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ML4903CT Datasheet(PDF) 10 Page - Micro Linear Corporation |
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ML4903CT Datasheet(HTML) 10 Page - Micro Linear Corporation |
10 / 12 page ML4903 10 DRV and P DRV switch the capacitive loads of the output MOSFET gates. At the same time, GND must not see the resulting switching spikes. If a current sensing resistor is used, the voltage across the resistor must be Kelvin-sensed. This ensures that the ML4903 monitors only the voltage across the resistor, and ignores the voltage drops and inductive transients in the PCB traces which carry current into and out of this resistor. The two pins of the ML4903 which must be Kelvin-connected to the sense resistor are ISENSE and GND. PWR GND should then return to the to the grounded end of RSENSE as well, using a high current Kelvin connection. This causes any noise across the resistor to appear primarily as a common-mode signal on ISENSE, GND, and PWR GND. Figure 4 shows a recommended implementation of these PCB layout requirements. When directly monitoring the voltage across the channel of the synchronous rectifier, the voltage across that MOSFET should be sensed as closely as possible to its drain. If a resistor divider is used to reduce the voltage at the ISENSE pin for a given current through (Q3||Q4)’s channel resistance, then the lower end of the divider should be returned to the immediate vicinity of its source. This ensures that the ML4903 monitors only the voltage across the synchronous rectifier, and not the voltage drops or inductive transients in the PCB traces which carry current into and out of it. If a PC board with a dedicated ground plane is used (recommended), the best return points for GND and PGND are directly into the ground plane. If the board does not have a dedicated ground plane, GND must be returned to a point near the IC which is relatively free from switching transients. Such a point may need to be empirically determined but will usually be near the ground connection of the output capacitor bank. MISCELLANEOUS POINTS ISENSE is the input to a medium-speed, high-sensitivity comparator (roughly comparable to an LM339-type comparator in terms of speed of response). Because of the leading-edge blanking on this comparator, it has a substantial ability to reject switching noise. Still, proper circuit function requires that the comparator not see significant noise at the time during which the synchronous rectifier MOSFET is on. The compensation components R3 and C13 are high- impedance nodes connected to the output of the voltage loop error amplifier. These components should be kept in close proximity to the ML4903. C13 should be returned to GND, not to PWR GND or the ground plane of the PC board. Keep the VREF bypass capacitor C14 close to the ML4903. Ensure that its ground connection is to GND, not to PWR GND. The 12V VDD input is both the converter's bulk power input, and the supply from which the internal circuitry of the ML4903 operates. VDD also provides the gate drive for P DRV and N DRV. The VDD pin of the ML4903 should be decoupled from the raw 12V system power to prevent noise on the 12V supply from affecting the circuit's switching action. A 10 W resistor between 12VIN and VDD DESIGN CONSIDERATIONS (Continued) SENSE RESISTOR TO ISENSE TO GND TO PWR GND POWER GROUND RETURN (GROUND PLANE) TO SYNCHRONOUS RECTIFIER MOSFET SOURCE Figure 4. Kelvin Sense Connections |
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