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The design uses a LM2651 synchronous controller utilizing integrated high side and low side MOSFETs operating at 300kHz. Synchronous rectification provides high efficiency for the design. At light loads LM2651 enters hysteretic control to keep the efficiency high. When the load is greater than the sleep mode threshold, LM2651 operates in PWM mode at fixed frequency of 300khz. When the load current decreases below the sleep mode threshold, LM2651 enters hysteretic mode operation. Cycle by Cycle current limit turns off high side Mosfet whenever the current reaches 2A.Careful PC board layout is critical to achieve low switching losses and clean, stable operation. The switching power stage requires particular attention.
Few points to note for this design are:
1. Decoupling capacitors are close to IC pins as possible. Keep separate power ground plane.
2. Input and output capacitors are connected to the power ground plane; all other capacitors are connected to the signal ground plane.
3. High current paths are very short.
4. Multiple MOSFETs have been used to reduce the conduction and switching losses.
5. Feedback connections are short and direct and routed away from any noisy traces (i.e. switch node).
6. High-side MOSFETs are as close as possible to the controller.
Notes: The LM2651 used here is in 1.8V option and provides the 1.8V 1.5A output. The 3.3V 2A output is provided by the LM2655 in 3.3V option. The LP2985 in 2.5V option provides the 2.5V 0.1A output.
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Example Schematic Showing Connection for all Components.
