APW7077 データシートの表示（PDF） - Anpec Electronics
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APW7077 Datasheet PDF : 21 Pages
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APW7077/A
Function Description (Cont.)
Step–up Converter Operating Mode (Cont.)
given by
V OUT − V IN
D=
V OUT
In higher output voltage or small output current
application, the step–up DC–DC controller operated in
discontinuous conduction mode almost. For a step-up
converter in a DCM, the duty cycle D is given by
D=
TS
2⋅L
⋅ RLOAD
⋅
VOUT
VIN
VOUT
VIN
− 1
External components values can be calculated from
these equations, however, the optimized value should
obtained through experimental results.
Critical Inductance Value
The minimum value of inductor to maintain continuous
conduction mode can be determined by the following
equation.
VOUT × D(1 − D)2
L≥
fsw × IO × Ratio
A system can be designed to operate in continuous
mode for load currents above a certain level usually
20 to 50% (Ratio define as 0.2~0.5) of full load at
minimum input voltage. When I smaller than (I *Ratio),
O
O
the controller system will into DCM.
∆IL is the ripple current flowing through the inductor,
which affects the output voltage ripple and core losses.
Based on 20%(Ratio=0.2) current ripple, VOUT=5V,
IO=1A and VIN =1.8V system, the inductance value is
calculated as 6.9uH and a 6.8uH inductor is used.
The inductor current ripple has an expression
VIN × D
∆IL =
fsw × L
The maximum DC input current can be calculated as
VOUT × IO(max)
IL(max) =
VIN (min)
The inductor peak current can be calculated as
VOUT × IO ∆IL
Ipk =
+
V IN
2
NOTES:
D – On–time duty cycle
IL – Average inductor current
IPK – Peak inductor current
IO – Desired dc output current
VIN – Nominal operating dc input voltage
VOUT – Desired dc output voltage
ESR – Equivalent series resistance of the output
capacitor
Inductor Selection
APW7077/A series are designed to work well with a
6.8 to 12uH inductors in most applications 10uH is a
sufficiently low value to allow the use of a small
surface mount coil, but large enough to maintain low
ripple. Lower inductance values supply higher output
current, but also increase the ripple and reduce
efficiency. Higher inductor values reduce ripple and
improve efficiency, but also limit output current. The
inductor should have small DCR, usually less than
1mΩ, to minimize loss. It is necessary to choose an
inductor with a saturation current greater than the peak
current which the inductor will encounter in the
application.
The inductor ripple current is important for a few
reasons. One reason is because the peak switch
current will be the average inductor current (I ) plus
L
∆IL.
As a side note, discontinuous operation occurs when
the inductor current falls to zero during a switching
cycle, or ∆IL is greater than the average inductor
current. Therefore, continuous conduction mode occurs
Copyright © ANPEC Electronics Corp.
13
Rev. A.4 - Sep, 2005
www.anpec.com.tw
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