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MIC24420 Datasheet(PDF) 13 Page - Micrel Semiconductor |
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MIC24420 Datasheet(HTML) 13 Page - Micrel Semiconductor |
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13 / 34 page  Micrel, Inc. MIC24420/MIC24421 June 2012 13 M9999-062012-C High-side Drive The internal high-side drive circuit is designed to switch the internal N-channel MOSFET. Figure 5 shows a diagram of the high-side MOSFET, gate drive and bootstrap circuit. D2 and CBST comprise the bootstrap circuit, which supplies drive voltage to the high-side MOSFET. Bootstrap capacitor CBST is charged through diode D2 when the low-side MOSFET turns on and pulls the SW pin voltage to ground. When the high-side MOSFET driver is turned on, energy from CBST charges the MOSFET gate, turning it on. Voltage on the SW pin increases to approximately VIN. Diode D2 is reversed biased and CBST flies high while maintaining gate voltage on the high-side MOSFET. A resistor should be added in series with the BST1 and BST2 pins. This will slow down the turn-on time of the high-side MOSFET while leaving the turn-off time unaffected. Slowing down the MOSFET risetime will reduce the turn-on overshoot at the switch node, which is important when operating with an input voltage close to the maximum operating voltage. The recommended capacitor for CBST is a 0.01µF ceramic capacitor. The recommended value for RBST is 20Ω to 60Ω. Figure 5. High-side Drive Circuitry Low-side Drive Output The LSD pin is used to drive an external MOSFET. This MOSFET is driven out of phase with the internal high- side MOSFET to conduct inductor current during the high-side MOSFETs off-time. Circuitry internal to the regulator prevents short circuit “shoot-through” current from flowing by preventing the high-side and low-side MOSFETs conducting at the same time. The low-side MOSFET gate voltage is supplied from PVDD. Turn off of the MOSFET is accomplished by discharging the gate through the LSD pin. The return path is through the PGND pin and back to the MOSFET’s Source pin. These circuit paths must be kept short to minimize noise. See the layout section for additional information. Driving the low-side MOSFET on and off dissipates power in the MIC24420/21 regulator. The power can be calculated by the equation below: S IN G DRIVER f V Q P × × = Where: PDRIVER is the power dissipated in the regulator by switching the MOSFET on and off. QG is the total Gate charge of the MOSFET at VGS = PVDD. VIN is the input voltage to the internal AVDD regulator. fS is the switching frequency of the regulator (1MHz/500kHz nominal). dV/dt Induced Turn-on of the Low-side MOSFET As the high-side MOSFET turns on, the rising dv/dt on the switch-node forces current through CGD of the low- side MOSFET causing a glitch on its gate. Figure 6 demonstrates the basic mechanism causing this issue. If the glitch on the gate is greater than the MOSFET’s turn- on threshold, it may cause an unwanted turn-on of the low-side MOSFET while the high-side MOSFET is on. A short circuit between input and ground would momentarily occur, which lowers efficiency and increases power dissipation in both MOSFETs. Additionally, turning on the low-side MOSFET during the off-time could interfere with overcurrent sensing. Figure 6. dV/dt induced turn-on of the low-side MOSFET |
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