Converter of switch mode DC-DC is the most efficient power source, efficiency of some parts of an apparatus can exceed 95% , but its cost is power source noise, put in noise problem inside wider bandwidth limits normally. Differ linear adjuster with low pressure normally (LDO) is cleared the noise in power supply course, but also need, balance is made between power comsumption and addition systematic thermic load. To alleviate these problems, when use LDO, can make input and exist to press difference lesserly between output voltage (abundant measures voltage) the influence that the article aims to discuss low abundant to quantity voltage is restrained to power source and always output noise.

 

 

LDO power source is restrained than (the difference that —— makes rich quantity voltage points to input and output voltage related PSRR) and abundant quantity voltage. Measure voltage to securing abundant, PSRR as load current rise and reduce, below big load current and condition of voltage of small abundant quantity especially such. The graph is shown 1 times exceed linear adjuster of low noise, 2.5 V to be in for ADM7160 the PSRR below condition of voltage of 200 MA load current and 200 MV, 300 MV, 500 MV and quantity of 1 V abundant.

 

Reduce as what abundant measures voltage, PSRR also can be reduced, the likelihood that press difference becomes very big. For example, below 100 KHz, abundant quantity voltage turns into from 1 V 500 MV, the result will make PSRR reduces 5 DB. However, abundant measures the minor change of voltage, turn into from 500 MV 300 MV, the result can bring about PSRR to drop 18 DB above.

 

 

Graph 1. ADM7160 PSRR and abundant are measured
 

Graph the 2 block diagram that showed LDO. As the addition of load current, PMOS adjusts the gain of component to be able to be reduced, it breaks away from saturated condition, enter triode working area. The result makes gain of total annulus road is reduced, bring about PSRR to drop. Abundant quantity tension is less, gain falls bigger. As abundant quantity voltage continues to reduce to nod, right now, the gain of operating ring road falls to 1, PSRR falls to 0 DB.

 

Bringing about another factor that annulus road gain reduces is the resistor of the component in access, include FET guide electrify block, piece inside interconnection resistance and solder line resistor. Can calculate this resistor according to pressing difference to push. For example, what the ADM7160 that uses WLCSP to enclose falls in 200 MA is the biggest press difference to be 200 MV. Use Ohm’s law, the resistor that adjusts component is 1 Ω about, can adjust component to regard as approximately fixed resistor and alterable resistor the sum.

 

The presses poor subtractive FET leakage that the load current that has shed this resistor brings about extremely voltage of source pole job. For example, in 1ΩBelow FET condition, the load current of 200 MA can make leak extremely source pole voltage drops 200 MV. When the PSRR of the LDO that makes rich in estimation the quantity is 500 MV or 1 V, must consider those who adjust component to go up to press difference, because adjust,the working report squeezing ramming of FET has 300 MV or 800 MV only on border.

 

Graph 2. Low pressure differs the block diagram of adjuster
 

 

The client asks to apply an engineer to help them choose appropriate LDO normally, so that fall to produce X of low noise voltage from input voltage Y in condition of load current Z, but when setting these parameter, often oversight input and output voltage tolerance this element. As abundant quantity voltage cost becomes less and less, input and the tolerance that output voltage may be right working requirement causes tremendous effect. Input and the tolerance of the poorest condition that output voltage can bring about abundant to measure voltage to drop from beginning to end. For example, the output voltage below the poorest condition may be tall 1.5% , input voltage may be low 3% . When the adjuster that passes 3.3 V of drive of source of a 3.8 V, the poorest condition makes rich quantity voltage is 336.5 MV, far under desired value 500 MV. Below the circumstance that is 200 MA in load current of the poorest condition, the leakage that adjusts FET extremely source pole voltage has 136.5 MV only. Below this kind of circumstance, the PSRR of ADM7160 is likely far under nominal value 55 DB (when 10 MA) .

 

 

Client classics regular meeting is in with respect to LDO the PSRR below the mode that press difference consults applied engineer. In the begining, this is a logical question it seems that, but should see simplifying block diagram only, know this problem is without a meaning. When LDO works to differ mode at pressing, the alterable resistor part that adjusts FET is 0, those who output voltage to be equal to input voltage and the load current that carry the RDSON that adjusts FET to bring about press falling difference. LDO does not undertake modulatory, and the noise that will restrain an input to carry without gain; Just act as a resistor. The RDSON of FET and output electric capacity form filter of a RC together, offer a few leftover PSRR, but magnetism bead can finish a simple resistor or ferrite same job, and more economy is efficient.

 

 

In low abundant quantity job mode falls, need consideration abundant measures the voltage effect to PSRR, otherwise, can bring about prep above of level of output voltage noise to anticipate. If pursue,curve of 3 shown PSRR and relation of abundant quantity voltage can be normally in data manual find, and can use give the noise that can come true below the condition to control an amount certainly.

 

Graph 3. PSRR and abundant measure the impact of voltage
 

However, occasionally, the PSRR through revealing LDO how effective filter divides the noise in source voltage, can see this kind of information easily use value. When the graph below revealed LDO to measure voltage to fall in different abundant, to always outputting the influence of noise.

