APW1172
Functional Description (Cont.)
Over-Voltage Protection (OVP) (Cont.)
current limit function acting and VOUT dropping. This
resultsthe switching frequencydecreased. In the prac-
tical application, when the load current increase big
enough such that current limit occurring. In this
situation,more load current cause the output voltage
get away the regulatory point and begin dropping until
it’s limitation. In this time, the actual duty was very
small in general. But the on time period limited by the
minimum on time limitation of the control circuit. This
on time limitation induce the load current runs away
the limiting boundary. To prevent this drawback, the
frequency fold back is used to ensure that load cur-
rent was limited by the setup value.
internal comparator. Theinput of the OVPcomparator
connects to the feedback, that turns off the pass
element when the OVP threshold is reached. This
threshold is typically 25% higher than the feedback
voltage.
Thermal protection
The thermal protection function generates a control
signal to shut off the APW1172. It prevents the
damages caused by over heat situation. The thermal
function was acted when the temperature of chip
reaching 160°C. A hysteresis of the thermal protection
function is approximately 30°C, in order to avoid pass
element turns on and off immediately.
Inhibit Function
The Inhibit function disables when the Inhibit voltage
lower than 1.3V. APW1172 entered the standby mode
with Inhibit voltage higher than 1.4V. The quiescent
current in the standby mode is less than 100uA to
saving power. If the Inhibit pin left floating, the Inhibit
voltage will be pull up by internal current source.
Voltage Feed Forward
The Voltage Feed Forward is acting when VCC goes
higher than 10V. This will increasesthe upper bond of
the internal sawtooth wave and results duty keeping
constant. The change of the upper bond is linear and
proportion with VCC.
Frequency Fold Back
The FrequencyFold Back function actswhen both the
Application Description
Input Capacitor
necessary to use low-ESR capacitors. More
capacitance reduce the variations of the input voltage
of VCC pin.
TheAPW1172 requiresproper input capacitorstosup-
ply current surge during stepping load transients to
prevent the input rail from dropping.Due to the wide
range of input voltage, the input capacitor must be
able to support the input operating voltage. Ultra-low-
ESR capacitors, such as ceramic chip capacitors,
are very good for the input capacitors. An aluminum
electrolytic capacitor(>100mF, ESR<300mW) is
recommended as the input capacitor. It is not
Inductor
Inductor is an important component in the application.
In the switching regulator, energy stored in the
inductor by magnetic field when the pass element
conducting. This behavior cause the ripple current
cycle by cycle, the ripple current flowing through the
Copyright ã ANPEC Electronics Corp.
12
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Rev. A.4 - Aug., 2005
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
