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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

assumed to be 30% of the input voltage.

The switch must also be able to conduct the repetitive peak

primary current as determined by:

The primary current waveform of a discontinuous mode

flyback converter is triangular in shape, therefore, its root

mean square(rms) current is calculated by:

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.

Irms prim

()

IPEAKprim

3

⁄

()

TONmax

() T

⁄

()

=

ISL6401