Specifications

8.3. REGULATOR SELECTION
8.3.2 ST Microelectronics’s L4976
ST Microelectronics’s L4976 devices operate at frequencies up to 300kHz
and are capable of delivering currents in excess of 1A [21]. Also included are
protection mechanisms for thermal overload, current limiting and protection
against feedback disconnection. A further advantage is that they are offered
in standard DIP8 packages, which aids prototyping in that they can be
housed on standard breadboards. An initial concern was the limited current
of 1A. ST provide a pin-compatible upgrade in the form of the L4978 which
specifies 2A continuous current [22]. This provides a safety upgrade path
should the L4976 prove insufficient. The L4978 was not available from lo cal
suppliers at the time of authoring, but it could be ordered with a projected
lead-time of 3 weeks.
However, our concerns were unfounded — the L4978 has been tested
to reliably deliver 2A over a short period of time. The limiting factor is
the power dissipation in the plastic package, rather than a peak current
limitation. Thus, the device operates reliably provided the total average
current remains below 1A and the power dissipated in the package below
1W. This is more than acceptable as the GSM mo dule only draws these
peaks every 5ms with pulse lengths in the order of microseconds. Average
consumption guaranteed to be less than 850mA [17]. Furthermore, the use
of large tantalum capacitors on the output rail smooth the load of these
transients on the switching converter. The operation at 300kHz requires an
inductor of approximately 80µH for sufficiently low noise on the output to
guarantee reliable operation of the digital circuitry. A high speed, low loss
Schottky recovery diode was used (with a 0.3V forward drop) rather than a
standard silicone diode (with a 0.7V forward drop) in an effort to improve
efficiency. Figure 8.1 shows the DC rail output of the switching converter
under light-load c onditions (only uplink module operating).
The recommended circuit outlined in the L4976 datasheet proved to be
sufficient. The following alterations were made:
• The capacitor and resistor which set the switching frequency were
changed to produce 300kHz instead of 100kHz to better respond to
the GSM module’s transient loads.
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