[introduction]Capacitance is having main effect in design of high speed PCB, also be the parts of an apparatus that uses at most on PCB normally. In PCB, capacitance divides capacitance of the wave that it is filter normally, go Ou capacitance, store can capacitance. We call the input of power source module, capacitance that outputs loop normally filter wave capacitance. Understand simply even if, assure the capacitance with steady power supply of input, output. In power source module, the principle that filter wave capacitance puts is ” first big hind small ” . If the graph is shown 2.48.1 times, filter wave capacitance presses arrowhead direction first big hind small put.

Capacitance is having main effect in design of high speed PCB, also be the parts of an apparatus that uses at most on PCB normally. In PCB, capacitance divides capacitance of the wave that it is filter normally, go Ou capacitance, store can capacitance.

1.Power source outputs capacitance, filter wave capacitance

We call the input of power source module, capacitance that outputs loop normally filter wave capacitance. Understand simply even if, assure the capacitance with steady power supply of input, output. In power source module, the principle that filter wave capacitance puts is ” first big hind small ” . If the graph is shown 2.48.1 times, filter wave capacitance presses arrowhead direction first big hind small put.

When power source is designed, should notice line and cupreous skin are enough wide, measure crossing aperture is adequate much, assure to connect shed ability to satisfy requirement. Width and bulk of electric current of union of amount crossing aperture will evaluate.

Power source inputs capacitance

Power source inputs capacitance and switch annulus road to form link of an electric current. The metabolic range of road of annulus of this electric current is big, the extent of Iout. Frequency is switch frequency. DCDC chip arises in switch process, the change of the electricity that annulus of this electric current produces, included faster Di/dt.

The means of synchronous BUCK, method of follow on current should pass the GND pin of chip, input capacitance should receive the GND in chip and Vin between, method unexpectedly likely short thick.

Area of annulus of this electric current is ample small, annulus of this electric current external radiation can have been jumped over.

2.Go Ou capacitance

The power source of high speed IC cites a base need is enough go more Ou capacitance, best can make sure every bring a foot to have. In be being designed actually, if do not have a space to put,go Ou capacitance, OK cutout of take into consideration the circumstances is decreased.

IC power source cites a base go the look value of Ou capacitance can be compared normally small, wait like 0.1 μ F, 0.01 μ F; Enclose correspondingly smaller also, be like 0402 enclose, 0603 enclose etc. Putting go when Ou capacitance, should notice the following.

(1) stand by power source to bring a foot to place as far as possible, may remove less than going to otherwise Ou action. Theoretic tell, capacitance has go certainly Ou radius limits, answer to carry out nearby principle strictly so.

(2) go Ou capacitance makes crural catalyst to power source as far as possible short, and down-lead should be added thick, the line is normally broad for 8 ～ 15mil (1mil = 0.0254mm) . Add thick purpose to depend on reducing down-lead inductance, assure power source function.

(3) go to the power source of Ou capacitance, ground citing a base from solder dish after derivative line, nearby stiletto, join arrives on power source, ground level. This down-lead should be added likewise thick, cross aperture to use large hole as far as possible, if can use the opening of bore diameter 10mil, need not the aperture of 8mil.

(4) assure to go Ou annulus road as far as possible small. Go if pursue,Ou capacitance puts give a demonstration commonly 2.48.2 ～ graph is shown 2.48.4 times. Graph 2.48.2 ～ graph is shown 2.48.4 times is the IC that SOP encloses goes of Ou capacitance put way, QFP those who enclose is as similar as this.

Common BGA is enclosed, its go Ou capacitance is put below BGA normally, namely rear. Because BGA encloses the density that cite a base big, because this goes to what Ou capacitance puts commonly,not be very much, but should put more as far as possible a few, if pursue,2.48.5 are shown.

3.Store can capacitance

Store can when the action of capacitance makes sure IC is using phone namely, electric energy can be provided inside the shortest time. Store can the look value of capacitance is compared commonly big, enclose correspondingly bigger also. In PCB, store can capacitance can leave parts of an apparatus a few further, but also cannot too far, if pursue,2.48.6 are shown. Common store can means of capacitance fan aperture, if pursue,2.48.7 are shown.

Principle of line of capacitance fan aperture, fan is as follows.

(1) down-lead as far as possible short and add thick, have lesser parasitism inductance so.

(2) to store can capacitance, perhaps cross electric current to compare big parts of an apparatus, answer when stiletto as far as possible many dozens a few.

(3) of course, the fan aperture with electric best function is Pan Zhongkong. Need integrated consideration actually

(Author: Zhu Xiaoming, origin: Hardware 100 thousand why)

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