APW7134 Просмотр технического описания (PDF) - Kingbright
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APW7134 Datasheet PDF : 16 Pages
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APW7134
Functional Descriptions (Cont.)
Short Circuit Protection Cont.
zero leads to the voltage across the inductor maximum
in the on period and the voltage across the inductor
minimum in the off period. In order to maintain the
volts-seconds balance, the off-time must be extended
to prevent the inductor current run away. Frequency
decay will extend the switching period to provide more
times to the off-period, then the inductor current have
to restrict to protect the electrical circuit in the short
situation.
Dropout Operation
An important detail to remember is that on resistance
of PMOSFET switch will increase at low input supply
voltage. Therefore, the user should calculate the power
dissipation when the APW7134 is used at 100% duty
cycle with low input voltage.
Slope Compensation
Slope compensation provides stability in constant
frequency current mode architecture by preventing
sub-harmonic oscillations at high duty cycle. It is
accomplished internally by adding a compensating
ramp to the inductor current signal at duty cycle in
excess of 40%. Normally, this results in a reduction
of maximum inductor peak current for duty cycles
greater than 40%. In the APW7134, the reduction of
inductor peak current recovered by a special skill at
high duty ratio. This allow the maximum inductor peak
current maintain a constant level through all duty ratio.
Application Description
Inductor Selection
Due to the high switching frequency as 1.5MHz, the
inductor value of the application field of APW7134 is
usually from 1µH to 4.7µH. The criterion to selecting a
suitable inductor is dependent on the worst current
ripple throughout the inductor. The worst current ripple
defines as 40% of the fully load capability. In the
APW7134 applications, the worst value of current
ripple is 240mA, the 40% of 600mA. Evaluate L by
equation (1):
( ) L = V IN − V OUT ⋅ V OUT ⋅
1
V IN
∆ IL ⋅ fS
(1)
where fS is the switching frequency of APW7134 and
∆IL is the value of the worst current ripple, it can be
any value of current ripple that smaller than the worst
value you can accept. In order to perform high efficiency,
selecting a low DC resistance inductor is a helpful way.
Another important parameter is the DC current rating
of the inductor. The minimum value of DC current
rating equals the full load value of 600mA, plus the
half of the worst current ripple, 120mA. Choose
inductors with suitable DC current rating to ensure
the inductors don’t operate in the saturation.
Input Capacitor Selection
The input capacitor must be able to support the
maximum input operating voltage and maximum RMS
input current. The Buck converter absorbs current from
input in pulses.
Copyright © ANPEC Electronics Corp.
10
Rev. A.1 - Aug., 2006
www.anpec.com.tw
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