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MP2188 Datasheet(PDF) 11 Page - Monolithic Power Systems |
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MP2188 Datasheet(HTML) 11 Page - Monolithic Power Systems |
11 / 17 page MP2188 – DUAL 3A, 5.5V, 1.2MHz, SYNCHRONOUS STEP-DOWN SWITCHER MP2188 Rev. 1.01 www.MonolithicPower.com 11 2/2/2016 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2016 MPS. All Rights Reserved. OPERATION The MP2188 uses constant on-time control with input voltage feed-forward to stabilize the switching frequency over its full input range. At light load, the MP2188 employs proprietary control over the low-side MOSFET (LS-FET) and inductor current to eliminate ringing on switching node and improve efficiency. The output voltages are fixed. The output voltages of two channels are 1.1V and 1.8V respectively. Constant-On–Time Control When compared to fixed-frequency PWM control, constant-on–time control offers advantages including simpler control loop and faster transient response. By using input voltage feed-forward, the MP2188 maintains a nearly constant switching frequency across the entire input and output voltage range. The on-time of the switching pulse can be estimated as: OUT ON IN V t0.833 s V =⋅ μ To prevent inductor current runaway during the load transient, the MP2188 has a fixed minimum off time of 50ns. However, this minimum off time limit does not affect the operation of the MP2188 in steady state in any way. Light-Load Operation Under light-load conditions, the MP2188 uses a proprietary control scheme to save power and improve efficiency: it gradually ramps down the LS-FET current to its minimum instead of turning off the LS-FET immediately when the inductor current starts to reverse. The gradual current drop avoids ringing at the switching node that always occurs in discontinuous conduction mode (DCM) operation. There is a zero current cross detect circuit (ZCD) to judge if the inductor current starts to reverse. When the inductor current touch ZCD threshold, the low side switch will start to be turned off. The DCM mode happens only after low side switch turned off by ZCD circuit. Considering the ZCD circuit propagation time, the typical delay is 30ns. It means the inductor current still fall after the ZCD is trigger during this delay. If the inductor current falling slew rate is fast (Output voltage is high or close to input Voltage), the low side MOSFET is turned off at the moment inductor current may be negative. This phenomena will cause MP2188 cannot enter DCM operation. If the DCM mode is required, the off time of low side MOSFET in CCM should be longer than 60ns. It means the maximum duty is 92% to guarantee DCM mode at light load. For example, Vin is 3.4V and Vo is 3.3V, the off time in CCM is 24.5ns. It is difficult to enter DCM at light load. And using smaller inductor can improve it and make it enter DCM easily. Enable When the input voltage exceeds the under- voltage lockout (UVLO) threshold—typically 2.2V—the MP2188 can be enabled by pulling the EN pin higher than 1.2V. Leaving EN pin floating or grounded will disable the MP2188. There is an internal 1MΩ resistor from the EN pin to ground. Soft-Start/Stop MP2188 has a built-in soft-start that ramps up the output voltage at a constant slew rate that avoids overshooting at startup. The soft-start time is typically about 1.3ms. When disabled, the MP2188 ramps down the internal reference voltage to allow the load to linearly discharge the output. Power GOOD Indictor MP2188 has an open drain with a 500kΩ pull- up resistor pin for power good (PG) indication. When the FB pin is within ±10% of the regulatory voltage (0.6V), the PG pin is pulled up to VIN by the internal resistor. If the FB pin voltage is outside the ±10% window, the PG pin is pulled to ground by an internal MOSFET. The MOSFET has a maximum Rdson of less than 100Ω. Current limit The MP2188 has a 5.3A minimum current limit for the high side switch (HS-FET). When the HS-FET hits its current limit, MP2188 enters |
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