 

Graph 4 those who reveal is 2.5 V ADM7160 in 500 MV abundant is measured and 100 MA laden condition falls, relative to the output noise at E3631A table power source, this table power source comes in 20 Hz the rated noise inside 20 MHz limits under 350 μV-rms. A lot of stray under 1 KHz are the harmonic related to commutate of frequency of 60 Hz circuitry. The wide stray of 10 KHz above comes from generation to output the DC-DC converter of voltage finally. The stray of 1 MHz above results from in the environment with source of RF of irrelvant of power source noise. Come in 10 Hz inside 100 KHz limits, these test place are 56 μV Rms with the actual measurement noise of power source, contain stray to be 104 μV. LDO restrains all noise on power source, output noise is 9 μV-rms about.

 

Graph 4. Density of ADM7160 noise spectrum (abundant quantity is 500 MV)
 

When abundant volume voltage drop comes when 200 MV, be close to 0 DB as high frequency PSRR, the noise stray of 100 KHz above begins to cross bottom of a confusion of voices. Noise rises slightly to 10.8 μV Rms. As abundant the volume falls to 150 MV, rectification harmonic begins an influence to output noise, latter rises to 12 μV Rms. Be in about the peak value that 250 KHz manages occurrence extent moderate, consequently although the addition of total noise is measured not big, way of Dan Min electrification gets possibly also adverse effect. As abundant quantity voltage drops further, function is affected, the stray related to rectification begins to be shown in noise spectrum. The graph shows the output that makes rich for 100-mV quantity condition falls 5 times. Noise already rose to 12.5 μV Rms. Harmonic place contains energy few, accordingly, stray noise just increases somewhat, for 12.7 μV Rms.

 

Graph 5. Density of ADM7160 noise spectrum (abundant quantity is 100 MV)
 

When making rich the quantity is 75 MV, output noise is affected badly, rectification harmonic appears in whole spectrum. Rms noise rises to 18 μV Rms, noise and stray the sum rise to 27μV Rms. The noise that exceeds ~200 KHz range by attenuation, because LDO annulus road does not have gain, act as filter of a passive RC. When making rich the quantity is 65 MV, ADM7160 uses pattern of the job that press difference. If the graph is shown 6 times, the output voltage noise of ADM7160 actually as identical as input noise. Now, rms noise is 53 μV Rms, noise and stray the sum are 109 μV Rms. The noise that exceeds ~100 KHz range by attenuation, because LDO acts as filter of a passive RC.

 

Graph 6. The noise frequency spectrum density that ADM7160 issues in the mode that press difference
 

 

If ADM7150 exceeds adjuster of low noise, tall PSRR couplet of a kind new-style LDO actual superior two LDO, accordingly, the PSRR that gets as a result is each class the sum about. These LDO requirements make rich high slightly quantity voltage, but the PSRR that can exceed 60 DB in the implementation below 1 MHz condition, below inferior frequency, PSRR is OK Yuan Chao 100 DB. The noise frequency spectrum density that the graph shows the ADM7150 that is a 5 V 7 times, its load current is 500 MA, make rich the quantity is 800 MV. 10 Hz comes inside 100 KHz limits, output noise is 2.2 μV Rms. As abundant the volume falls to 600 MV, rectification harmonic begins to show, but when output noise rises to 2.3 μV Rms when, its are very small to the influence of noise.

 

Graph 7. Density of ADM7150 noise spectrum (abundant quantity is 800 MV)
 

When making rich the quantity is 500 MV, can see rectification harmonic and peak value apparently in 12 KHz place, if pursue,8 are shown. Output voltage noise rises to 3.9 μV Rms.

 

Graph 8. Density of ADM7150 noise spectrum (abundant quantity is 500 MV)
 

When making rich the quantity is 350 MV, LDO uses pattern of the job that press difference. Right now, LDO also cannot adjust again output voltage, act as a resistor, output noise rises to close 76 μVrms, if pursue,9 are shown. The RDSON that has FET only and the utmost attenuation that output the capacitance of end to form input noise.

 

Graph 9. The noise frequency spectrum density that ADM7150 issues in the mode that press difference
 

 

Contemporary LDO divides the noise in power supply course increasingly with Yu Qing, these power supply course pass the switch conditioner that can play generation noise in more broadband chart to come true normally. Switch adjuster forms these voltage course with superhigh efficiency, but itself bad news can LDO can reduce noise already, also can bring about efficiency to drop. Accordingly, the working abundant that should reduce LDO as far as possible measures voltage.

 

The place before be like is narrated, the PSRR of LDO is load current and the function that abundant measures voltage, can measure voltage along with the addition of load current or abundant decrease and decrease, because, the working place that adjusting a canal changes from saturated work area to triode working zone time, annulus road gain can drop.

 

Input tolerance of source noise character, PSRR and the poorest condition through the consideration, stylist can optimize power comsumption and output noise, for sensitive model imitate circuit makes an efficient low noise power supply.

 

In abundant quantity voltage exceeds low requirement to fall, input and the tolerance of the poorest condition that output voltage may form an influence to PSRR. Tolerance of mature when the design the poorest condition can ensure reliable design, the has inferior PSRR power source solution that devises otherwise, its total noise also is met prep above anticipates.