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MP9447 Datasheet(PDF) 12 Page - Monolithic Power Systems |
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MP9447 Datasheet(HTML) 12 Page - Monolithic Power Systems |
12 / 19 page MP9447–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER MP9447 Rev. 1.2 www.MonolithicPower.com 12 1/5/2016 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2016 MPS. All Rights Reserved. Figure 7 shows an equivalent circuit in PWM mode with the HS-FET OFF and without an external ramp circuit. The ESR ripple dominates the output ripple. The VFB downward slope is: REF SLOPE1 ESR V V L (7) From equation 7, the VFB downward slope is proportional to ESR/L. Therefore, it’s necessary to know the minimum ESR value of the output capacitors without an external ramp. There is also an inductance limit: A smaller inductance leads to more stability. Based on bench experiments, keep VSLOPE1 around 15V/ms to 30V/ms. In skip mode, the external ramp does not affect the downward slope, and VFB ripple’s downward slope is the same with or without the external ramp. Figure 8 shows an equivalent circuit with the HS-FET off and the current modulator regulating the LS-FET. IMOD Figure 8: Simplified Circuit in Skip Mode The VFB ripple’s downward slope is: REF SLOPE2 12 OUT V V RR C (8) To keep the system stable during light loads, avoid large VFB resistors. Also, keep the VSLOPE2 value around 0.4V/ms to 0.8mV/ms. Note that IMOD is excluded from the equation because it does not impact the system’s light-load stability. Soft-Start The MP9447 employs soft start (SS) to ensure a smooth output during power-up. When the EN pin goes HIGH, an internal current source (8.5μA) charges up the SS capacitor (CSS). The CSS voltage takes over the REF voltage to the PWM comparator. The output voltage smoothly ramps up with VSS. Once VSS reaches the same level as VREF, it continues ramping up while VREF takes over the PWM comparator. At this point, soft-start finishes and the MP9447 enters steady-state. CSS is then: SS SS SS REF tms I A CnF VV (9) If the output capacitors have large capacitance values, avoid setting a short SS or risk hitting the current limit during SS. Select a minimum value of 4.7nF if the output capacitance value exceeds 330μF. Over-Current Protection (OCP) and Short- Circuit Protection (SCP) The MP9447 has cycle-by-cycle over-current limit control. The inductor current is monitored during the ON state. Once the inductor current exceeds the current limit, the HS-FET turns OFF. At the same time, the OCP timer starts. The OCP timer is set at 100μs. Hitting the current limit during each cycle during this 100μs time frame will trigger hiccup SCP. If a short circuit occurs, the MP9447 will immediately hit its current limit and VFB will drop below 50%×VREF (0.815V). The device considers this an output dead short and will trigger hiccup SCP immediately. Under-Voltage Protection (UVP) The MP9447 monitors the output voltage through the tap of a resistor divider to the FB pin to detect output under-voltage conditions.. A VFB drop below 50% ×VREF triggers UVP as well as a current-limit that triggers SCP. |
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