"FPGA full-step---Practical Walkthrough" chapter 21st Power supply Common type: Ldo and DCDC

Design of power supply in high-speed circuit

The design of power supply in high-speed circuit is probably divided into two kinds, one is lumped architecture and one is distributed architecture. A lumped architecture is a power input that generates a variety of required voltages. As shown in 1. This architecture will increase the number of DC modules, so the cost is not controllable, the PCB area also needs to increase, but the lumped distribution architecture can improve the overall power conversion efficiency.

Figure 1 Total distribution architecture of the set

A distributed architecture is the voltage required to generate an intermediate voltage from a module before it is converted to another single board, as shown in 2. The first level of output can require a large number of noise and ripple, the second level of power output required by a variety of power, you must consider the ripple and noise problems. But there is also a problem with the distribution of multi-level conversion, may be less efficient than the lumped distribution architecture.

In high-speed circuit design, it is recommended to adopt distributed architecture.

Figure 2 Distributed architecture

Ldo Power Type

Figure 3 is an internal LDO structure diagram. Vout is captured by the R1 and R2 two resistors and then sent to op amp A, where the voltage value of the v+ is compared to Vref, and the difference is amplified by the OP amp and sent to the transistor or MOSFET to achieve the adjustment to Vout, basically based on the feedback principle, The change in VIN and the change in load will have an effect on vout.

The Ldo power chip needs to note that the pressure difference (dropout), the larger the load current if necessary, its pressure should be greater.

Another problem with the Ldo power supply chip is that the power dissipation is too high, P = (vin-vout) *i, in the design of the PCB is best to make some vias at the bottom of the ldo to increase heat dissipation.

The Ldo chip should also be aware that the ground current (GND Pin is the current) Ignd this refers to the quiescent current, which is the internal consumption of the Ldo.

when calculating the Ldo power consumption, p should include static and dynamic power dissipation, dynamic power dissipation PD = (vin-vout) *i, static power PS = Vin *ignd (this current is the internal consumption of current)

Figure 3 LDO Internal structure diagram

In the design of power, the power should be placed in a region, for multilayer boards, its adjacent layer is best not to go high-speed signal line, to avoid the chip of the Ldo chip.

The filter of the Ldo power supply chip is recommended to use a tantalum capacitor with a larger ESR and a smaller ESR ceramic capacitor, or separate the noise with a ceramic capacitor with a smaller ESR. It is not recommended to replace tantalum capacitors with aluminum electrolysis because the ESR of the aluminum electrolytic capacitors is too large to exceed the Ldo's maximum ESR requirements.

DCDC power supply Type

The DC-V power supply is modulated by PWM (pulse width modulation), PFM (pulse frequency modulation), and PWM and PFM mixing. The current market above is based on PWM majority.

The DCDC circuit for buck is called the buck Circuit, and the circuit for the purpose of boosting is called the boost circuit, and the DCDC circuit is called the Buck-boost circuit for the purpose of the inverting phase.

DCDC Power supply Ripple Noise

In the high-end chip tends to the power supply ripple and noise have certain requirements, ripple general control within 1% of the supply voltage, noise control in the supply voltage 3%~5%. The ripple and noise of DCDC is always a difficult problem in circuit design.

Ripple is the low-frequency component of the power supply fluctuation component, the frequency band within 5MHz, produces the switch action from the MOSFET.

Noise refers to the high frequency components in power supply fluctuations, the general frequency is higher than 5mhz,mosfet switching noise, random white noise, and surrounding interference.

There are two ways to deal with Ripple and noise: 1) absorption filter, composed of magnetic bead and capacitance combination circuit, 2) reflective filter, filter circuit composed of Π,T,LC, the reflective type does not remove noise fundamentally, but returns the noise to the source end. As shown in 4.

Figure 4 LC Filter Circuit

DCDC Power output

1) Sometimes the inductance will produce a noise, which may be caused by the inductance of the current is too large, the improvement method, the input increases the power-up slow circuit, the output reduces inductance value. This slow-up circuit can be built using RC. t = 2.2 RC.

2) in the DCDC circuit, should not be a simple focus on the output voltage section, but also to ensure that the input voltage in the power-up process can not be too slow, nor too fast.

3) in the design of the power supply to determine the large current circuit, in this way, noise, ripple, current are the largest, need to be aware that can not be connected to other three of the above impact of the signal pins.

Fuse

1) The rated current is the maximum current that can withstand the normal operation of the safety tube. Insurance Tube selection can not be selected too big, also can not choose too small, too big on the circuit unprotected, too small to the normal work of the circuit has an impact. The fuse should be used when using the rated current derating to 75%. When selecting the rated current of the fuse, the maximum current IMAX (full negative load) required in the entire system should be calculated first, and after determination, the rated current of the fuse is selected for imax/75% use.

2) DC resistance P = I2 R, if the power dissipation is too large, consider the thermal problem.

3) The rated voltage refers to the maximum voltage that can be sustained when the fuse is blown, and should generally be larger than the entire system input voltage. According to the UL standard, when choosing the rated voltage of the fuse, should reduce the amount 80%, even if system normal operating voltage is U, the fuse rated voltage must be at least u/80%;

4) Pressure drop, if the chip requires a voltage of u, fuse voltage drop is u, then the power supply input voltage should be U + u to meet to go.

5) Thermal value i2t refers to the thermal energy required for fuse fusing.

"FPGA full-step---Practical Walkthrough" chapter 21st Power supply Common type: Ldo and DCDC