APW1173 Ver la hoja de datos (PDF) - Anpec Electronics
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APW1173 Datasheet PDF : 21 Pages
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APW1173
Functional Description (Cont.)
Over-Voltage Protection (OVP) (Cont.)
internal comparator. The input of the OVP comparator
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 APW1173. It prevents the
damages caused by over heat situation. The thermal
func-tion 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.
Voltage Feed Forward
The Voltage Feed Forward is acting when VCC goes
higher than 10V. This will increases the 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 current limit function acting and VOUT dropping.
This results the switching frequency decreased. In the
practical 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 current
was limited by the setup value.
Inhibit Function
The Inhibit function disables when the Inhibit voltage
lower than 1.3V. APW1173 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.
The Frequency Fold Back function acts when both
Application Description
Input Capacitor
The APW1173 requires proper input capacitors to sup-
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 (>100µF, ESR<300mΩ) is
recommended as the input capacitor. It is not neces-
sary to use low-ESR capacitors. More capacitance
reduce the variations of the input voltage of VCC pin.
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 output capacitor
induce the output ripple voltage. In general, the ripple
Copyright © ANPEC Electronics Corp.
12
Rev. A.5 - Aug., 2005
www.anpec.com.tw
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