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MAX8791B Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX8791B Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 12 page 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 switching-loss equation above. If the high-side MOSFET chosen for adequate RDS(ON) at low battery voltages becomes extraordinarily hot when biased from VIN(MAX), consider choosing another MOSFET with lower parasitic capacitance. For the low-side MOSFET (NL), the worst-case power dissipation always occurs at the maximum input voltage: The worst case for MOSFET power dissipation occurs under heavy load conditions that are greater than ILOAD(MAX), but are not quite high enough to exceed the current limit and cause the fault latch to trip. The MOSFETs must have a good-sized heatsink to handle the overload power dissipation. The heat sink can be a large copper field on the PCB or an externally mounted device. An optional Schottky diode only conducts during the dead time when both the high-side and low-side MOSFETs are off. Choose a Schottky diode with a forward voltage low enough to prevent the low-side MOSFET body diode from turning on during the dead time, and a peak current rating higher than the peak inductor current. The Schottky diode must be rated to handle the average power dissipation per switching cycle. This diode is optional and can be removed if effi- ciency is not critical. IC Power Dissipation and Thermal Considerations Power dissipation in the IC package comes mainly from driving the MOSFETs. Therefore, it is a function of both switching frequency and the total gate charge of the selected MOSFETs. The total power dissipation when both drivers are switching is given by: where IBIAS is the bias current of the 5V supply calcu- lated in the 5V Bias Supply (VDD) section. The rise in die temperature due to self-heating is given by the following formula: where PD(IC) is the power dissipated by the device, and ΘJA is the package’s thermal resistance. The typi- cal thermal resistance is 42°C/W for the 3mm x 3mm TQFN package. Avoiding dV/dt Turning on the Low-Side MOSFET At high input voltages, fast turn-on of the high-side MOSFET can momentarily turn on the low-side MOSFET due to the high dV/dt appearing at the drain of the low- side MOSFET. The high dV/dt causes a current flow through the Miller capacitance (CRSS) and the input capacitance (CISS) of the low-side MOSFET. Improper selection of the low-side MOSFET that results in a high ratio of CRSS/CISS makes the problem more severe. To avoid this problem, minimize the ratio of CRSS/CISS when selecting the low-side MOSFET. Adding a 1 Ω to 4.7 Ω resistor between BST and CBST can slow the high-side MOSFET turn-on. Similarly, adding a small capacitor from the gate to the source of the high-side MOSFET has the same effect. However, both methods work at the expense of increased switching losses. Layout Guidelines The MAX8791/MAX8791B MOSFET driver sources and sinks large currents to drive MOSFETs at high switch- ing speeds. The high di/dt can cause unacceptable ringing if the trace lengths and impedances are not well controlled. The following PCB layout guidelines are rec- ommended when designing with the MAX8791/ MAX8791B: 1) Place all decoupling capacitors as close as possi- ble to their respective IC pins. 2) Minimize the length of the high-current loop from the input capacitor, the upper switching MOSFET, and the low-side MOSFET back to the input-capacitor negative terminal. 3) Provide enough copper area at and around the switching MOSFETs and inductors to aid in thermal dissipation. 4) Connect GND of the MAX8791/MAX8791B as close as possible to the source of the low-side MOSFETs. A sample layout is available in the MAX8786 evaluation kit. ∆Θ TPD IC JJA =× () PD IC I V BIAS () =× 5 PD N RESISTIVE V V I R L OUT IN MAX LOAD TOTAL DS ON ( ) () () =− ⎛ ⎝⎜ ⎞ ⎠⎟ ⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ⎛ ⎝⎜ ⎞ ⎠⎟ 1 2 η Single-Phase, Synchronous MOSFET Drivers 10 ______________________________________________________________________________________ |
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