1. abstract the purpose of this application note is to present the electrical schematic that allows the use of the vb409 as a -5v output regulator. the vb409 is a fully protected high voltage positive regulator designed in vipower m1-2 technology. it can be directly connected to the rectified mains supplying a double output: +5v5% dc and +16v (maximum) non regulated voltage. it has a built-in over current (80ma minimum value) and thermal shut-down (140c minimum) protection. in the following sections we will explain if and how the vb409 is well suited to be used in application where a -5v is requested. 2. operation description. figure 1: vb409 block diagram the vb409 block diagram (see figure 1) is divided in two stages: the first stage (on top of the block diagram) is a preregulator which transforms the pulsed voltage, derived from the rectified main on the input (1), to a lower voltage (typically 16v) used to charge an external electrolytic capacitor. the second stage is a standard low voltage regulator able to guarantee 5% precision on the output regulated voltage (+5v). the first stage of the vb409 is a trilinton that is driven in such a way to provide current to the connected second stage. the device works by setting the conduction angle (the on time of the trilinton). this means that the current is delivered from the mains only during a low voltage portion of each positive half cycle july 2000 1/6 AN1293 application note using the vb409 to supply a negative regulated output voltage g. nuzzarello - g. di stefano + main input input current limiter cap (out2) c1 + vref1 thermal protection vref2 vref3 gnd output1 load r1 r2 threshold output current limiter
AN1293 - application note 2/6 0-t 1 ; t 2 -t/ 2 (see the waveform in figure 2b). the result is a drastic reduction of the power dissipated on the device. during the conduction period the current flowing through the trilinton icl(in) charges the external capacitor and supplies the connected loads. when the voltage on the capacitor reaches a fixed voltage vcap (max), the trilinton is switched off. the load is supplied by the discharge current of the external capacitor c 1 . the proper setting of the conduction angle can be done by dimensioning the r 1 - r 2 divider. to do this two parameters have to be considered: 1) the voltage drop between the discharged capacitor and the output1 pin has to be over 2.5v; 2) it is necessary not to exceed the soa limits of the bipolar power stage. a good compromise will be reached by dimensioning the ratio r 1 /r 2 <11. to have more information about the operation of the vb409 you can consult application note an1219. figure 2: vb409 operation waveform rectified mains v max v 1 v (output2) cap v cap(max) v cap(min) i in i cl(in) v out1 t t t t t1 t2 t/2 t (figure 2a) (figure 2b) (figure 2c) (figure 2d)
AN1293 - application note 3/6 vb409 + 470 m f 1m w 1n4007 100k w out1 out2(vcap) in adj gndpin r1 load=820 w +v plus r2 load=100 w -5v i 1 i 2 3. electrical schematic for -5v output voltage. this application note focuses on the way to obtain a negative voltage from the vb409. we are referring to a load made of two resistors, r1 and r2, with the common point connected to the earth (see scheme in figure 3). the electrical scheme is browsed in picture 3: the earth of the supplied load must be connect with the output1 pin. the output2 (v out2 ) voltage is referred to the load earth; so the positive voltage on the r 1 =820ohm resistor is given by the formula: with the v out2 value of 16v (typical value), the equation is vplus=(16-5)v=11v. the voltage on the load resistor r 2 =100ohm is always -5v5%. figure 3: -5v application schematic by indicating the currents flowing through r 1 and r 2 with i 1 and i 2 , respectively, the vb409 must provide a total current itot of i 1 +i 2 . the itot current does not exceed the value of 80ma specified in the datasheet. with the data in figure 3: as well we can use the vb409 to supply, together with the -5v regulated voltage one +5v (also regulated) from output 2. to do this we propose the electrical scheme in figure 4. in order to have the +5v regulated voltage on r1, the +5v is generated from the +11v of output 2 by using the positive low voltage regulator l7805 or equivalent. by using this configuration the total current in the absolute value will be: 5v/820ohm+5v/100ohm=(6+50)ma=56ma. on the output pin of the l7805 it is suggested to connect a capacitor of 0.1 m f, in order to avoid self- oscillation: i 1 = v plus / 820ohm = 13ma i 2 = 5v / 100ohm = 50ma i tot = i 1 + i 2 = 63ma i 1 = 5v / 820ohm = 6ma i 2 = 5v / 100ohm = 50ma i tot = i 1 + i 2 = 56ma v plus v out 2 5 v C =
AN1293 - application note 4/6 vb409 + 470 m f 1m w 1n4007 100k w l7805 lm337 out1 out2(vcap) in adj 0.1 m f +5v -2v -5v gndpin r1 load=820 w r2 load=100 w r3=100 w r4=trim.470 w -5v vb409 + 470 m f 1m w 1n4007 100k w l7805 r1 load=820 w out1 out2(vcap) in adj 0.1 m f +5v gndpin r2 load=100 w vmain, 50hz figure 4: 5v application schematic 4. adjustable negative voltage regulator. if an adjustable negative output voltage is requested the schematic in figure 5 can be implemented. this schematic uses the negative adjustable voltage regulator lm337. in this way the negative output voltage may be adjusted from -2v to -5v. this device can be connected to the resistive load used in the former case. figure 5: adjustable negative voltage regulator the lm337 is connected in the standard configuration with the resistor divider made of r 3 =100ohm and r 4 =470ohm (trimmer). we connected three different loads: 1)r 1 =820ohm; r 2 =100ohm (see the waveform in figure 6); 2)r 1 =470ohm; r 2 =100ohm (see the waveform in figure 7); 3)r 1 =220ohm; r 2 =100ohm (see the waveform in figure 8).
AN1293 - application note 5/6 the currents (in absolute value) are in the table below: figure 6: output waveforms with r1=820ohm and r2=100ohm (circuit in figure 5) figure 7: output waveforms with r1=470ohm and r2=100ohm (circuit in figure 5) figure 8: output waveforms with r1=220ohm and r2=100ohm (circuit in figure 5) i 1 = v plus / 820ohm = 13ma i 2 = 2v / 100ohm = 20ma i tot = i 1 + i 2 = 33ma i 1 = v plus / 470ohm = 23ma i 2 = 2v / 100ohm = 20ma i tot = i 1 + i 2 = 43ma i 1 = v plus / 220ohm = 50ma i 2 = 2v / 100ohm = 20ma i tot = i 1 + i 2 = 70ma
AN1293 - application note 6/6 information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specification mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of stmicroelectronics. the st logo is a trademark of stmicroelectronics ? 2000 stmicroelectronics - printed in italy - all rights reserved stmicroelectronics group of companies australia - brazil - china - finland - france - germany - hong kong - india - italy - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - u.s.a. http://www.st.com
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