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AP5100_1011 Datasheet(PDF) 8 Page - Diodes Incorporated |
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AP5100_1011 Datasheet(HTML) 8 Page - Diodes Incorporated |
8 / 10 page AP5100 1.2A Step-Down Converter with 1.4MHz Switching Frequency AP5100 Document number: DS32130 Rev. 2 - 2 8 of 10 www.diodes.com November 2010 © Diodes Incorporated Applications Information (Continued) Internal Soft Start Soft start is traditionally implemented to prevent the excess inrush current. This in turn prevents the converter output voltage from overshooting when it reaches regulation. The AP5100 has an internal current source with a soft start capacitor to ramp the reference voltage from 0V to 0.810V. The soft start time is internally fixed at 200us (TYP). The soft start sequence is reset when there is a Thermal Shutdown, Under Voltage Lockout (UVLO) or when the part is disabled using the EN pin. Current Limit The AP5100 has cycle-by-cycle current limiting implementation. The voltage drop across the internal high- side mosfet is sensed and compared with the internally set current limit threshold. This voltage drop is sensed at about 30ns after the HS turns on. When the peak inductor current exceeds the set current limit threshold, current limit protection is activated. During this time the feedback voltage (VFB) drops down. When the voltage at the FB pin reaches 0.4V, the internal oscillator shifts the frequency from the normal operating frequency of 1.4MHz to a fold- back frequency of 480kHz. The current limit is reduced to 70% of nominal current limit when the part is operating at 480kHz. This low Fold-back frequency prevents runaway current. Thermal Shutdown The AP5100 has on-chip thermal protection that prevents damage to the IC when the die temperature exceeds safe margins. It implements a thermal sensing to monitor the operating junction temperature of the IC. Once the die temperature rises to approximately 140°C, the thermal protection feature gets activated .The internal thermal sense circuitry turns the IC off thus preventing the power switch from damage. A hysteresis in the thermal sense circuit allows the device to cool down to approximately 120°C before the IC is enabled again. This thermal hysteresis feature prevents undesirable oscillations of the thermal protection circuit. PC Board Layout This is a high switching frequency converter. Hence attention must be paid to the switching currents interference in the layout. Switching current from one power device to another can generate voltage transients across the impedances of the interconnecting bond wires and circuit traces. These interconnecting impedances should be minimized by using wide, short printed circuit traces. The input capacitor needs to be as close as possible to the IN and GND pins. The external feedback resistors should be placed next to the FB pin. |
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