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ADP3419JRM-REEL Datasheet(PDF) 9 Page - ON Semiconductor |
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ADP3419JRM-REEL Datasheet(HTML) 9 Page - ON Semiconductor |
9 / 13 page ADP3419 Rev. 2 | Page 9 of 13 | www.onsemi.com THEORY OF OPERATION The ADP3419 is a dual MOSFET driver optimized for driving two N-channel MOSFETs in a synchronous buck converter topology. A single PWM input signal is all that is required to properly drive the high-side and the low-side MOSFETs. Each driver is capable of driving a 3 nF load at speeds up to 1 MHz. A more detailed description of the ADP3419 and its features follows. Refer to the detailed block diagram in Figure 17. 5 ADP3419 VCC CROWBAR IN VCC BST DRVH GND SW DRVL UVLO AND BIAS OVERLAP PROTECTION AND TIME-OUT CIRCUIT RBST C BST VDCIN 5V D1 Q1 Q2 + DRVLSD SD 5 4 1 2 3 7 6 8 9 10 Figure 17. Detailed Block Diagram of the ADP3419 UNDERVOLTAGE LOCKOUT The undervoltage lockout (UVLO) circuit holds both MOSFET driver outputs low during VCC supply ramp-up. The UVLO logic becomes active and in control of the driver outputs at a supply voltage of no greater than 1.5 V. The UVLO circuit waits until the VCC supply has reached a voltage high enough to bias logic level MOSFETs fully on before releasing control of the drivers to the control pins. DRIVER CONTROL INPUT The driver control input (IN) is connected to the duty ratio modulation signal of a switch-mode controller. IN can be driven by 2.5 V to 5.0 V logic. The output MOSFETs are driven so that the SW node follows the polarity of IN. LOW-SIDE DRIVER The low-side driver is designed to drive a ground-referenced low RDS(ON) N-channel synchronous rectifier MOSFET. The bias to the low-side driver is internally connected to the VCC supply and GND. Once the supply voltage ramps up and exceeds the UVLO threshold, the driver is enabled. When the driver is enabled, the driver’s output is 180° out of phase with the IN pin. Table 4 shows the relationship between DRVL and the different control inputs of the ADP3419. HIGH-SIDE DRIVER The high-side driver is designed to drive a floating low RDS(ON) N-channel MOSFET. The bias voltage for the high-side driver is developed by an external bootstrap supply circuit, which is connected between the BST and SW pins. The bootstrap circuit comprises a diode, D1, and bootstrap capacitor, CBST. When the ADP3419 is starting up, the SW pin is at ground, so the bootstrap capacitor charges up to VCC through D1. Once the supply voltage ramps up and exceeds the UVLO threshold, the driver is enabled. When IN goes high, the high-side driver begins to turn on the high-side MOSFET (Q1) by transferring charge from CBST. As Q1 turns on, the SW pin rises up to VDCIN, forcing the BST pin to VDCIN + VC(BST), which is enough gate-to-source voltage to hold Q1 on. To complete the cycle, Q1 is switched off by pulling the gate down to the voltage at the SW pin. When the low-side MOSFET (Q2) turns on, the SW pin is pulled to ground. This allows the bootstrap capacitor to charge up to VCC again. When the driver is enabled, the driver’s output is in phase with the IN pin. Table 4 shows the relationship between DRVH and the different control inputs of the ADP3419. OVERLAP PROTECTION CIRCUIT The overlap protection circuit prevents both main power switches, Q1 and Q2, from being on at the same time. This is done to prevent shoot-through currents from flowing through both power switches and the associated losses that can occur during their on-off transitions. The overlap protection circuit accomplishes this by adaptively controlling the delay from Q1’s turn-off to Q2’s turn-on, and the delay from Q2’s turn-off to Q1’s turn-on. To prevent the overlap of the gate drives during Q1’s turn-off and Q2’s turn-on, the overlap circuit monitors the voltage at the SW pin and DRVH pin. When IN goes low, Q1 begins to turn off. The overlap protection circuit waits for the voltage at the SW and DRVH pins to both fall below 1.6 V. Once both of these conditions are met, Q2 begins to turn on. Using this method, the overlap protection circuit ensures that Q1 is off before Q2 turns on, regardless of variations in temperature, supply voltage, gate charge, and drive current. There is, however, a timeout circuit that overrides the waiting period for the SW and DRVH pins to reach 1.6 V. After the timeout period has expired, DRVL is asserted high regardless of the SW and DRVH voltages. In the opposite case, when IN goes high, Q2 begins to turn off after a propagation delay. The overlap protection circuit waits for the voltage at DRVL to fall below 1.6 V, after which DRVH is asserted high and Q1 turns on. |
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