RT8205L RICHTEK [Richtek Technology Corporation], RT8205L Datasheet - Page 24

• Manufacturer Part Number: RT8205L
• Description: High Efficiency, Main Power Supply Controller for Notebook Computer
• Manufacturer: RICHTEK [Richtek Technology Corporation]

RT8205L/M
Output Capacitor Selection
The capacitor value and ESR determine the amount of
output voltage ripple and load transient response. Thus,
the capacitor value must be greater than the largest value
calculated from below equations :
where V
undershoot voltage and overshoot voltage in the load
transient, V
minimum off-time, and K is a factor listed in from Table 1.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
P
where T
the ambient temperature, and θ
thermal resistance.
For recommended operating condition specifications of
the RT8205L/M, the maximum junction temperature is
125°C and T
ambient thermal resistance, θ
WQFN-24L 4x4 packages, the thermal resistance, θ
52°C/W on a standard JEDEC 51-7 four-layer thermal test
board. The maximum power dissipation at T
be calculated by the following formula :
P
WQFN-24L 4x4 package
www.richtek.com
24
V
V
V
D(MAX)
D(MAX)
P P
SAG
SOAR
−
=
=
= (T
LIR I
=
J(MAX)
2 C
= (125°C − 25°C) / (52°C/W) = 1.923W for
SAG
2 C
×
×
p-p
×
J(MAX)
( I
Δ
A
OUT
LOAD(MAX)
and V
is the maximum junction temperature, T
is the ambient temperature. The junction to
is the output ripple voltage, t
OUT
LOAD
( I
Δ
LOAD
− T
×
×
V
)
SOAR
OUTx
V
A
2
) / θ
OUTx
×
)
2
L
×
× ×
JA
are the allowable amount of
⎛
⎜
⎝
×
L (K
ESR
⎡
⎢
⎣
K
JA
JA
⎛
⎜
⎝
, is layout dependent. For
V
+
is the junction to ambient
V
IN
OUTx
8 C
V
IN
×
−
V
V
IN
OUT
OUTx
1
+
t
OFF(MIN)
×
OFF(MIN)
A
⎞
⎟
⎠
f
= 25°C can
−
⎞
⎟
⎠
t
OFF(MIN)
)
is the
JA
A
, is
is
⎤
⎥
⎦
The maximum power dissipation depends on the operating
ambient temperature for fixed T
resistance, θ
curve in Figure 7 allows the designer to see the effect of
rising ambient temperature on the maximum power
dissipation.
Layout Considerations
Layout is very important in high frequency switching
converter designs, the PCB could radiate excessive noise
and contribute to the converter instability with improper
layout. Certain points must be considered before starting
a layout using the RT8205L/M.
Keep current limit setting network as close as possible
Place the filter capacitor close to the IC, within 12 mm
Connections from the drivers to the respective gate of
All sensitive analog traces and components such as
Figure 7. Derating Curve for the RT8205L/M Package
(0.5 inch) if possible.
to the IC. Routing of the network should avoid coupling
to high voltage switching node.
the high side or the low side MOSFET should be as
short as possible to reduce stray inductance. Use 0.65
mm (25 mils) or wider trace.
V
should be placed away from high voltage switching
nodes such as PHASEx, LGATEx, UGATEx, or BOOTx
nodes to avoid coupling. Use internal layer(s) as ground
plane(s) and shield the feedback trace from power traces
and components.
OUTx
2.0
1.6
1.2
0.8
0.4
0.0
, FB
0
X
JA
, GND, ENTRIPx, PGOOD, and TONSEL
. For the RT8205L/M package, the derating
25
Ambient Temperature (°C)
50
DS8205L/M-05 June 2011
75
J (MAX)
Four-Layer PCB
and thermal
100
125