Richtek RT8238A, Руководство пользователя

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RT8238A
14
www.richtek.com
RT8238A-07 January 2014
© Copyright 2014 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Layout Considerations
Layout is very important in high frequency switching
converter design. If the layout is designed improperly, the
PCB could radiate excessive noise and contribute to the
converter instability. The following points must be followed
for a proper layout of RT8238A.
`
Connect a filter capacitor to VCC, 1
μ
F to 4.7
μ
F range is
recommended. Place the filter capacitor close to the
IC.
`
Keep current limit setting network as close as possible
to the IC. Routing of the network should avoid coupling
to high voltage switching node.
`
Connections from the drivers to the respective gate of
the high side or the low side MOSFET should be as
short as possible to reduce stray inductance.
`
All sensitive analog traces and components such as
MODE, FB, GND, EN, PGOOD, CS, VCC, and TON
should be placed away from high voltage switching
nodes such as PHASE, LGATE, UGATE, or BOOT
nodes to avoid coupling. Use internal layer (s) as ground
plane (s) and shield the feedback trace from power traces
and components.
`
Current sense connections must always be made using
Kelvin connections to ensure an accurate signal, with
the current limit resistor located at the device.
`
Power sections should connect directly to ground plane
(s) using multiple vias as required for current handling
(including the chip power ground connections). Power
components should be placed to minimize loops and
reduce losses.
Figure 5. Derating Curves for RT8238A Packages
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0 25 50 75 100 125
Ambient Temperature (°C)
Ma
xi
mu
m P
o
we
r Di
ss
ip
a
tio
n
(
W
)
1
Single-Layer PCB
IC package, PCB layout, the rate of surroundings airflow
and temperature difference between junction to ambient.
The maximum power dissipation can be calculated by
following formula :
P
D(MAX)
= (T
J(MAX)
− T
A
) / θ
JA
where T
J(MAX)
is the maximum operation junction
temperature 125
°
C, T
A
is the ambient temperature and
the θ
JA
is the junction to ambient thermal resistance.
For recommended operating conditions specification of
RT8238A, the maximum junction temperature is 125
°
C
and T
A
is the ambient temperature. The junction to ambient
thermal resistance, θ
JA
, is layout dependent. For WQFN-
12L 2x2 packages, the thermal resistance, θ
JA
, is 165 ° C /
W on a standard JEDEC 51-3 single-layer thermal test
board. The maximum power dissipation at
T
A
= 25
°
C can
be calculated by the following formula:
P
D(MAX)
= (125 ° C − 25 ° C ) / (165 ° C W) = 0.606W for
WQFN-12L 2x2 package
The maximum power dissipation depends on the operating
ambient temperature for fixed
T
J(MAX)
and thermal
resistance,
θ
JA
. For the RT8238A package, the derating
curve in Figure 5 allows the designer to see the effect of
rising ambient temperature on the maximum power
dissipation.