The first circuit is a capacitive charge-pump design based on a TC1044S<\/a> charge pump. I chose the Microchip product over the industry standard 7660 device because it is cheap, it works with a supply voltage up to 12V, and you can double the switching frequency of the original to 20kHz. Here’s the schematic:-<\/p>\n The TC1044S – along with D3, D4, C7 and C8 – forms a conventional charge-pump voltage doubler as figure 8 on page 6 of the Microchip data sheet.\u00a0 In this case we have added a further six stages of charge pumpage to give a no-load voltage of about 63V at the cathode of D16.\u00a0 However, as soon as you load up the output of the charge pump, the voltage starts to droop – so we need no-load voltage above 48V and linear regulator to iron out the droop.<\/p>\n The regulator is an LM317AHV (Fairchild) or LM317HV (National Semiconductor), which is the high-voltage version of the ‘317<\/strong>.\u00a0 Also, the LM317 is a floating design so – given that the LM317 drop-out voltage is about 1.5V – as the input voltage drops below 49.5V the output tracks at approximately 1.5V less than the input voltage. Consequently, if you do run the circuit on batteries then as the input voltage drops below 9V, so the output voltage falls off in step. D3~D16 can be any small signal Schottky diode (the lower the Vf, the better). D1 and D2 can be almost any general rectifier at all, e.g. 1n914, 1n4148, 1n4001~7.\u00a0 The BAV21 is a beefy 1N4148.<\/p>\n There are some drawbacks to this design.\u00a0 Firstly, it has significant switching losses.\u00a0 Whilst the quiescent current for a 1-stage charge pump is about zero, the quiescent current for the above design (bearing in mind that the LM317 has a minimum load current of 3.5mA to maintain output regulation) is 27mA @ 9.2V.\u00a0 Also when you put a significant load on the output, the conversion efficiency for the load current is about 65%, which is very poor.\u00a0 Secondly, in order to provide 10mA at 48V (as per IEC61938 spec.), you need an input voltage of at least 9.5V.\u00a0 Thirdly, it will only do one ‘lot’ of phantom power, and fourthly, it switches at 20kHz, which may be a problem for you if you’re doing high-quality digital audio.<\/p>\n So, cue design number two.\u00a0 This is straight off the Texas Instruments WEBENCH online design package.\u00a0 I specified a switcher with 8.5V to 14.5V input, 48V output at 20mA.\u00a0 Here’s the schematic: –<\/p>\n It is worth noting that Ti have a number of switching devices which are less overkill than the above circuit, but – without exception – all of the devices are only available in tiny surface-mount packages and as such, not Stompville style. So this design has the good old LM2585 <\/a>at its heart.\u00a0 The switching frequency is 100kHz, which is good for digital audio sample rates up to 48kHz.\u00a0 If you’re sampling at 96kHz, good luck to you.\u00a0 There’s not much to say about the circuit but it is worth noting that you have a wide choice of suitable inductors.\u00a0 Basically you will likely get away with any inductor that has inductance of 0.56 ~ 1.5 mH providing the inductor has a current rating in excess of 0.5A and a d.c. resistance below about 2 Ohms.\u00a0 Note that an iron-powder toroidal-cored inductor will have better EMC performance than a ferrite-cored bobbin inductor and is therefore preferable.\u00a0 I used a Bourns 2124-H-RC from Farnell (1mH, toroidal, 0.4 Ohm, 1.3A), because it was the best-value option they had in stock on the day.\u00a0 I have a Panasonic ELC12D561E (560 uH ferrite bobbin inductor) to try as an absolute-minimum-spec. part, but I haven’t got round to trying it yet. Also, D1 can be any Schottky diode with a Vrrm of 100V (minimum) and a current rating of 1 A (minimum).<\/p>\n As you would expect, this design works flawlessly, maintaining a steady output of 48V\/10mA from a supply between about 3.6 ~ 24V d.c. (You can go higher, but 24V was the highest voltage I had available when I wrote this article).<\/p>\n Here are photo’s of the two designs.\u00a0 The PCB’s are designed to fit a Hammond 27134PSLA or 1590B stomp box in a similar manner to my re-Stomp<\/a> re-amping project: –<\/p>\n Note that the LM2585 design has a surface-mount LM2585S on the rear.\u00a0 This is just about the smallest surface-mount item I am prepared to solder by hand!<\/p>\n Finally, here’s a table comparing the specification and relative performance of the two designs:-<\/p>\n Notes:<\/p>\n Two final caveats:<\/p>\n Firstly, the LM317HV data sheet specifies a standard value for R1 of 240 Ohms.\u00a0 This low value is specified because the LM317 requires a minimum output current of 3.5mA for proper regulation. In this design, R1 is increased to 1k2 Ohm and PR1 and R2 increased accordingly.\u00a0 This gives a quiescent current through the R1-PR2-R2 network of approximately 1mA at 48V.\u00a0 However, the LED current is 2.1 mA @Vf 1.7V. On the sample I had, 3.1 mA (i.e. 1 mA + 2.1 mA) quiescent load current was enough to get the regulator working properly.\u00a0 So, if you build the TC1044S design, don’t omit the LED and if you find that the LM317 is not regulating, reduce the value of R3 to increase the LED current<\/a>.<\/p>\n Secondly, pay careful attention to the voltage ratings of the electrolytic capacitors as shown on the schematics.\u00a0 If you over-voltage an electrolytic it will overheat and its useful life will be severely shortened. It may even explode!\u00a0 Here are a couple of options for generating proper 48V phantom power from a 9V supply. Before we get stuck into the details I should point out that the quiescent current for the first option is about 27mA and for the second about 52mA, so don’t be thinking you can run this from a PP3\/1604\/6LR61\u2026 Read More »<\/a><\/span><\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5,7],"tags":[],"_links":{"self":[{"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/387"}],"collection":[{"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=387"}],"version-history":[{"count":2,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/387\/revisions"}],"predecessor-version":[{"id":1427,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/387\/revisions\/1427"}],"wp:attachment":[{"href":"https:\/\/stompville.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=387"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=387"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=387"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"127SV\"](\"http:\/\/stompville.co.uk\/wp-content\/uploads\/2013\/02\/127SV1-1024x666.png\")
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