RT7080
Application Information
Buck Description
Thermal Considerations
The RT7080 includes a step-down voltage regulator which
is a high-efficiency synchronous step-down DC-DC
converter that can deliver up to 60mAoutput current from
a 8V to 28V input supply. This Buck converter employs
a Hysteresis Mode control by using the Boundary
Conduction Mode (BCM) with low quiescent current and
high-side peak current limit, and hereby providing high
efficiency over a wide range of load current.
The junction temperature should never exceed the
absolute maximum junction temperature TJ(MAX), listed
under Absolute Maximum Ratings, to avoid permanent
damage to the device. The maximum allowable power
dissipation depends on the thermal resistance of the IC
package, the PCB layout, the rate of surrounding airflow,
and the difference between the junction and ambient
temperatures. The maximum power dissipation can be
calculated using the following formula :
FB
VIN
C
PD(MAX) = (TJ(MAX) − TA) / θJA
IN
Current Sense
0.8V
where TJ(MAX) is the maximum junction temperature, TA is
the ambient temperature, and θJA is the junction-to-ambient
thermal resistance.
FB
Comparator
L
1
SW
V
OUT
Buck
Controller
R
1
For continuous operation, the maximum operating junction
temperature indicated under Recommended Operating
Conditions is 125°C. The junction-to-ambient thermal
resistance, θJA, is highly package dependent. For a
WQFN-40L 6x6 package, the thermal resistance, θJA, is
27°C/W on a standard JEDEC 51-7 high effective-thermal-
conductivity four-layer test board. The maximum power
dissipation at TA = 25°C can be calculated as below :
C
OUT
FB
R
2
C
F
SWGND
DVSS
Figure 1. Buck Regulator
Table 1. Recommended Value for Buck Regulator
VOUT (V)
COUT (F)
L1 (H)
6.1
4.7
22
PD(MAX) = (125°C − 25°C) / (27°C/W) = 3.7W for a WQFN-
40L 6x6 package.
The maximum power dissipation depends on the operating
ambient temperature for the fixed TJ(MAX) and the thermal
resistance, θJA. The derating curves in Figure 2 allows
the designer to see the effect of rising ambient temperature
on the maximum power dissipation.
R1 (k)
100
15
R2 (k)
Output Voltage Setting and Feedback Network
The resistive divider allows the FB pin to sense the output
voltage. The output voltage is set by the external resistive
voltage divider according to the following equation :
R1
VOUT VREF 1
R2
Where VREF is the reference voltage (0.8V typ.).
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is a registered trademark of Richtek Technology Corporation.
www.richtek.com
12
DS7080-00 August 2019
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
