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ISL6522 Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL6522 Datasheet(HTML) 11 Page - Intersil Corporation |
11 / 15 page 11 FN9030.8 March 10, 2006 switch realizes most of the switching losses when the converter is sinking current (see the equations below). These equations assume linear voltage-current transitions and do not adequately model power loss due the reverse- recovery of the upper and lower MOSFET’s body diode. The gate-charge losses are dissipated by the ISL6522 and do not heat the MOSFETs. However, large gate-charge increases the switching interval, tSW which increases the upper MOSFET switching losses. Ensure that both MOSFETs are within their maximum junction temperature at high ambient temperature by calculating the temperature rise according to package thermal-resistance specifications. A separate heatsink may be necessary depending upon MOSFET power, package type, ambient temperature and air flow. Standard-gate MOSFETs are normally recommended for use with the ISL6522. However, logic-level gate MOSFETs can be used under special circumstances. The input voltage, upper gate drive level, and the MOSFETs absolute gate-to- source voltage rating determine whether logic-level MOSFETs are appropriate. Figure 9 shows the upper gate drive (BOOT pin) supplied by a bootstrap circuit from VCC . The boot capacitor, CBOOT develops a floating supply voltage referenced to the PHASE pin. This supply is refreshed each cycle to a voltage of VCC less the boot diode drop (VD) when the lower MOSFET, Q2 turns on. A logic-level MOSFET can only be used for Q1 if the MOSFETs absolute gate-to-source voltage rating exceeds the maximum voltage applied to VCC. For Q2, a logic-level MOSFET can be used if its absolute gate-to- source voltage rating exceeds the maximum voltage applied to PVCC. Figure 10 shows the upper gate drive supplied by a direct connection to VCC. This option should only be used in converter systems where the main input voltage is +5VDC or less. The peak upper gate-to-source voltage is approximately VCC less the input supply. For +5V main power and +12VDC for the bias, the gate-to-source voltage of Q1 is 7V. A logic-level MOSFET is a good choice for Q1 and a logic-level MOSFET can be used for Q2 if its absolute gate-to-source voltage rating exceeds the maximum voltage applied to PVCC. Schottky Selection Rectifier D2 is a clamp that catches the negative inductor swing during the dead time between turning off the lower MOSFET and turning on the upper MOSFET. The diode must be a Schottky type to prevent the lossy parasitic MOSFET body diode from conducting. It is acceptable to omit the diode and let the body diode of the lower MOSFET clamp the negative inductor swing, but efficiency will drop one or two percent as a result. The diode's rated reverse breakdown voltage must be greater than the maximum input voltage. PLOWER = Io2 x rDS(ON) x (1 - D) Where: D is the duty cycle = VOUT / VIN, tSW is the switching interval, and FS is the switching frequency. Losses while Sourcing Current Losses while Sinking Current P LOWER Io 2 r DS ON () × 1D – () × 1 2 --- Io ⋅ V IN × t SW F S × × + = PUPPER Io 2 r DS ON () × D × 1 2 --- Io ⋅ V IN × t SW F S × × + = PUPPER = Io2 x rDS(ON) x D +12V PGND ISL6522 GND LGATE UGATE PHASE BOOT VCC +5V OR +12V FIGURE 9. UPPER GATE DRIVE - BOOTSTRAP OPTION NOTE: VG-S ≈ VCC - VD NOTE: VG-S ≈ PVCC CBOOT DBOOT Q1 Q2 PVCC +5V OR +12V D2 + - VD + - +12V PGND LGATE UGATE PHASE BOOT VCC +5V OR LESS FIGURE 10. UPPER GATE DRIVE - DIRECT VCC DRIVE OPTION NOTE: VG-S ≈ VCC - 5V NOTE: VG-S ≈ PVCC Q1 Q2 PVCC +5V OR +12V D2 ISL6522 GND + - ISL6522 |
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Similar Description - ISL6522 |
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