Article analysis and assessment target are to use manostat of step-down of voltaic mode pilot, because it is the most commonly used in application. Signal analysis is character of noise of understanding switch ripple, current broadband noise (reach its origin) , the main way of the noise of high frequency aiguille that switch causes. PSRR of manostat of article general discussion switch (power source is restrained than, its are very important to inputting noise to restrain) and signal analyses a method.

 

Switch ripple noise

 

Cadre divides theory of wave of harmony of basis radical wave to introduce step-down converter to output ripple computation formula. According to switch manostat develop attacks structure and main operation, ripple is the main noise in switch manostat from beginning to end, because range of voltage of peak peak value is general,arrive for a few MV a few MV. It should be regarded as periodic and the signal that can forecast. If work in order to secure switch frequency, pass oscillograph in time domain, or decompose through Fu Lixie in frequency domain, identify its very easily and undertake metrical.

 

The graph shows the step-down manostat that is a model 1 times. Two switch put through alternately and disconnect, because VSW of voltage of this SW node is an ideal square wave, this character is delivered then occupy sky comparing and input voltage. VSWVSW can express with the formula below:

 

Graph 1. Step-down manostat develop attacks

 

Among them:

 

VIN is input voltage. D is compared for nothing to occupy; To step-down manostat, its are equal to VOUT/VIN

 

After VIN is affirmatory, the VSW base wave and harmonic composition depend on only occupy sky comparing. Graph 2 showed with occupy the VSW related sky comparing base extent of wave harmony wave. When occupying sky comparing to approach an in part, ripple extent with base wave give priority to.

 

Graph 2. Extent of step-down manostat VSWW concerns with what occupy sky comparing

 

Class of LC of step-down manostat output transfer function is as follows:

 

 

Among them, l is worth to output inductance, DCR is inductance resistance, CL is value of capacitance of inductance paralell connection.

 

COUT holds value of a quantity for output. ESL is value of inductance of capacitance series connection. ESR is resistance of capacitance series connection.

Accordingly, VOUT can express as follows:

 

 

Calculate to simplify, we output LC class to be Cheng of 20 DB/ decuple frequency suppose, it is next with occupy the VOUT related sky comparing extent of wave of harmony of ripple radical wave, if pursue,3 are shown. When occupying sky comparing to approach an in part, 3 or odd harmonic general prep above is even second harmonic. Because LC is restrained, taller harmonic will have inferior range, and be compared with photograph of total ripple extent, its proportion is very small. Same, base wave extent is the bases in ripple of switch manostat output.

 

Graph 3. Extent of ripple of step-down manostat VOUT concerns with what occupy sky comparing

 

To step-down manostat, base wave extent and input voltage, occupy frequency of sky comparing, switch and LC class to concern; But, all these parameter can affect application ask, be like dimension of efficiency and solution to wait. To reduce ripple further, the proposal increases postposition filter.

 

Broadband noise

 

The broadband noise in switch manostat is the random extent noise on output voltage. It can express with the noise density inside whole frequency limits, the unit is V/√Hz Z, or will express with Vrms, the density inside its and frequency limits is impartible. As a result of the limitation of silicon craft and design of filter of fiducial voltage source, the 10Hz that broadband noise basically is located in switch manostat comes inside 1MHz frequency limits, reduce its very hard inside low frequency limits through increasing filter.

 

Voltage of extent of peak value of peak of noise of broadband of typical step-down manostat comes for 100μV about 1000μV, far under switch ripple noise. If use additional filter to reduce switch ripple noise, criterion the main noise that broadband noise may make switch manostat output voltage. Graph 4 showed when should doing not have additional filter, the main source of noise of step-down manostat output is switch ripple. Graph 5 showed when should using additional filter, the main source that outputs noise is broadband noise.

 

Graph 4. Without the VOUT of additional filter

 

Graph 5. The VOUT that has additional filter (use 1000 times preamplifier to undertake metrical)

 

Output broadband noise to identify and analyse switch manostat, must obtain manostat to dominate plan and module noise news. For example, graph the manostat of 6 voltaic mode step-down that showed a pattern control plan and infuse of module noise source.

