Normally, nicety measures a system to need low pressure manostat (the one part that LDO) regards his as power source plan, use it to produce course of low noise power source for accurate ADC. However, the power output efficiency of LDO is very low, major power often regards caustic of quantity of heat as lose. Article discussion is approached for accurate successive model (SAR) ADC implementation is more efficient the way of power source solution. Implementing a method is use below arrearage mode exceed manostat of switch of low power comsumption, analyse —— of performance gain and loss to include to come through some kind of means manostat of intelligent control switch, those who make is as synchronous as SAR changeover, improve noise performance thereby.

 

Be in in tall load current (the hundreds MA measurement to several A) in the system, wide modulation of fixed frequency or arteries and veins (PWM) switch manostat but effectively (often be more than 90%) to produce power source course. However, although efficiency is tall, but cost is to be able to have switch ripple, its frequency is fixed normally, arrive from hundreds KHz several MHz. If the graph is shown 1 times, the power source of typical and accurate SAR ADC is restrained than (when making an appointment with 100 KHz, PSRR) comes in low frequency is first-rate when —— exceeds this frequency, PSRR drops quickly.

 

Graph 1. Power source of SAR ADC imitate controls the concern with frequency

 

Accurate SAR ADC with inferior when taking in and send out in large quantities rate moves, the typical load current that supplies VDD line is counting MA or μA limits —— accordingly, compare at LDO, use manostat of fixed frequency switch to be ADC power supply to do not have an advantage on efficiency directly.

 

However, efficient, exceed manostat of switch of step-down of low power comsumption to be able to leave the job in arrearage mode, electric current of its static state is very small.

 

Below arrearage mode, through adjusting constant electric current of peak value inductance, manostat uses PWM pulse to make output voltage summary prep above is nominal output voltage. When output voltage raises output to detect when signal exceeds arrearage upper limit, manostat is entered bide one’s time mode. In bide one’s time below mode, high end and low end MOSFET and major circuit are banned with, static electric current is very small, efficiency performance is very high, if pursue,2 are shown. Bide one’s time during mode, output electric capacity sends load energy, output voltage is reduced under arrearage comparator floor level till. Manostat is waked up, produce PWM pulse, charge to output again.

 

Graph 2. The graph on PWM() with arrearage mode (next graphs) the relation of —— efficiency and load current

 

Below arrearage circumstance, switch ripple frequency and load current and LC network are concerned; To counting the load of MA, its are inside KHz limits. When several KHz, the PSRR of accurate ADC is first-rate, can restrain well / the switch ripple that output of attenuation ADC number carries.

 

To in an attempt to shows circuit 3 times to be exemple, it uses AD7980ADC; In take in and send out in large quantities completely rate (when 1 MSPS) , its VDD electric current uses up typical cost to be 1.5 MA; If reduce,take in and send out in large quantities rate, electric current is used up meeting in proportion linear drops. This but from the graph 4 see: When using power supply of course of 5 V voltage, the switch frequency ripple that output of 2.5 V stabilized voltage carries is 4.5 KHz and peak value of 50 MV peak. End is outputted in ADC number, ADC with PSRR rating attenuation this ripple. In ADC FFT output, it behaves the stray that is 4.5 KHz of extent −120 DBFS, frequency. Input limits to the 5 V of ADC, this is equivalent to

 

 

Graph 3. AD7980 and ADP5300 use circuit

 

Graph 4. The arrearage switch ripple when ADP5300 is AD7980 power supply (communication coupling) , and 1 MSPS takes in and send out in large quantities the ADC FFT when rate outputs medium grain Boeing

 

The level of this kind of ripple that appears in ADC to output end is opposite is low-down 16 converter; The 0.07 LSB that correspondence of peak value of 5 μV peak issues at 16. The ripple of this kind of level can be buried in bottom of ADC a confusion of voices, need all is worth operation ability to discover in great quantities, won’t see it in a lot of application. The PSRR of correspondence of this output ripple is

 

 

This requirement and graph show likeness 1 times, AD7980 is 77 DB about in the PSRR when 4.5 KHz.

 

If ADC takes in and send out in large quantities rate reduces 10 KSPS, criterion the electric current of ADC is used up in proportion linear drops to make an appointment with 100 times) to 15 μA (, the switch frequency ripple of ADP5300 reduces 46.5 Hz (accordingly to make an appointment with 100 times) , extent is peak value of 55 MV peak, if pursue,5 are shown. When 46 Hz, this ripple appears in ADC FFT output again, extent is) of peak value of peak of −120 DB (5 μV, because the PSRR in this frequency is similar. Evidence makes clear 2 harmonic of existence 93 Hz, its extent is lower, for −125 DB.

