APW7085K View Datasheet(PDF) - Anpec Electronics
Part Name
Description
Manufacturer
APW7085K Datasheet PDF : 24 Pages
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APW7085
Application Information (Cont.)
Output Capacitor Selection (Cont.)
∆VCOUT =
∆I
(V)
8 ⋅ FOSC ⋅ COUT
........... (4)
For the applications, using bulk capacitors, the ∆VCOUT is
much smaller than the V and can be ignored. Therefore,
ESR
the AC peak-to-peak output voltage (∆VOUT ) is shown as
below:
∆VOUT = ∆ I ⋅ ESR (V)
........... (5)
For the applications, using ceramic capacitors, the VESR is
much smaller than the ∆VCOUT and can be ignored.
Therefore, the AC peak-to-peak output voltage (∆VOUT ) is
close to ∆VCOUT .
The load transient requirements are the functions of the
slew rate (di/dt) and the magnitude of the transient load
current. These requirements are generally met with a
mix of capacitors and careful layout. High frequency ca-
pacitors initially supply the transient and slow the current
load rate seen by the bulk capacitors. The bulk filter ca-
pacitor values are generally determined by the ESR
(Effective Series Resistance) and voltage rating require-
ments rather than actual capacitance requirements.
High frequency decoupling capacitors should be placed
physically as close to the power pins of the load as
possible. Be careful not to add inductance in the circuit
board wiring that could cancel the usefulness of these
low inductance components. An aluminum electrolytic
capacitor’s ESR value is related to the case size with lower
ESR available in larger case sizes. However, the Equiva-
lent Series Inductance (ESL) of these capacitors increases
with case size and can reduce the usefulness of the ca-
pacitor to high slew-rate transient loading.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies permit the
use of a smaller inductor for the same amount of inductor
ripple current. However, this is at the expense of efficiency
due to an increase in MOSFET gate charge losses. The
equation (2) shows that the inductance value has a direct
effect on ripple current.
Accepting larger values of ripple current allows the use of
low inductances but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ∆I ≤ 0.4. I . OUT(MAX) Remember, the
maximum ripple current occurs at the maximum input
voltage. The minimum inductance of the inductor is cal-
culated by using the following equation:
VOUT ·(VIN - VOUT) ≤ 1.2
380000 ·L ·VIN
L ≥ VOUT ·(VIN - VOUT)
(H)
456000 ·VIN
where V = V
IN
IN(MAX)
........... (6)
Output Diode Selection
The Schottky diode carries load current during the off-
time. Therefore, the average diode current is dependent
on the P-channel power MOSFET duty cycle. At high input
voltages, the diode conducts most of the time. As VIN ap-
proaches VOUT, the diode conducts only a small fraction of
the time. The most stressful condition for the diode is
when the output is short-circuited. Therefore, it is impor-
tant to adequately specify the diode peak current and av-
erage power dissipation so as not to exceed the diode
ratings.
Under normal load conditions, the average current con-
ducted by the diode is:
ID = VIN - VOUT ⋅IOUT
VIN + VD
The APW7085 is equipped with whole protections to re-
duce the power dissipation during short-circuit condition.
Therefore, the maximum power dissipation of the diode
is calculated from the maximum output current as:
PDIODE(MAX) = VD ·ID(MAX)
where VOUT= VOUT(MAX)
Remember to keep lead length short and observe proper
grounding to avoid ringing and increased dissipation.
Copyright © ANPEC Electronics Corp.
18
Rev. A.7 - Sep., 2010
www.anpec.com.tw
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