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SS6845GGTR Datasheet(PDF) 10 Page - Silicon Standard Corp. |
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SS6845GGTR Datasheet(HTML) 10 Page - Silicon Standard Corp. |
10 / 13 page www.SiliconStandard.com 10 of 13 SS6845G 7/21/2005 Rev.3.01 2. Package Size: A ceramic capacitor with large volume (0805), gets a lower ESR than a small one (0603). Therefore, larger devices provide improved transient response over smaller ones. Table 1 lists the recommended components for use with the SS6845G. Table.1 Bill of Material Design- ator Part Type Description Vendor CIN 2.2 µ CELMK212BJ- 225MG (X5R) TAIYO YUDEN CFLY 0.22 µ CEEMK212BJ -224KG (X7R) TAIYO YUDEN COUT 2.2 µ CELMK212BJ- 225MG (X5R) TAIYO YUDEN Power Dissipation Now, le t’s look at the power dissipation in R DS-ON and ESR. Assume that the RDS-ON of each internal switching element in the SS6845G is equal and ESR is the equivalent series resistance of CFLY (refer to Fig. 20 and Fig. 21). The approximation of the power losses of RDS-ON and ESR are given below: R D) - D(1 2 I ) R D - 1 2 ( I ) R D 2 ( I D) - (1 2R ) D - 1 I ( D 2R ) 2D 2I ( D) - (1 2R ) D - 1 I ( D 2R ) 2D I ( ) D (1 2R I D 2R I P ON - DS 2 OUT ON - DS 2 OUT ON - DS 2 OUT ON - DS 2 OUT ON - DS 2 OUT ON - DS 2 OUT ON - DS 2 IN ON DS 2 AVE - OFF ON DS 2 AVE - ON R ON DS × × = × + × = × × + × × = × × + × × = − × × + × × ≅ − − − ..........(7) ESR + AVE OFF AVE OFF ON DS IN OUT AVE OFF C I I 2R V V V − − − − − × × + − = ………………..(12) D) - D(1 1 ESR I D - 1 1 ESR I D 1 ESR I D) (1 ESR ) D 1 I ( D ESR ) 2D I ( D) (1 ESR I D ESR I P 2 OUT 2 OUT 2 OUT 2 OUT 2 IN 2 AVE OFF 2 AVE ON ESR × × = × × + × × = − × × − + × × = − × × + × × ≅ − − With a duty-cycle of 0.5, the power loss of RDS-ON is 8R I R 0.5) 0.5(1 2 I P ON DS 2 OUT ON - DS 2 OUT R ON DS − × = × − × ≅ − 4ESR I 0.5) 0.5(1 1 ESR I P 2 OUT 2 OUT ESR × = − × × ≅ In fact, whether the current is the on-state or the off- state, it decays exponentially rather than flows steadily, and as the root mean square value of exponential decay is not equal to that of steady flow, then we must use an approximation. Let’s use another approach to look at the charge pump circuit and focus on the flying capacitor C1. Referring to Fig. 20, when the circuit is in the on state, the voltage across C1 is: (t) I ESR - (t) I 2R V (t) V ON ON ON DS IN ON - C × × − = − …(9) The average of VC1 during the on-state is: AVE ON AVE ON ON DS IN AVE ON C I ESR I 2R V V − − − − − × − × − = ……………………….(10) Similarly, referring to Fig. 21, when the circuit is in the off-state, the voltage of C1 is: (t) I ESR (t) I 2R V V (t) V OFF OFF ON - DS IN OUT OFF - C × + × + − = ……………………………(11) The average of VC1 during the off-state is: The difference in charge stored in C1 between the on- state and off-state is the net charge transferred to the output in one cycle. |
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