APW1172
Application Description (Cont.)
PD = VD ´ ID ´ (1- D)
(8)
Output Capacitor (Cont.)
generate a zeroto provide a positive phase for control
loop.Thiszero improved the stabilitywithout extra PID
compensator, if the zero is lower enough.
Where VD is the forward voltage of the wheeling diode,
ID is current flowing through the wheeling diode when
it conducting. In the PCB layout,usually place the
wheeling diode near the APW1172, the power
dissipation of wheeling diode will increase the ambient
temperature and limit the maximum power dissipation
of APW1172.These power dissipations are the major
energy loss in the voltage conversion.
Switch diode
APW1172 is an non-synchronous type buck regulator
and needs a Shottky diode as the wheeling diode.
This diode will conduct when the pass element turned
off.Current flows through the diode in the conducted
period, the order of the maximum peak current
reaches few Amperes. The diode requires the ability
to flow the great forward current. The peak forward
current of the diode denote in the specification must
great than 15A, and the conducting time in this
situation must great than 8ms. 1N5818 is a suitable
component.
To improve the thermal resistance by increasing
copper area is a suitable method. Design a copper
area according to the following curve to improve the
thermal resistance.
48
46
44
42
40
38
36
34
32
30
Thermal Consideration
APW1172 is a switching regulator whose pass element
inside, it have the ability to provide 3 Amperes.As the
show in the block diagram, the structure of the pass
element consist of a NPN and a PNP transistors. The
voltage across the pass element, VCE, is about 0.8V
to 1.3V in the light load to heavy load. The product of
VCE and IL, where IL is current flowing through the
inductor, generate thermal cause the junction
temperature increased. The thermal stream conduct
via the thermal pad of SOP-8-P to the printed circuit
0
2
4
6
8
10
12
TopCopperArea (cm^2)
Frequency Compensation
board.The power dissipation of APW1172 can be
approximated by (7)
In the Buck converter,there is a LPF (Low Pass Filter)
in the output stage to filtering the switching noise.
The LPF consist of an inductor and a capacitor. These
two components generate the double poles in the
(7)
P = (VCE ´ IL ´ D) + (VIN ´ IL ´ FS )(TR +TF )
Where VCE is the voltage across the pass element, IL
is the current flowing through the inductor, D is the
duty. TR and TF are the transition time.
frequencydomain.
1
fnatural
=
(9)
2p LC
The wheeling diode is another thermal source. It’s
power dissipation approximated by (8)
Where L is the inductance of the LPF and C is the
capacitanceof theoutputcapacitor.Thesedoublepoles
Copyright ã ANPEC Electronics Corp.
14
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Rev. A.4 - Aug., 2005
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
