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SS6621 Datasheet(PDF) 9 Page - Silicon Standard Corp. |
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SS6621 Datasheet(HTML) 9 Page - Silicon Standard Corp. |
9 / 12 page www.SiliconStandard.com 9 of 12 SS6620/21/2 Synchronous Rectification Using the internal synchronous rectifier eliminates the need for an external Schottky diode. Therefore, the cost and board space is reduced. During the cycle of off-time, the P-MOSFET turns on and shunts the N- MOSFET. Due to the low turn-on resistance of the MOSFET, the synchronous rectifier significantly im- proves efficiency without the addition of an external component. Thus, the conversion efficiency can be as high as 93%. Reference Voltage The reference voltage (REF) is nominally 1.23V for excellent T.C. performance. In addition, the REF pin can source up to 100 µA to an external circuit with good load regulation (<10mV). A bypass capacitor of 0.1 µF is required for proper operation and good per- formance. Shutdown The whole circuit is shutdown when SHDN V is low. During shutdown mode, current can flow from the bat- tery to the output through the body diode of the P- MOSFET. VOUT falls to approximately (Vin - 0.6V) and LX remains high impedance. The capacitance and load at OUT determine the rate at which VOUT decays. Shutdown can be pulled as high as 6V, regardless of the voltage at OUT. Current Limit Select Pin The SS6622 series allows a selectable inductor current limit of either 0.45A or 0.8A. This allows flexi- bility in designing for higher current or smaller appli- cations. CLSEL draws 1.4 µA when connected to OUT. BATT/Damping Switch The SS6622 is designed with an internal damping switch (Fig. 15) to reduce ringing at LX. The damping switch supplies a path to quickly dissipate the energy stored in the inductor and reduces the ringing at LX. Damping LX ringing does not reduce VOUT ripple, but does reduce EMI. A value of R1=200 Ω works well for most application while reducing efficiency by only 1%. Larger R1 values provide less damping, but less im- pact on efficiency. In principle, lower values of R1 are needed to fully damp LX when the VOUT /VIN ratio is high. Selecting the Output Voltage VOUT can be simply set to 3.3V by connecting the FB pin to OUT due to the internal resistor divider (Fig. 16). In order to adjust the output voltage, a resistor divider is connected to VOUT, FB, GND (Fig. 17). Use the following equation to calculate: R5=R6 [(VOUT / VREF )-1] where VREF =1.23V and VOUT may range from 1.8V to 4V. Low-Battery Detection The SS6620 series contain an on-chip comparator with 50mV internal hysteresis (REF, REF+50mV) for low battery detection. If the voltage at LBI falls below the internal reference voltage, LBO (an open-drain output) sinks current to GND. Rev.2.01 6/06/2003 |
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