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SS6845GGTR Fiches technique(PDF) 11 Page - Silicon Standard Corp. |
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SS6845GGTR Fiches technique(HTML) 11 Page - Silicon Standard Corp. |
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11 / 13 page  www.SiliconStandard.com 11 of 13 SS6845G 7/21/2005 Rev.3.01 ] D) D(1 1 I ESR) (2R V [2V C ) D - 1 I ESR - D I ESR - D 1 I 2R D I 2R V (2V C ) I ESR - I ESR - I 2R - I 2R - V - (2V C ) V (V C Q - Q Q OUT ON DS OUT IN 1 OUT OUT OUT ON DS OUT ON DS OUT IN 1 AVE - OFF AVE ON AVE - OFF ON - DS AVE - ON ON - DS OUT IN 1 AVE OFF C1 AVE ON C1 1 OFF ON − × × + − − × = × × − × − × − − × = × × × × × = − × = = ∆ − − − − − − − − ………(13) Thus the output current can be written as ] D) - D(1 1 I ) ESR (2R - V [2V C f ) Q (Q f Q f I OUT ON - DS OUT IN 1 OFF ON OUT × × + − × × = − × = ∆ × = (14) When the duty cycle is 0.5, the output current can be written as: ] I 4ESR) (8R V [2V fC ] 0.5) 0.5(1 1 I ESR) (2R V [2V C f I OUT ON DS OUT IN 1 OUT ON DS OUT IN 1 OUT × + − − × = − × × + − − × × = − − (15) And equation (15) can be re-written as: OUT ON DS OUT 1 OUT IN I 4ESR) (8R I fC 1 V 2V × + + × = − − (16) VOUT 2VIN COUT 1/fC1 8RDS-ON IOUT LOAD 4ESR Fig. 22 The equivalent circuit of charge pump According to equation (16), when the duty cycle is 0.5, the equivalent circuit of the charge pump is shown in Fig. 22. The term 8RDS-ON is the total effect of switching resistance, 1/fC1 is the effect of flying capacitor and 4ESR is its equivalent resistance. From the equivalent circuit shown in Fig. 22, it is seen that the terms 1/fC1, 4ESR and 8RDS-ON should be as small as possible to get large output current. However, since the R DS-ON is internal to the SS6845G , all that can be done is to lower the values of 1/fC1 and ESR. However even if the values of 1/fC1 and ESR can be kept as small as possible, the term 8RDS-ON still dominates the limit of the maximum output current. Layout Considerations With the high switching frequency and transient currents of the SS6845G, careful consideration of PCB layout is important. To achieve the best performance, it is necessary to minimize the distance between every component and also to minimize the length of every connection and maximize the trace width. Make sure each device connects to an immediate ground plane. Fig. 23 to Fig. 25 show a recommended layout. |
Numéro de pièce similaire - SS6845GGTR |
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Description similaire - SS6845GGTR |
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