RT8008
curves. Thus, to obtain I2R losses, simply add RSW to RL
and multiply the result by the square of the average output
current.
Where TC is the package case (Pin 2 of package leads)
temperature measured by thermal sensor, PD isthe power
dissipation defined by user's function and the qJC is the
junction to case thermal resistance provided by IC
manufacturer. Therefore it's easy to estimate the junction
temperature by any condition.
Other losses including CIN and COUT ESR dissipative
losses and inductor core losses generally account for less
than 2% of the total loss.
Checking Transient Response
Thermal Considerations
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, VOUT immediately shifts by an amount
equal to DILOAD (ESR), where ESR is the effective series
resistance of COUT. DILOAD also begins to charge or
discharge COUT generating a feedback error signal used
by the regulator to return VOUT to its steady-state value.
During this recovery time, VOUT can be monitored for
overshoot or ringing that would indicate a stability problem.
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 :
PD(MAX) = ( TJ(MAX) - TA ) / qJA
Where TJ(MAX) is the maximum operation junction
temperature 125°C, TA is the ambienttemperature and the
qJA is the junction to ambient thermal resistance.
For recommended operating conditions specification of
RT8008DC/DC converter, whereTJ (MAX) is the maximum
junction temperature of the die (125°C) and TA is the
maximum ambient temperature. The junction to ambient
thermal resistance qJA is layout dependent. For
SOT-23-5/TSOT-23-5 packages, the thermal resistance qJA
is 250°C/W on the standard JEDEC 51-3 single-layer
thermal test board. The maximum power dissipation at
TA = 25°C can be calculated by following formula :
450
Single Layer PCB
400
350
SOT-23-5, TSOT-23-5 Packages
300
250
200
150
100
50
PD(MAX) = ( 125°C - 25°C ) / 250 = 0.4 W for SOT-23-5/
0
TSOT-23-5 packages
0
20
40
60
80
100
120
140
The maximum power dissipation depends on operating
ambient temperature for fixed TJ(MAX) and thermal
resistance qJA. For RT8008 packages, the Figure 5 of
derating curves allows the designer to see the effect of
rising ambient temperature on the maximum power
allowed.
Ambient Temperature (°C)
Figure 5. Derating Curves for RT8008 Package
Layout Considerations
Follow the PCB layout guidelines for optimal performance
of RT8008.
The value of junction to case thermal resistance qJC is
popular for users. This thermal parameter is convenient
for users to estimate the internal junction operated
temperature of packages while IC operating. It's
independent of PCBlayout, the surroundings airflow effects
and temperature difference between junction to ambient.
The operated junction temperature can be calculated by
following formula :
} For the main current paths as indicated in bold lines in
Figure 6, keep their traces short and wide.
} Put the input capacitor as close as possible to the device
pins (VIN andGND).
}LX node is with high frequency voltage swing and should
be kept small area. Keep analog components away from
LX node to prevent stray capacitive noise pick-up.
TJ = TC + PD x qJC
DS8008-04 March 2007
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