APW7179A Просмотр технического описания (PDF) - Anpec Electronics
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APW7179A Datasheet PDF : 16 Pages
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APW7179A
Application Information (Cont.)
Output Capacitor Selection
The output voltage ripple is a significant parameter to
estimate the performance of a convertor. There are two
discrete components that affect the output voltage ripple
to be bigger or smaller. It is recommended to use the
criterion mentioned in the "Inductor Selection" to choose
a suitable inductor. Then, based on this known inductor
current ripple, the value and equivalent-series-resistance
(ESR) of output capacitor will affect the output voltage ripple
to be smaller or larger. The output voltage ripple consists
of two portions, one is the product of ESR and inductor
current ripple, the other portion is the function of the in-
ductor current ripple and the output capacitance. Figure-3
illustrates the waveform of the ripple voltage which is
generated when the inductor ripple current charges or
discharges the pure capacitor without the ESR.
0A
∆IL
I(COUT)
0.5TS
∆VOUT1
VOUT
Figure-3
Evaluate the ∆VOUT1 by the ideal of energy equalization.
According to the definition of Q,
Q
=
1
2
1
2
∆IL
⋅
1
2
TS
=
COUT
⋅ ∆VOUT1
....(6)
where TS is the inverse of switching frequency and the ∆IL
is the inductor current ripple. Move the C to the left side
OUT
to estimate the value of ∆VOUT1 as equation (7).
∆VOUT1
=
∆IL ⋅ TS
8 ⋅ COUT
................................(7)
As mentioned above, one part of output voltage ripple is
the product of the inductor current ripple and ESR of out-
put capacitor. The equation (8) explains the output volt-
age ripple estimation.
∆VOUT
=
∆IL
⋅ ESR +
TS
8 ⋅ COUT
.................(8)
Thermal Consideration
APW7179A is a high efficiency switching converter, it
means less power loss transferred into heat. Due to the
on resistance difference between internal power
PMOSFET and NMOSFET, the power dissipation at high
duty cycle is greater than the low duty cycle. The worst
case in the dropout operation is the conduction loss dis-
sipate mainly on the internal power PMOSFET. The power
dissipation is nearly defined as:
[ ] ( ) ( ) PD = IOUT 2 RDS−P ⋅D + RDS−N ⋅ 1− D .......(9)
The APW7179A provides internal over-temperature
protection. When the junction temperature reaches 150
degrees centigrade, the APW7179 will turn off both inter-
nal power PMOSFET and NMOSFET. The estimation of
the junction temperature, T , is defined as:
J
TJ = PD ⋅ θJA............................................(10)
where the θJA is the thermal resistance of the package
utilized by the APW7179A.
Output Voltage Setting
Then APW7179A has the adjustable version for output
voltage setting by the users. A suggestion of maximum
value of R2 is 300kΩ to keep the minimum current that
provides enough noise rejection ability through the re-
sistor divider. The output voltage is programmed by the
equation as below:
VOUT
=
0.6 ⋅ 1+
R1
R2
...............................(11)
VOUT
APW7179A
R1
FB
R2
Copyright © ANPEC Electronics Corp.
10
Rev. A.1 - Oct., 2011
www.anpec.com.tw
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