rt8020 Richtek Technology Corporation, rt8020 Datasheet - Page 12

• Manufacturer Part Number: rt8020
• Description: Dual High-efficiency Step-down Dc-dc Converter
• Manufacturer: Richtek Technology Corporation

www.richtek.com
12
RT8020
and the synchronous switch. Thus, the series resistance
looking into the LX pin is a function of both top and bottom
MOSFET R
follows :
R
The R
obtained from the Typical Performance Characteristics
curves. Thus, to obtain I
and multiply the result by the square of the average output
current. Other losses including C
dissipative losses and inductor core losses generally
account for less than 2% of the total loss.
Thermal Considerations
The maximum power dissipation depends on the thermal
resistance of 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
Where T
the ambient temperature and the
ambient thermal resistance. For recommended operating
conditions specification of RT8020 DC/DC converter,
where T
the die and T
to ambient thermal resistance
For WDFN-12L 3x3 packages, the thermal resistance
thermal test board. The maximum power dissipation at
T
P
WDFN-12L 3x3 packages
The maximum power dissipation depends on operating
ambient temperature for fixed T
resistance
rating curves allows the designer to see the effect of rising
ambient temperature on the maximum power allowed.
JA
A
D(MAX)
D(MAX)
SW
= 25 C can be calculated by following formula :
is 60 C/W on the standard JEDEC 51-7 four-layers
= R
DS(ON)
= ( T
= (125 C
DS(ON)TOP
J(MAX)
J(MAX)
JA
DS(ON)
for both the top and bottom MOSFETs can be
J(MAX)
A
. For RT8020 packages, the Figure 4 of de-
is the maximum junction temperature, T
is the maximum junction temperature of
is the ambient temperature. The junction
x DC + R
and the duty cycle (DC) is shown as
T
A
25 C) / (60 C/W) = 1.667W for
) /
2
R losses, simply add R
JA
DS(ON)BOT
JA
is layout dependent.
JA
J(MAX)
x (1 DC)
IN
is the junction to
and C
and thermal
OUT
SW
to R
ESR
A
is
L
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V
equal to I
resistance of C
discharge C
by the regulator to return V
During this recovery time, V
overshoot or ringing that would indicate a stability problem.
Layout Considerations
Follow the PCB layout guidelines for optimal performance
of RT8020.
} For the main current paths, keep their traces short and
} Put the input capacitor as close as possible to the device
} LX node is with high frequency voltage swing and should
} Connect feedback network behind the output capacitors.
} Connect all analog grounds to a command node and
wide.
pins (VIN and GND).
be kept small area. Keep analog components away from
LX node to prevent stray capacitive noise pick-up.
Keep the loop area small. Place the feedback
components near the RT8020.
then connect the command node to the power ground
behind the output capacitors.
Figure 4. De-rating Curves for RT8020 Package
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
LOAD
OUT
(ESR), where ESR is the effective series
generating a feedback error signal used
OUT
25
Ambient Temperature (°C)
.
OUT
I
LOAD
immediately shifts by an amount
50
OUT
also begins to charge or
OUT
to its steady-state value.
DS8020-03 August 2007
can be monitored for
75
100
125