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LTC1479IG Datasheet(PDF) 10 Page - Linear Technology |
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LTC1479IG Datasheet(HTML) 10 Page - Linear Technology |
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10 / 24 page  10 LTC1479 OPERATION The LTC1479 is responsible for low-loss switching at the “front end” of the power management system, where up to two battery packs and a DC power source can be indiscriminately connected and disconnected. Smooth switching between input power sources is accomplished with the help of low-loss N-channel switches driven by special gate drive circuitry which limits the inrush current in and out of the battery packs and the system power supply capacitors. All N-Channel Switching The LTC1479 drives external back-to-back N-channel MOSFET switches to direct power from the three main power sources: the external DC power source, the pri- mary battery and the secondary battery connected to the main supply pins—DCIN, BAT1 and BAT2 respectively. (N-channel MOSFET switches are more cost effective and provide lower voltage drops than their P-channel counterparts.) Gate Drive (VGG) Power Supply The gate drive for the low-loss N-channel switches is supplied by a micropower boost regulator which is regu- lated at approximately 36.5V. The VGG supply provides sufficient headroom above the maximum 28V operating voltage of the three main power sources to ensure that the MOSFET switches are fully enhanced. The power for this inductor based regulator is taken from three internal diodes as shown in Figure 1. The three Figure 1. VGG Switching Regulator BAT1 BAT2 DCIN V + SW GND 1479 F01 VGG + + L1 1mH C1 1 µF 35V C2 1 µF 50V TO GATE DRIVERS (36.5V) LTC1479 VGG SWITCHING REGULATOR diodes are connected to each of the three main power sources, DCIN, BAT1 and BAT2. The highest voltage potential is directed to the top of the boost regulator inductor to maximize regulator efficiency. C1 provides filtering at the top of the 1mH switched inductor, L1, which is housed in a small surface mount package. A fourth internal diode directs the current from the 1mH inductor to the VGG output capacitor, C2, further reducing the external parts count. In fact, as demonstrated in Figure 1, only three external components are required by the VGG regulator, L1, C1 and C2. Inrush Current Limiting The LTC1479 uses an adaptive inrush current limiting scheme to reduce current flowing in and out of the three main power sources and the DC/DC converter input ca- pacitor during switch-over transitions. The voltage across a single small-valued resistor, RSENSE, is measured to ascertain the instantaneous current flowing through the three main switch pairs, SW A/B, SW C/D, and SW E/F during the transitions. Figure 2 is a block diagram showing only the DCIN switch pair, SW A/B. (The gate drive circuits for switch pairs SW C/D and SW E/F are identical). A bidirectional current sensing and limiting circuit determines when the voltage drop across RSENSE reaches plus or minus 200mV. The gate-to-source voltage, VGS, of the appropriate switch is limited during the transition period until the inrush current subsides, generally within a few milliseconds, depending upon the value of the DC/DC converter input capacitor. VSENSE+ VSENSE– GA GB SAB SW A SW B RSENSE VGG LTC1479 1479 F02 DCIN + OUTPUT TO DC/DC CONVERTER COUT 6V 6V ±200mV THRESHOLD SW A/B GATE DRIVERS BIDIRECTIONAL INRUSH CURRENT SENSING AND LIMITING Figure 2. SW A/B Inrush Current Limiting |
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