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LM3489MM Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM3489MM Datasheet(HTML) 9 Page - National Semiconductor (TI) |
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9 / 15 page  Simplified Functional Block Diagram 20186938 Functional Description OVERVIEW The LM3489 is a buck (step-down) DC-DC controller that uses a hysteretic control scheme. The control comparator is designed with approximately 10mV of hysteresis. In re- sponse to the voltage at the FB pin, the gate drive (PGATE pin) turns the external PFET on or off. When the inductor current is too high, the current limit protection circuit en- gages and turns the PFET off for approximately 9µs. Hysteretic control does not require an internal oscillator. Switching frequency depends on the external components and operating conditions. The operating frequency reduces at light loads resulting in excellent efficiency compared to other architectures. The output voltage can be programmed by two external resistors. The output can be set in a wide range from 1.239V (typical) to V IN. HYSTERETIC CONTROL CIRCUIT When the FB input to the control comparator falls below the reference voltage (1.239V), the output of the comparator switches to a low state. This results in the driver output, PGATE, pulling the gate of the PFET low and turning on the PFET. With the PFET on, the input supply charges C OUT and supplies current to the load via the series path through the PFET and the inductor. Current through the Inductor ramps up linearly and the output voltage increases. As the FB voltage reaches the upper threshold, which is the internal reference voltage plus 10mV, the output of the comparator changes from low to high, and the PGATE responds by turning the PFET off. As the PFET turns off, the inductor voltage reverses, the catch diode turns on, and the current through the inductor ramps down. Then, as the output volt- age reaches the internal reference voltage again, the next cycle starts. The LM3489 operates in discontinuous conduction mode at light load current or continuous conduction mode at heavy load current. In discontinuous conduction mode, current through the inductor starts at zero and ramps up to the peak, then ramps down to zero. The next cycle starts when the FB voltage reaches the reference voltage. Until then, the induc- tor current remains zero and the output capacitor supplies the load. The operating frequency is lower and switching losses reduced. In continuous conduction mode, current always flows through the inductor and never ramps down to zero. The output voltage (V OUT) can be programmed by 2 external resistors. It can be calculated as follows: V OUT = 1.239 x (R1 + R2) / R2 20186923 FIGURE 1. Hysteretic Window www.national.com 9 |
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