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SS6577CGOTB Datasheet(PDF) 10 Page - Silicon Standard Corp. |
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SS6577CGOTB Datasheet(HTML) 10 Page - Silicon Standard Corp. |
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10 / 14 page  www.SiliconStandard.com 10 of 14 SS6577 Rev.1.02 3/26/2004 Output Diode Selection + = 1 R 2 R 1 V 8 . 0 VOUT In order not to exceed the diode ratings, it is impor- tant to specify the diode peak current and average power dissipation. The feedback reference voltage 0.8V allows low output voltages from 0.8V to input voltage. A small capacitor at 1nF in parallel to the upper feedback resistor is required for a stable feedback. CIN and COUT Selection To prevent the high voltage spike resulted from high frequency switching, a low ESR input capacitor for the maximum RMS current must be used. Usually capacitors may be paralleled to meet size or height requirements in the design. ITH/RUN Function The ITH/RUN pin, also as a dual-purpose pin, pro- vides loop compensation as well as shutdown func- tion. An internal current source at 2.5 µA charges up the external capacitor C5. When the voltage on ITH/RUN pin reaches 0.8V, the SS6577 begins to operate. The selection of COUT depends on the required ef- fective series resistance (ESR). In general once the ESR requirement is met, the capacitance is suitable for filtering. The output ripple voltage (ΔVOUT) is determined by: C5 330pF R3 24k D2 LL4148 C7 1 µF R4 1.2M VIN 4.5V~24V ITH/RUN + ∆ ≈ ∆ OUT L OUT fC 4 1 ESR I V where f = operating frequency, COUT = output ca- pacitance and ΔIL = ripple current of the inductor. Once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. Fig. 12 ITH/RUN pin interfacing Topside MOSFET Driver Supply (C3) External bootstrap capacitor C3 connecting to BOOST pin supplies the gate drive voltage for high- side MOSFET. C3 is charged from INTVCC when SW pin is low. When the high-side MOSFET turns on, the driver places the C3 voltage across the gate to the source of MOSFET. It will enhance the MOSFET and turn on the high-side switch. Then the switch node voltage SW rises to VIN and BOOST pin rises to VIN + INTVCC. In general, 0.1 µF is acceptable. Over Current Protection Over current protection occurs when the peak in- ductor current reaches maximum current sense threshold divided by sense resistor. The maximum current under over current protection can be calcu- lated by the following formula. S R threshold) sense current mum 150mV(Maxi MAX I = At the same time, the frequency of oscillator will be reduced to sixteenth of original value, 500kHz. This lower frequency allows the inductor current to safely discharge, thereby preventing current run- away. The frequency of oscillator will automatically Output Voltage Programming The typical SS6577 application circuit is shown in figure17. A resistive divider, as in the following for- mula, sets the output voltage. |
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