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MP2233 Datasheet(PDF) 11 Page - Monolithic Power Systems |
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MP2233 Datasheet(HTML) 11 Page - Monolithic Power Systems |
11 / 18 page MP2233 – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS MP2233 Rev. 1.01 www.MonolithicPower.com 11 12/6/2012 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2012 MPS. All Rights Reserved. enough hysteresis to allow for any input supply variations. Figure 3: Adjustable UVLO External Soft-Start The MP2233 employs a soft-start (SS) mechanism to ensure smooth output during power-up. When the EN pin goes high, an internal current source (14μA) charges the SS capacitor. The SS capacitor voltage overtakes the REF voltage to the PWM comparator. The output voltage smoothly ramps up with the SS voltage. Once the SS voltage reaches the same level as the REF voltage, it keeps ramping up while VREF takes over the PWM comparator. At this point, the soft-start finishes and the device enters steady state operation. If the output is pre-biased to a certain voltage during startup, the IC will disable the switching of both high-side and low-side switches until the voltage on the internal soft-start capacitor exceeds the sensed output voltage at the FB pin. The SS capacitor value can be determined as follows: () ( ) ( ) () SS SS SS REF ms I A CnF VV τ× μ = (1) If the output capacitors have large capacitances, avoid setting a short SS time to avoid hitting the current limit during SS. Use a minimum value of 4.7nF if the output capacitance value is larger than 330μF. Over-Current-Protection and Hiccup The MP2233 has a cycle-by-cycle over-current limit that protects against the inductor current peak value exceeding the set current limit threshold. Under-voltage protection (UVP) triggers if the FB voltage drops below the under-voltage (UV) threshold—typically 50% below the reference. Once UVP triggers, the MP2233 enters hiccup mode to periodically restart the part. This protection mode is especially useful when the output is dead- shorted to ground. The average of short-circuit current falls to alleviate thermal issues and to protect the regulator. The MP2233 exits hiccup mode once the over-current condition is removed. Thermal Shutdown Thermal shutdown prevents the chip from operating at exceedingly high temperatures. When the silicon die temperature exceeds 150°C, it shuts down the whole chip. When the temperature drops below its lower threshold (typically 130°C) the chip is enabled again. Floating Driver and Bootstrap Charging An external bootstrap capacitor powers the floating power MOSFET driver. This floating driver has its own UVLO protection with a rising threshold of 2.2V and a hysteresis of 150mV. The bootstrap capacitor voltage is regulated internally by VIN through D1, M1, R3, C4, L1 and C2 (Figure 4). If (VIN-VSW) exceeds 5V, U1 will regulate M1 to maintain a 5V BST voltage across C4. A 20Ω resistor placed between SW and BST capacitor is strongly recommended to reduce SW voltage spikes. R3 Figure 4: Internal Bootstrap Charging Circuit |
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