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LTC1871 Datasheet(PDF) 11 Page - Linear Technology |
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LTC1871 Datasheet(HTML) 11 Page - Linear Technology |
11 / 24 page 11 LTC3783 3783f INTVCC Regulator Bypassing and Operation An internal, P-channel low dropout voltage regulator pro- duces the 7V supply which powers the gate drivers and logic circuitry within the LTC3783 as shown in Figure 3. The INTVCC regulator can supply up to 50mA and must be bypassed to ground immediately adjacent to the IC pins with a minimum of 4.7 µF low ESR or ceramic capacitor. Good bypassing is necessary to supply the high transient currents required by the MOSFET gate driver. For input voltages that don’t exceed 8V (the absolute maximum rating for INTVCC is 9V), the internal low drop- out regulator in the LTC3783 is redundant and the INTVCC pin can be shorted directly to the VIN pin. With the INTVCC pin shorted to VIN, however, the divider that programs the regulated INTVCC voltage will draw 15µA from the input supply, even in shutdown mode. For applications that require the lowest shutdown mode input supply current, do not connect the INTVCC pin to VIN. Regardless of whether the INTVCC pin is shorted to VIN or not, it is always necessary to have the driver circuitry bypassed with a 4.7 µF low ESR ceramic capacitor to ground immediately adjacent to the INTVCC and GND pins. In an actual application, most of the IC supply current is used to drive the gate capacitance of the power MOSFET. As a result, high input voltage applications in which a large power MOSFET is being driven at high frequencies can cause the LTC3783 to exceed its maximum junction tem- perature rating. The junction temperature can be esti- mated using the following equations: IQ(TOT) = IQ + f • QG PIC = VIN • (IQ + f • QG) TJ = TA + PIC • θJA The total quiescent current IQ(TOT) consists of the static supply current (IQ) and the current required to charge and discharge the gate of the power MOSFET. The 16-lead FE package has a thermal resistance of θJA = 38°C/W and the DHD package has an θJA = 43°C/W As an example, consider a power supply with VIN = 12V and VOUT = 25V at IOUT = 1A. The switching frequency is 300kHz, and the maximum ambient temperature is 70 °C. The power MOSFET chosen is the Si7884DP, which has a maximum RDS(ON) of 10mΩ (at room temperature) and a OPERATIO 1.230V R2 R1 P-CH 7V DRIVER GATE CVCC 4.7 µF X5R CIN INPUT SUPPLY 6V TO 36V GND PLACE AS CLOSE AS POSSIBLE TO DEVICE PINS M1 3783 F03 INTVCC VIN GND LOGIC 6V-RATED POWER MOSFET Figure 3. Bypassing the LDO Regulator and Gate Driver Supply |
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