.4ms; and when the value of the capacitor C10 between DELAY and ground is 22nF, the delay time from low level to high level of its PG00D terminal is about 12ms. Table 2 lists the specific parameters of each component in the circuit. The following is a specific description of the specific selection of several major peripheral components of the circuit.

2.1 Selection of Power MOSFET

MOSFET tubes should meet the following aspects.

(1) Have a low drain-source on-resistance. The RDS(ON) of the selected MOSFET should be at least less than 10mΩ and the lower the better;

(2) Choose a device package with better temperature performance:

(3) The rated drain-source voltage should be greater than 15V;

(4) The selected MOSFET should have a low gate charge, especially when operating at high frequencies.

For low-side MOSFET, the first thing to consider when selecting is the on-resistance RDS(ON), because its high-side duty cycle is relatively small, and the on-resistance has a greater impact on the power consumption of the low-side MOSFET, which will affect the DC/DC conversion efficiency of the circuit. For the circuit in Figure 2, since the output current required is large, two low-side MOSFETs are used in each channel of the circuit. S2~S3 and S5~S6 in this design are selected from Fairchild's FDS6676S MOSFET tubes, which have an on-resistance RDS(ON) of 6mΩ.

When selecting a high-side MOSFET, its gate charge and on-resistance are equally important. This is because the gate charge of the high-side MOSFET will affect the switching speed and thus the power consumption. Therefore, the gate charge and on-resistance of the device should be considered comprehensively. In fact, for high-current output applications, if the switching frequency of the circuit is high, the high-side MOSFET can also be designed using two MOSFETs. The circuit in Figure 2 uses a FDS6694 from Fairchild as the high-side MOSFET.

2.2 Inductor selection

This circuit uses two output inductors, and the two output inductors are distributed on each channel. The main function of the output inductor is to reduce the output voltage ripple. However, a larger inductor will not only increase the system cost, but also increase the valuable circuit board space. In addition, the selection of the output inductor must also consider the switching operating frequency, input voltage and output voltage of the circuit. When the operating frequency is 600kHz (300kHz per channel), the input voltage is 20V, and the output voltage is 1.5v, the output inductor is about 1.6μH.

2.3 Setting of current limiting resistor

The setting of the current limiting resistor R4 in the circuit should take into account the current detection resistor RSENSE in the circuit, the on-resistance RDS(ON) of the MOSFET and the current limiting threshold of the circuit. If you want to set the current limiting threshold of the circuit at a level of about 42A, and the current detection resistor is 1kΩ, then for the MOSFET on-resistance RSD(ON) of 3mΩ per channel, the value of the current limiting resistor R4 should be 56kΩ. The specific calculation formula is as follows.

R4=7.2RSENSE/ILIMT RDS(ON)

3 Conclusion

Compared with previous DC/DC converters, the main feature of FAN5240 is that its output voltage can be programmed through the 5-bit DAC on the device instead of being set by a resistor divider. Therefore, in addition to the high output voltage accuracy and large output current, FAN5240 also has all the other functions and features of previous resistor divider DC/DC converters, so it can be used to design stable, reliable and high-precision power supply systems for new low-voltage and high-current microprocessors.