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SP7648TR Datasheet(PDF) 5 Page - Sipex Corporation |
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SP7648TR Datasheet(HTML) 5 Page - Sipex Corporation |
5 / 12 page 5 Date: 7/20/05 SP7648 Ultra-low Quiescent Current, High Efficiency Boost Regulator © Copyright 2005 Sipex Corporation _________________________DETAILED DESCRIPTION The SP7648 is a step-up DC-DC converter with an input voltage operation range from 2.7V to 4.7V. In addition to the main 0.3Ω internal NMOSFET switch the SP7648 has an internal synchronous rectifier, thereby increasing efficiency and reducing the space and cost of an external diode. An internal inductive-damping switch significantly re- duces inductive ringing for low noise-high efficiency operation. If the supply voltage drops below 0.61V the SP7648 goes into under voltage lockout, thus opening both internal switches. The inductor peak current is externally programmable to allow for a range of inductor values. __________________________ CIRCUIT LAYOUT Printed circuit board layout is a critical part of a power supply design. Poor designs can result in excessive EMI on the feedback paths and on the ground planes with applica- tions involving high switching frequencies and large peak currents. Excessive EMI can result in instability or regulation errors. All power components should be placed on the PC board as closely as possible with the traces kept short, direct, and wide (>50mils or 1.25mm). Extra copper on the PC board should be integrated into ground as a pseudo- ground plane. On a multilayer PC board, route the star ground using component-side copper fill, then connect it to the internal ground plane using vias. For the SP7648 THEORY OF OPERATION devices, the inductor and input & output filter capacitors should be soldered with their ground pins as close together as possible in a star-ground configuration. The VOUT pin must be bypassed directly to ground as close to the SP7648 devices as possible (within 0.2in or 5mm). The DC-DC converter and any digital circuitry should be placed on the opposite corner of the PC board as far away from sensitive RF and analog input stages. Noisy traces, such as from the LX pin, should be kept away from the voltage- feedback VFB node and separated from it using grounded copper to minimize EMI. See the SP7648EB Evaluation Board Manual for PC Board Layout design details. ________________ CONTROL SCHEME A minimum off-time, current limited pulse frequency modulation (PFM) control scheme combines the high output power and effi- ciency of a pulse width modulation (PWM) device with the ultra low quiescent current of the traditional PFM. At low to moderate output loads the PFM control provides higher efficiency than traditional PWM converters are capable of delivering. At these loads the switching frequency is determined by a mini- mum off-time (TOFF, MIN) and a maximum on-time (TON, MAX) where: TOFF < KOFF / (VOUT - VBATT) TON > KON / VBATT KOFF = 1.0Vµs KON = 3.5 Vµs |
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