FEATURES

Fixed +5-V or +3.3-V Output

Integrated Floating Feedback Amplifier

On-Chip 70-V, 1.5- N-Channel

MOSFET Switch

Integrated High Voltage Start-Up Circuit,

with VCC Regulator

−10-V to −60-V Input Voltage Range

95-kHz PWM Operation

Integrated Soft-Start and Oscillator

High Efficiency Over Full Load Range

Under Voltage Lockout

Current Mode Control

Hiccup Mode Short Circuit

Protection

Thermal Shutdown

SOIC-8 Narrow-Body Package

DESCRIPTION

The Si9121 simplifies the −48-V to +5-V or +3.3-V converter

design for ISDN application by integrating the floating

feedback error amplifier providing direct output voltage

regulation. This approach eliminates the need for an external

shunt regulator. The Si9121 also integrates a high voltage

depletion mode MOSFET which allows the converter to be

powered directly from the high input bus voltage without

requiring an external start-up circuit. Combined with simple

magnetic design due to its non-isolated topology, the Si9121

provides a one-chip solution for complete ISDN power supply.

In order to reduce external component count, the Si9121 has

a fully integrated 95-kHz oscillator and soft-start circuit.

The Si9121 is available in both standard and lead (Pb)-free

SOIC-8 pin packages, and is offered in either +5-V or +3.3-V

fixed output options (Si9121DY-5 or Si9121DY-3,

respectively). In order to satisfy the stringent ambient

temperature requirements in many applications, the Si9121 is

rated to the industrial temperature range of −40 C to 85 C.