 

Graph 5. The arrearage switch ripple when ADP5300 is AD7980 power supply (communication coupling) , and 10 KSPS takes in and send out in large quantities the ADC FFT when rate outputs medium grain Boeing

 

Graph 6 contrast showed ADP5300 and LDO take in and send out in large quantities in different ADC the efficiency when rate, two kinds of circumstances all use power supply of course of 5 V voltage, adjust 2.5 V output. Like anticipating together, switch manostat carries the efficiency outclass LDO of power, be 90% pairs of 50%(input) in the light of 5 V when 1 MSPS, take in and send out in large quantities in inferior ADC rate / it is better also to when inferior electric current is used up, maintain, from beginning to end prep above 80% , till 5 KSPS.

 

Graph 6. The efficiency of ADP5300 and LDO and ADC take in and send out in large quantities the relation of rate

 

ADC takes in and send out in large quantities rate is 1 MSPS and when use LDO, the electric current that uses up from course of 5 V voltage is 1.5 MA or 7.5 MW. When use ADP5300, the electric current that uses up from course of 5 V voltage is 828 μA or 4.1 MW, namely the power comsumption of ADC power source reduces 3.4 MW or 45% .

 

Use ADP5300 regards VDD power source and 10 KHz as signal of input of close full span (−0.5 DB) , AD7980 is like a graph in the function when 1 MSPS 7 are shown. In SNR (91.5 DB) and respect of THD (−103 DB) , ADC still accords with data manual norms. However, the ripple of 4.5 KHz switch of ADP5300 is inputting signal aspirant travel modulation, show for 10 KHz–The stray of 4.5 KHz (5.5 KHz) and 10 KHz + 4.5 KHz (14.5 KHz) . These stray still are in low-down level (−116 DBFS) , far be less than base the THD(that 2 harmonic of wave signal introduce its are −103.8 DBFS) when 20 KHz. When 16 levels, these bogus resemble is a LSB only is very small one part, it is consequently in a lot of application, considering the advantage omiting report of ADP5300 manostat, this can be accepted completely.

 

Graph 7. The function when AD7980 uses ADP5300 to regard VDD as power source base the Bian Pin that can see switch ripple modulation is caused all round wave signal is taken (10 KHz ±4.5 KHz)

 

ADP5300 switch manostat has a STOP (to stop) switch is characteristic, it can eliminate switch ripple bogus completely to resemble, make its won’t appear in ADC FFT output. When STOP is in tall n, the STOP of ADP5300 brings a foot to be able to prevent SW to make crural switch. In sensitive to noise ADC transition, can use this character to prevent switch happening. To realize this one function, CNV signal and STOP signal should be together repeatedly (refer to graph 3) , the CNV signal that and be opposite comes from processor undertakes timing, make its maintain tall n when ADC is changed. To AD7980, this time maximum is 710 Ns, changeover is started in CNV ascendant edge. If pursue,as a result 8 are shown. The ripple frequency below this kind of circumstance more change is errant, because be in only specific time, SW node just is met open and adjust. Ask an attention additionally, turn into from STOP signal low n may be hundreds Ns to the time of SW open. Graph 8 in, STOP drops after the edge about 850 Ns, SW brings crural open. This is meant, when ADC takes in and send out in large quantities rate is 1 MSPS, we cannot use STOP character, because SW brings a foot to will turn into without enough time,tall n adjusts, VDD power source will be lost adjust and break down. STOP function is in take in and send out in large quantities rate is 500 KSPS or lower from time to tome effect.

 

Graph 8. The ADP5300 switch ripple that STOP function uses when 500 KSPS (yellow) , CNV/STOP signal (blue) the SW that reachs ADP5300 cites a base (pink)

 

From the graph 9 can see, after use STOP signal, in bottom of ADC a confusion of voices stray of ripple of completely nonexistent switch. Should bring to bear on when the input signal of 10 KHz, except general harmonic, base there is modulation all round wave. However, bring a foot to ban when SW with (STOP is tall n) when, because SW brings the ringing on the foot, the efficiency of use STOP character can drop. Take in and send out in large quantities to the ADC of 500 KHz rate, the efficiency of ADP5300 falls to about 75% . The efficiency that LDO of this remarkable still prep above can achieve (<50%) , and offerred > of a power comsumption

 

Graph 9. AD7980 uses the property when ADP5300 regards VDD as power source, STOP characteristic sequential undertakes adjustment according to changing cycle

 

The switch manostat solution such as ADP5300 and LDO solution are had in cost and PCB area respect certain can compare a gender. BOM basically increases an inductance, 2.2 μH inductance is OK small to 0603 enclose, and to input and outputting capacitance, LDO solution needs likewise. Accordingly, in power comsumption sensitive model in application, it is the strong replacement scheme of LDO, do not have apparent effect to precision function.

 

When waiting for accurate ADC power supply for AD7980 from course of 5 V power source, ADP5300 efficient, exceed manostat of switch of low power comsumption and LDO photograph to compare, can save the power comsumption of 45% . This applies to content couplet net (lengthen wireless sensor node to may apparel the battery service life of equipment) , power comsumption is sensitive model system of segregation type industry, 4 MA to road of 20 MA annulus power supply system has a lot of profit.