RT8008 RICHTEK [Richtek Technology Corporation], RT8008 Datasheet - Page 11

• Manufacturer Part Number: RT8008
• Description: 1.5MHz, 600mA, High Efficiency PWM Step-Down DC/DC Converter
• Manufacturer: RICHTEK [Richtek Technology Corporation]

curves. Thus, to obtain I
and multiply the result by the square of the average output
current.
Other losses including C
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
temperature 125 C, T
For recommended operating conditions specification of
RT8008 DC/DC converter, where T
junction temperature of the die (125 C) and T
maximum ambient temperature. The junction to ambient
thermal resistance
SOT-23-5/TSOT-23-5 packages, the thermal resistance
is 250 C/W on the standard JEDEC 51-3 single-layer
thermal test board. The maximum power dissipation at
T
P
TSOT-23-5 packages
The maximum power dissipation depends on operating
ambient temperature for fixed T
resistance
derating curves allows the designer to see the effect of
rising ambient temperature on the maximum power
allowed.
The value of junction to case thermal resistance
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 PCB layout, the surroundings airflow effects
and temperature difference between junction to ambient.
The operated junction temperature can be calculated by
following formula :
T
DS8008-04 March 2007
JA
A
J
D(MAX)
D(MAX)
= 25 C can be calculated by following formula :
= T
is the junction to ambient thermal resistance.
C
= ( T
= ( 125 C - 25 C ) / 250 = 0.4 W for SOT-23-5/
+ P
J(MAX)
D
J(MAX)
JA
x
. For RT8008 packages, the Figure 5 of
JC
is the maximum operation junction
- T
A
A
) /
is the ambient temperature and the
JA
2
R losses, simply add R
JA
IN
is layout dependent. For
and C
J (MAX)
OUT
J(MAX)
ESR dissipative
is the maximum
and thermal
SW
A
is the
to R
JC
JA
is
L
Where T
temperature measured by thermal sensor, P
dissipation defined by user's function and the
junction to case thermal resistance provided by IC
manufacturer. Therefore it's easy to estimate the junction
temperature by any condition.
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 RT8008.
} For the main current paths as indicated in bold lines in
} Put the input capacitor as close as possible to the device
} LX node is with high frequency voltage swing and should
Figure 6, keep their traces short and wide.
pins (VIN and GND).
be kept small area. Keep analog components away from
LX node to prevent stray capacitive noise pick-up.
Figure 5. Derating Curves for RT8008 Package
450
400
350
300
250
200
150
100
50
0
C
0
is the package case (Pin 2 of package leads)
LOAD
OUT
(ESR), where ESR is the effective series
20
generating a feedback error signal used
OUT
Ambient Temperature (°C)
.
OUT
40
I
LOAD
immediately shifts by an amount
SOT-23-5, TSOT-23-5 Packages
OUT
60
also begins to charge or
OUT
to its steady-state value.
can be monitored for
80
Single Layer PCB
RT8008
100
www.richtek.com
D
is the power
120
JC
is the
140
11