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EL7585ILZ Datasheet(PDF) 11 Page - Intersil Corporation |
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EL7585ILZ Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 18 page  11 FN7345.1 July 1, 2005 Cascaded MOSFET Application A 20V N-channel MOSFET is integrated in the boost regulator. For the applications where the output voltage is greater than 20V, an external cascaded MOSFET is needed as shown in Figure 22. The voltage rating of the external MOSFET should be greater than VBOOST. Linear-Regulator Controllers (VON, VLOGIC, and VOFF) The EL7585 includes three independent linear-regulator controllers, in which two are positive output voltage (VON and VLOGIC), and one is negative. The VON, VOFF, and VLOGIC linear-regulator controller functional diagrams, applications circuits are shown in Figures 23, 24, and 25 respectively. Calculation of the Linear Regulator Base-Emitter Resistors (RBL, RBP and RBN) For the pass transistor of the linear regulator, low frequency gain (Hfe) and unity gain freq. (fT) are usually specified in the datasheet. The pass transistor adds a pole to the loop transfer function at fp=fT/Hfe. Therefore, in order to maintain phase margin at low frequency, the best choice for a pass device is often a high frequency low gain switching transistor. Further improvement can be obtained by adding a base-emitter resistor RBE (RBP, RBL, RBN in the Functional Block Diagram), which increase the pole frequency to: fp=fT*(1+ Hfe *re/RBE)/Hfe, where re=KT/qIc. So choose the lowest value RBE in the design as long as there is still enough base current (IB) to support the maximum output current (IC). We will take as an example the VLOGIC linear regulator. If a Fairchild FMMT549 PNP transistor is used as the external pass transistor, Q5 in the application diagram, then for a maximum VLOGIC operating requirement of 500mA the data sheet indicates Hfe_min = 100. The base-emitter saturation voltage is: Vbe_max = 1.25V (note this is normally a Vbe ~ 0.7V, however, for the Q5 transistor an internal Darlington arrangement is used to increase it's current gain, giving a 'base-emitter' voltage of 2xVBE). (Note that using a high current Darlington PNP transistor for Q5 requires that VIN > VLOGIC + 2V. Should a lower input voltage be required, then an ordinary high gain PNP transistor should be selected for Q5 so as to allow a lower collector-emitter saturation voltage). For the EL7585, the minimum drive current is: I_DRVL_min = 8mA The minimum base-emitter resistor, RBL, can now be calculated as: RBL_min = VBE_max/(I_DRVL_min - Ic/Hfe_min) = 1.25V/(8mA - 500mA/100) = 417 Ω This is the minimum value that can be used - so, we now choose a convenient value greater than this minimum value; say 500 Ω. Larger values may be used to reduce quiescent current, however, regulation may be adversely affected, by supply noise if RBL is made too high in value. EL7585 FB LX VBOOST VIN FIGURE 22. CASCADED MOSFET TOPOLOGY FOR HIGH OUTPUT VOLTAGE APPLICATIONS - + - + 36V ESD CLAMP GMP LDO_ON PG_LDOP 1: Np FBP DRVP 7k Ω RBP VBOOST 0.1µF 0.1µF CP (TO 36V) 20k Ω RP2 RP1 CON VON (TO 35V) LX 0.9V Q3 FIGURE 23. VON FUNCTIONAL BLOCK DIAGRAM EL7585 |
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