RT8048
10
DS8048-01 June 2011www.richtek.com
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
D(MAX)
= (T
J(MAX)
− T
A
) / θ
JA
where T
J(MAX)
is the maximum junction temperature, T
A
is
the ambient temperature, and θ
JA
is the junction to ambient
thermal resistance.
For recommended operating condition specifications of
the RT8048, 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 WDFN-
6L 2x2 packages, the thermal resistance, θ
JA
, is 120°C/
W on a standard JEDEC 51-7 four-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) / (120°C/W) = 0.833W for
WDFN-6L 2x2 package
The maximum power dissipation depends on the operating
ambient temperature for fixed T
J(MAX)
and thermal
resistance, θ
JA
. For the RT8048 packages, the derating
curve in Figure 3 allows the designer to see the effect of
rising ambient temperature on the maximum power
dissipation.
Figure 3. Derating Curve for the RT8048 Packages
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
0 25 50 75 100 125
Ambient Temperature (°C)
Maximum Power Dissipation (W)
Four-Layer PCB
polymer, aluminum electrolytic and ceramic capacitors are
all available in surface mount packages. Special polymer
capacitors offer very low ESR, but have lower capacitance
density than other types. Tantalum capacitors have the
highest capacitance density, but it is important to only
use types that have been surge tested for use in switching
power supplies. Aluminum electrolytic capacitors have
significantly higher ESR but can be used in cost sensitive
applications provided that consideration is given to ripple
current ratings and long term reliability. Ceramic capacitors
have excellent low ESR characteristics, but can have a
high voltage coefficient and audible piezoelectric effects.
The high Q of ceramic capacitors with trace inductance
can also lead to significant ringing.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
input, VIN. At best, this ringing can couple to the output
and be mistaken as loop instability. At worst, a sudden
inrush of current through the long wires can potentially
cause a voltage spike at VIN large enough to damage the
part.
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
OUT
immediately shifts by an amount
equal to ΔI
LOAD
(ESR), where ESR is the effective series
resistance of C
OUT
. ΔI
LOAD
also begins to charge or
discharge C
OUT
, generating a feedback error signal used
by the regulator to return V
OUT
to its steady-state value.
During this recovery time, V
OUT
can be monitored for
overshoot or ringing, which would indicate a stability
problem.
