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ISL6401CBZ Datasheet(PDF) 7 Page - Intersil Corporation |
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ISL6401CBZ Datasheet(HTML) 7 Page - Intersil Corporation |
7 / 11 page 7 FN9007.7 April 13, 2005 signal generation and off-hook loop current supply are two analog functions that are performed by Subscriber Line Interface Circuits (SLICs). A SLIC is the primary interface between the 4-wire (ground referenced) low voltage switch environment and the 2 wire (floating) high voltage loop environment. It performs a number of important functions including battery feed, overvoltage protection, ringing, signaling, coding, hybrid balancing and testing. The Ringing SLIC (RSLIC) typically requires two high voltage power supply inputs. The first is a tightly regulated voltage around -24V or -48V for off-hook voice transmission. The second is a loosely regulated -70 to -100V for ring tone generation. When the switch hook is released the phone puts approximately 200 Ω of resistance across the phone terminals. Once voice transmission begins, the SLIC requires a lower voltage input to establish a current loop of approximately 25mA. The loop feeds the 200 Ω, protection resistors, and line resistances within the phone. ISL6401 Flyback Reference Design The Typical Application Schematic shows a current mode power supply using the Intersil ISL6401 in a standard flyback topology. The IC requires +5V Bias. The application circuit is intended for wall adapters that power home gateway/router boxes. This circuit input voltage can be 9V to 20V with the selected transformer and external components. The output voltages are -24V at 120mA and -72V at 120mA. The circuit uses inexpensive transformers to generate both outputs using a single controller. The transformer turns ratio is such that 24V appear across each secondary winding and the primary during the switch off- time. The remaining secondary windings are stacked in series to develop -48V. The -48V section is then stacked on the -24V section to get the -72V. This technique provides good cross regulation, lowers the voltage rating required for the output capacitors, and lowers the RMS current, allowing the use of less expensive output capacitors. Also, the selection of a transformer with multifilar winding lowers the leakage inductance and cost. The -24V output is precisely regulated by feeding back this output to the controller. The -72V output is derived from the third pair of windings. Regulation of this output is obtained by the turn’s ratio of the transformer with -24V output, as well as with split feedback. Circuit Element Descriptions • Transformers T1, MOSFET Q1, Schottky diode D1, D2, and input capacitor C1 and C2 form the power stage of the converter. Power resistor R5 senses the switch current and converts this current into a voltage to be sensed by the primary side controller feedback comparator. • Capacitors C9 to C12 filter out high frequency noise on the output bus directly at the output diode. • R7 and C8 provide secondary side snubbing. • R6 and C7 filter out the leading edge voltage spikes resulting from the leakage inductance of the transformer. • C4 sets the switching frequency of the converter. • C3 is a decoupling capacitor, which should always be a good quality low-ESR/ESL type capacitor, placed as close to the IC pins as possible and returned directly to the IC ground reference. • The gate drive circuitry can be composed of a small gate drive resistor, necessary for damping any oscillations resulting from the input capacitance of Q1 and any parasitic stray inductance. • The voltage sense feedback loop is comprised of R4 and R3. Feedback components R1, C6, and C5 provide the necessary gain and pole to stabilize the control loop. Component Selection Guidelines Power MOSFET The MOSFET switch is selected to meet the drain to source voltage stress resulting from the maximum input voltage (VIN(max)), the reflected secondary voltages, equal to the output voltage (VOUT), plus the output diode voltage drop (VF), and the voltage spike due to the leakage inductance, assumed to be 30% of the input voltage. Vds (stress) = [(VIN(max)) + (N)(Vout +Vf)] + (0.3)(VIN(max)) The switch must also be able to conduct the repetitive peak primary current as determined by: Ipeak (primary) = (Vinmin - Vds) (tON(max)) / Lp The primary current waveform of a discontinuous mode flyback converter is triangular in shape, therefore, its root mean square(rms) current is calculated by: The chosen device should also have a low RDS(ON) value, because the conduction losses of the device are proportional to the square of the primary rms current through the device. Selection of a device that has a peak current rating of at least three times the peak current usually insures acceptably low conduction losses. Pconduction = (Iprms2) (RDS(on)) Irms prim () IPEAKprim 3 ⁄ () TONmax () T ⁄ () = ISL6401 |
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