APW7080 Datasheet, PDF (18/23 Pages) Anpec Electronics Coropration – 4A, 26V, 380kHz, Asynchronous Step-Down Converter

APW7080
Application Information (Cont.)
Output Capacitor Selection (Cont.)
∆VCOUT =
∆I
(V)
8 ⋅ FOSC ⋅ COUT
........... (4)
For the applications using bulk capacitors, the ∆V_COUT
is much smaller than the V_ESR and can be ignored.
Therefore, the AC peak-to-peak output voltage (∆V_OUT ) is
shown below:
∆VOUT = ∆ I ⋅ ESR (V)
........... (5)
For the applications using ceramic capacitors, the V is
ESR
much
smaller
than
the
∆
V
COU
T
and
can
be
ignored.
Therefore, the AC peak-to-peak output voltage (∆V_OUT ) is
close to ∆V_COUT .
The load transient requirements are a function 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
capacitors initially supply the transient and slow the
current load rate seen by the bulk capacitors. The bulk
filter capacitor 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
as close to the power pins of the load as physically
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
Equivalent Series Inductance (ESL) of these capacitors
increases with case size and can reduce the usefulness
of the capacitor to high slew-rate transient loading.
Inductor Value Calculation
The operating frequency and inductor selection are
interrelated 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
calculated by using the following equation:
VOUT ·(VIN - VOUT) _{≤ 1.2}
380000 ·L ·VIN
L ≥ ^{VOUT ·(VIN - VOUT)}
(H)
456000 ·VIN
where ^VIN ^{= V}IN(MAX)
........... (6)
Output Diode Selection
The Schottky diode carries load current during the off-time.
The average diode current is therefore dependent on the
P-channel power MOSFET duty cycle. At high input voltages
the diode conducts most of the time. As V_IN approaches
V_OUT 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 important to
adequately specify the diode peak current and average
power dissipation so as not to exceed the diode ratings.
Under normal load conditions, the average current
conducted by the diode is:
ID = ^{VIN - VOUT} ⋅IOUT
VIN + VD
The APW7080 is equipped with whole protections to
reduce the power dissipation during short-circuit
condition. Therefore, the maximum power dissipation of
the diode is calculated from the maximum output current
as:
P_DIODE(MAX) = V_D ·I_D(MAX)
where I_OUT = I_OUT(MAX)
Remember to keep lead length short and observe proper
grounding to avoid ringing and increased dissipation.
Copyright © ANPEC Electronics Corp.
18
Rev. A.6 - Jun., 2008
www.anpec.com.tw