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MAX8725ETI Fiches technique(PDF) 27 Page - Maxim Integrated Products |
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MAX8725ETI Fiches technique(HTML) 27 Page - Maxim Integrated Products |
27 / 30 page Multichemistry Battery Chargers with Automatic System Power Selector ______________________________________________________________________________________ 27 Choose a low-side MOSFET that has the lowest possi- ble on-resistance (RDS(ON)), comes in a moderate- sized package, and is reasonably priced. Make sure that the DLO gate driver can supply sufficient current to support the gate charge and the current injected into the parasitic gate-to-drain capacitor caused by the high-side MOSFET turning on; otherwise, cross-con- duction problems can occur. The MAX1909/MAX8725 have an adaptive dead-time cir- cuit that prevents the high-side and low-side MOSFETs from conducting at the same time (see the MOSFET Drivers section). Even with this protection, it is still possi- ble for delays internal to the MOSFET to prevent one MOSFET from turning off when the other is turned on. Select devices that have low turn-off times. To be conservative, make sure that P1(tDOFF(MAX)) - N1(tDON(MIN)) < 40ns. Failure to do so may result in efficiency-killing shoot-through currents. If delay mis- match causes shoot-through currents, consider adding extra capacitance from gate to source on N1 to slow down its turn-on time. MOSFET Power Dissipation Worst-case conduction losses occur at the duty factor extremes. For the high-side MOSFET, the worst-case power dissipation (PD) due to resistance occurs at the minimum supply voltage: Generally, a small high-side MOSFET is desired to reduce switching losses at high input voltages. However, the RDS(ON) required to stay within package power-dissipation limits often limits how small the MOSFET can be. The optimum occurs when the switch- ing (AC) losses equal the conduction (I2RDS(ON)) losses. High-side switching losses do not usually become an issue until the input is greater than approxi- mately 15V. Switching losses in the high-side MOSFET can become an insidious heat problem when maximum AC adapter voltages are applied, due to the squared term in the CV2 f switching-loss equation. If the high- side MOSFET that was chosen for adequate RDS(ON) at low supply voltages becomes extraordinarily hot when subjected to VDCIN(MAX), then choose a MOSFET with lower losses. Calculating the power dissipation in P1 due to switching losses is difficult since it must allow for difficult quantifying factors that influence the turn-on and turn-off times. These factors include the internal gate resistance, gate charge, threshold voltage, source inductance, and PC board layout characteristics. The following switching-loss calculation provides only a very rough estimate and is no substitute for breadboard evaluation, preferably including a verification using a thermocouple mounted on P1: where CRSS is the reverse transfer capacitance of P1, and IGATE is the peak gate-drive source/sink current. For the low-side MOSFET (N1), the worst-case power dissipation always occurs at maximum input voltage: Choose a Schottky diode (D1, Figure 2) with a forward voltage low enough to prevent the N1 MOSFET body diode from turning on during the dead time. As a gen- eral rule, a diode with a DC current rating equal to 1/3rd the load current is sufficient. This diode is optional and can be removed if efficiency is not critical. Inductor Selection The charge current, ripple, and operating frequency (off-time) determine the inductor characteristics. Inductor L1 must have a saturation current rating of at least the maximum charge current plus 1/2 of the ripple current (∆IL): ISAT = ICHG + (1/2) ∆IL PD N V V I R BATT DCIN LOAD DS ON () () 11 2 2 = × − PD P Switching VC f I I DCIN MAX RSS SW LOAD GATE (_ ) () 1 2 2 = ×× × PD P V V I R BATT DCIN LOAD DS ON () () 1 2 2 = × 0 1.0 0.5 1.5 810 111213 914 15161718 VBATT (V) VDCIN = 19V VCTL = ICTL = LDO 3 CELLS 4 CELLS Figure 11. Ripple Current vs. Battery Voltage (MAX1909) |
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