APL5331
plane reduces the resistance cCA . The relationship
between power dissipation and temperatures is the
following equation :
A capacitor of 1µF (ceramic chip capacitor) or greater
(aluminum electrolytic capacitor) is recommended to
connect near VCNTL pin. For VIN pin, an aluminum
electrolytic capacitor (>50µF) is recommended. It is
not necessary to use low-ESR capacitors.
PD = (TJ - TA) / cJA
where,
PD : Power dissipation
TJ : Junction Temperature
TA : Ambient Temperature
cJA : Junction-to-Ambient Thermal Resis-
tance
Layout and Thermal Consideration
The input capacitors for VIN and VCNTL pins are
normally placed near each pin for good
performances. Ceramic decoupling capacitors at
output must be placed as close to the load to reduce
the parasitic inductors of traces. It is also recom-
mended that the APL5331 and output capacitors are
placed near the load for good load regulation and
load transient response. The negative pins of the in-
put and output capacitors and the GND pin of the
APL5331 should connect to analog ground plane of
the load.
102 mil
SOP-8-P
118 mil
Thermal
Top
ground
pad
D ie
See figure 1. The SOP-8-P utilizes a bottom thermal
pad to minimize the thermal resistance of the package,
making the package suitable for high current
applications. The thermal pad is soldered to the top
ground pad and is connected to the internal or bot-
tom ground plane by several vias. The printed circuit
board (PCB) forms a heat sink and dissipates most
of the heat into ambient air. The vias are recom-
mended to have proper size to retain solder, helping
heat conduction.
pad
Ambient
Air
Vias Internal
ground
Printed
circuit
board
plane
Figure 1 Package Top and side view
Figure 2 shows a board layout using the SOP-8-P
package. The demo board is made of FR-4 material
and is a two-layer PCB. The size and thickness are
65mm* 65mm and 1.6mm. An area of 140mil*105mil
on the top layer is use as a thermal pad for the
APL5331 and this is connected to the bottom layer
by vias. The bottom layer using 2 oz. copper acts as
the ground plane for the system. The PCB and all
components on the board form a heat sink. The cJA
of the APL5331(SOP-8-P) mounted on this demo
board is about 37oC/W in free air. Assuming the
TA=25oC and the maximum TJ=150oC (typical thermal
limit temperature), the maximum power dissipation is
calculated as :
Thermal resistance consists of two main elements,
cJC (junction-to-case thermal resistance) and cCA
(case-to-ambient thermal resistance). cJC is speci-
fied from the IC junction to the bottom of the thermal
pad directly below the die. cCA is the resistance from
the bottom of thermal pad to the ambient air and it
includes cCS (case-to-sink thermal resistance) and
cSA (sink-to-ambient thermal resistance). The speci-
fied path for heat flow is the lowest resistance path
and it dissipates majority of the heat to the ambient
air. Typically, cCA is the dominant thermal resistance.
Therefore, enlarging the internal or bottom ground
Copyright ANPEC Electronics Corp.
Rev. A.8 - Oct., 2003
12
www.anpec.com.tw
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