 

Graph 6. Manostat of step-down of mode of typical electric current dominates program

 

To acquisition operating ring way transfer function is mixed information of module noise character, have two kinds of different noise: Annulus road inputs the noise inside noise and annulus. The annulus road inside bandwidth of operating ring road inputs noise to be able to transmit output, and the noise besides annulus road bandwidth is met by attenuation. To switch manostat, EA of design low noise and fiducial voltage source are crucial, because of the unit feedback gain can maintain noise level changeless, is not voltage n increases and raise it as output. The biggest challenge is the biggest noise source in finding out whole system, reduce this noise in circuit design. ADP5014 undertook optimizing in the light of low noise technology, use voltaic mode to control plan and a simple LC exterior filter, come in 10Hz the noise performance under 20μVrms achieved inside 1MHz frequency limits. The output noise function of ADP5014 is shown 7 times like the graph.

 

Graph 7. The ADP5014 that uses additional LC filter outputs noise function.

 

High frequency aiguille and ringing

 

The 3rd kind of noise is high frequency aiguille and ringing noise, because output voltage is,guide by switch manostat connect or close twinkling to change of generation. Consideration silicon circuit and PCB take the parasitism inductance in the line and electric capacity; To step-down manostat, twinkling of fast electric current changes aiguille of high frequency voltage and ringing will be caused in SW node place. Aiguille and ringing noise can rise ladenly as electric current and rise. Graph 8 showed how step-down manostat forms aiguille. According to switch manostat guide / close to press place rate, highest aiguille and ringing frequency will come in 20MHz inside 300MHz limits, suffer parasitism inductance and capacitance effect, output LC filter is controlling an aspect may not be very effective. With afore-mentioned compare about conducting all discussion photographs of method, the poorest is the radiation noise that comes from SW and VIN node, because its frequency is very tall, output voltage and other imitate circuit can be affected.

 

Graph 8. Step-down manostat is high frequency aiguille and ringing noise

 

To reduce high frequency aiguille and ringing noise, the proposal uses effective method to carry out application and chip design. Above all, additional LC filter or magnetism should be used on terminal load bead. Normally, this meeting makes the aiguille noise on output far be less than ripple noise, but can increase more high-frequency composition. Next, ying Bing my SW and the noise source that input node or let its be far from output side and sensitive imitate circuit, and screen outputs inductance. Elaborate composition and wiring are very important to the design. The 3rd, those who optimize switch manostat guide / close to press place rate, reduce the parasitism inductance of switch manostat and resistance as far as possible, reduce SW node noise effectively thereby. ADISilentSwitchr® technology also conduces to through chip the design reduces VIN node noise.

 

Switch manostat PSRR

 

PSRR mirrors switch manostat to restrain noise of input power source to transmit the ability of output. The function of step-down manostat PSRR inside limits of this partial analysis low frequency. Voltage of output of influence of high frequency noise basically is to adopt radiation method, is not to pass in front discuss conduct method.

 

The step-down small signal that the graph shows the basis 9 times pursues, step-down PSRR can express as follows:

 

 

Graph 9. From the input report overwhelms the graph of small signal of voltaic mode step-down of output.

 

Among them:

 

 

signal pattern computation has comparative with emulation result. Small signal mode is effective, agree with emulation result.

 

The PSRR function of T switch manostat depends on the performance of annulus road gain inside low frequency limits. The inherent LC filter of switch manostat can restrain intermediate frequency limits (100Hz comes 10MHz) the input noise inside. The inhibition inside this limits performance is better than LDO PSRR much. Accordingly, switch manostat has good PSRR performance, because its have gain of road of tall be surrounded by mountains when low frequency, and inherent LC filter can affect intermediate frequency range.

 

Graph 10. Use the PSRR computation result of mode of step-down small signal

 

 

Graph 11. The PSRR of SIMPLIS mode is emulated

 

Conclusion

 

More and more imitate circuit, wait like ADC/DAC, clock and PLL, need the clean power source that can provide tall electricity. Every parts of an apparatus has diverse demand and norms to the power source noise inside different frequency limits. The switch manostat noise that is necessary to know different type in the round is cognitive requirement of power source noise, be designed thereby and realize manostat of switch of efficient, low noise, in order to satisfy the low noise norms of power source of most imitate circuit. Compare with LDO manostat, solution of switch of this kind of low noise will have taller effect to compare, smaller solution size and lower cost.