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MAX8725ETI Arkusz danych(PDF) 25 Page - Maxim Integrated Products |
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MAX8725ETI Arkusz danych(HTML) 25 Page - Maxim Integrated Products |
25 / 30 page Multichemistry Battery Chargers with Automatic System Power Selector ______________________________________________________________________________________ 25 The loop transfer function is given by: Since: the loop transfer function simplifies to: The crossover frequency is given by: For stability, choose a crossover frequency lower than 1/10th the switching frequency: CCS = GMS / (2π fCO_CS) Choosing a crossover frequency of 30kHz and using the component values listed in Figure 1 yields CCS > 5.4nF. Values for CCI greater than 10 times the mini- mum value may slow down the current-loop response excessively. Figure 10 shows the Bode plot of the input current-limit loop frequency response using the values calculated above. MOSFET Drivers The DHI and DLO outputs are optimized for driving moderately-sized power MOSFETs. The MOSFET drive capability is the same for both the low-side and high- side switches. This is consistent with the variable duty factor that occurs in the notebook computer environ- ment where the battery voltage changes over a wide range. An adaptive dead-time circuit monitors the DLO output and prevents the high-side FET from turning on until DLO is fully off. There must be a low-resistance, low-inductance path from the DLO driver to the MOSFET gate for the adaptive dead-time circuit to work properly. Otherwise, the sense circuitry in the MAX1909/MAX8725 interpret the MOSFET gate as “off” while there is still charge left on the gate. Use very short, wide traces measuring 10 squares to 20 squares or less (1.25mm to 2.5mm wide if the MOSFET is 25mm from the device). Unlike the DLO output, the DHI output uses a fixed-delay 50ns time to prevent the low-side FET from turning on until DHI is fully off. The same lay- out considerations should be used for routing the DHI signal to the high-side FET. Since the transition time for a p-channel switch can be much longer than an n-channel switch, the dead time prior to the high-side PMOS turning on is more pro- nounced than in other synchronous step-down regula- tors, which use high-side n-channel switches. On the high-to-low transition, the voltage on the inductor’s “switched” terminal flies below ground until the low-side switch turns on. A similar dead-time spike occurs on the opposite low-to-high transition. Depending upon the magnitude of the load current, these spikes usually have a minor impact on efficiency. The high-side driver (DHI) swings from SRC to 5V below SRC and typically sources 0.9A and sinks 0.5A from the gate of the p-channel FET. The internal pull- down transistors that drive DHI high are robust, with a 2.0Ω (typ) on-resistance. The low-side driver (DLO) swings from DLOV to ground and typically sources 0.5A and sinks 0.9A from the gate of the n-channel FET. The internal pulldown transistors that drive DLO low are robust, with a 1.0Ω (typ) on- resistance. This helps prevent DLO from being pulled up when the high-side switch turns on, due to capaci- tive coupling from the drain to the gate of the low-side MOSFET. This places some restrictions on the FETs that can be used. Using a low-side FET with smaller gate-to-drain capacitance can prevent these problems. f GMS C CO CS CS _ = 2π LTF GMS R sR C OGMS OGMS CS = +× 1 GM ARS IN CSS = × 1 1 LTF GM A RS GMS R sR C IN CSS OGMS OGMS CS =× × × +× 1 1 FREQUENCY (Hz) 100k 10M 1k 10 -20 0 20 40 60 100 80 -40 -45 0 -90 0.1 MAG PHASE Figure 10. CCS Loop Response |
Podobny numer części - MAX8725ETI |
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Podobny opis - MAX8725ETI |
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