SA57003 Datasheet, PDF (9/16 Pages) NXP Semiconductors – Five-output composite voltage regulator

Philips Semiconductors
Five-output composite voltage regulator
Product data
SA57003
TECHNICAL DESCRIPTION
The SA57003 is a monolithic composite five-output regulator
developed to power the RF sections of mobile telephones. It
contains three independent full-featured voltage regulator circuits.
Each regulator circuit incorporates individual feedback error
amplifiers for output voltage regulation, output On/Off Control, Noise
Bypass Pin, Current Limiting, and Thermal Shutdown. The Noise
Bypass Pins provide the option of externally bypassing an internal
voltage reference node for enhanced noise reduction.
The output of one of the three regulator circuits, in addition to being
pinned out, feeds two dependent switched output regulators. Both
switched output regulators incorporate individual feedback error
amplifiers for output voltage regulation but have no thermal
shutdown or current limiting feature.
The three full-featured regulators have typical dropout voltages of
200 mV at 30 mA of output current. The two switched outputs have
a minimum current capacity of 80 mA each.
Each independent regulator in the SA57003 is a series pass
regulator incorporating a bandgap reference, two feedback
amplifiers, thermal shutdown circuit, and output current limiting.
See the device block diagram shown in Figure 10 and the equivalent
circuit in Figure 11. Both feedback amplifiers are referenced to the
same bandgap reference. A PNP transistor is used in the device’s
output and serves as a series pass element. The output PNP pass
transistor incorporates a dual collector. The first feedback amplifier
monitors the first collector’s output voltage through the use of a
voltage divider network fed from the output. The second collector
monitors the output current and produces a small output current
proportional to that current delivered to the output. This small
proportional current is used to generate a second feedback voltage
fed to the second feedback amplifier to fold back the output current
to a safe level in the event of an output short. Both feedback
amplifiers act on the same control node to control the PNP pass
transistor. Dual path output monitoring in this manner maintains a
constant output voltage while adding the feature enhancement of
output current limiting.
Operating stability of the SA57003 linear regulator is determined by
start-up delay, transient response to loading, and stability of the
feedback loop. The SA57003 has a fast transient loop response. No
built-in delay is incorporated.
Capacitors play an important part in compensating the regulator’s
output. A 4.7 µF aluminum electrolytic capacitor is recommended for
most applications. This consideration is made primarily on a basis of
minimal cost with good performance.
A tantalum capacitor could also be used. Tantalum capacitors have
the advantage of being smaller size than electrolytic capacitors of
the same value of capacitance. Tantalum capacitors are also not
prone to dry-out. The electrolyte used in electrolytic capacitors tends
to dry-out with time causing degradation in capacitance value. Avoid
using low ESR film or ceramic capacitors to avoid instability problems.
Keep in mind that the output capacitor tries to supply any
instantaneous increase in load current. Using higher values of
capacitance will enhance transient load performance as well as
stability. Lowering the ESR of the capacitors will also improve the
transient response to load current changes but at the expense of
stability.
2003 Oct 13
9