A new multiwinding transformer (configurable by the user for a variety of applications) enables an inverting controller to produce the high negative voltages required by an ISDN board or other telephone line card. Such line cards use a subscriber-line interface circuit (SLIC), such as the 79R79 ringing SLIC from AMD. This IC generates the off-hook and on-hook signal transmission, ring-tone generation, and ring-tip detection that constitute an analog telephone interface. For off-hook signal transmission, it requires a tightly regulated -24 or -48 V; to generate ring tones, it requires a loosely regulated -70 V. The "five -ringer-equivalent" requirement demands 9 to 10 W from the -70-V output, which translates to a full-load lout of about 150 mA. IC1 is an inverting switching regulator that usually converts a 3- to 16-V input to a fixed output of -5 V or an adjustable output. In the circuit shown, three pairs of windings in series (provided by a single off-the-shelf multiwinding transformer) enable IC1 to generate the high voltages needed by a SLIC IC1 (D1). Connecting a diode and output capacitors (C7 and C9) at the first or second pair of windings produces -24 V (as shown) or -48 V, respectively. Feedback to the IC via R1 and R2 achieves tight regulation at this output. The trans-former turns ratios establish a loose regulation at the -70-V output. The circuit can service a five telephone load (10 W) from an input of 12 V, ±10 percent. It operates down to 3 V and produces about 2.4 W at 3.3 V and 3.9 W at 5 V. The -70-V output depends on cross-regulation with respect to the -24-V output. It is, therefore, affected by relative loading on the two outputs (i.e., whether one is heavily loaded and the other lightly loaded, or vice versa).
The MAX774/MAX775/MAX776 inverting switching regulators deliver high efficiency over three decades of load current. A unique current-limited, pulsefrequency- modulated (PFM) control scheme provides the benefits of pulse-width modulation (high efficiency with heavy loads), while using less than 100μA of supply current (vs. 2mA to 10mA for PWM converters). The result is high efficiency over a wide range of loads.
These ICs also use tiny external components; their high switching frequency (up to 300kHz) allows for less than 5mm diameter surface-mount magnetics.
The MAX774/MAX775/MAX776 accept input voltages from 3V to 16.5V, and have preset output voltages of -5V, -12V, and -15V, respectively. Or, the output voltage can be user-adjusted with two resistors. Maximum VIN - VOUT ifferential voltage is limited only by the breakdown voltage of the chosen external switch transistor.
These inverters use external P-channel MOSFET switches, allowing them to power loads up to 5W. If less power is required, use the MAX764/MAX765/MAX766 inverting switching regulators with on-board MOSFETs.
ABSOLUTE MAXIMUM RATINGS
Supply Voltages
V+ to OUT ...........................................................................21V
V+ to GND ..............................................................-0.3V, +17V
OUT to GND ........................................................-0.3V, to -17V
REF, SHDN, FB, CS......................................-0.3V to (V+ + 0.3V)
EXT ...............................................(VOUT - 0.3V) to (V+ + 0.3V)
Continuous Power Dissipation (TA = +70°C)
Plastic DIP (derate 9.09mW/°C above +70°C) ...........727mW
SO (derate 5.88mW/°C above +70°C)........................471mW
CERDIP (derate 8.00mW/°C above +70°C) ................640mW
Operating Temperature Ranges:
MAX77_C__ .......................................................0°C to +70°C
MAX77_E__ ....................................................-40°C to +85°C
MAX77_MJA ..................................................-55°C to +125°C
Maximum Junction Temperatures:
MAX77_C__/E__...........................................................+150°C
MAX77_MJA...................................................................+175°C
Storage Temperature Range ........................-65°C to +160°C
Lead Temperature (soldering, 10sec) ...........................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. hese are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating onditions for extended periods may affect device reliability.
Required Field 
