RT7271A
Thermal Shutdown (OTP)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Four-Layer PCB
The device implements an internal thermal shutdown
function when the junction temperature exceeds 160°C.
The thermal shutdown forces the device to stop switching
when the junction temperature exceeds the thermal
shutdown threshold. Once the die temperature decreases
below the hysteresis of 10°C, the device reinstates the
power up sequence.
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 :
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 2. Derating Curve of Maximum PowerDissipation
Layout Considerations
Layout is very important in high frequency switching
converter design. The PCB can radiate excessive noise
and contribute to converter instability with improper layout.
Certain points must be considered before starting a layout
using the RT7271A.
P
D(MAX) = (TJ(MAX) − TA) / θJA
where TJ(MAX) is the maximum junction temperature, TAis
the ambient temperature, and θJA is the junction to ambient
thermal resistance.
For recommended operating condition specifications of
the RT7271A, the maximum junction temperature is 125°C
and TA is the ambient temperature. The junction to ambient
thermal resistance, θJA, is layout dependent. For
WL-CSP-12B 1.65x1.95 (BSC), the thermal resistance,
θJA, is 53°C/W on a standard JEDEC 51-7 four-layer
thermal test board. The maximum power dissipation at
TA = 25°C can be calculated by the following formulas :
ꢀ Make the traces of the main current paths as short and
wide as possible.
ꢀPut the input capacitor as close as possible to the device
pins (VIN andGND).
ꢀLX node encounters high frequency voltage swings so it
should be kept in a small area. Keep sensitive
components away from the LX node to prevent stray
capacitive noise pick-up.
PD(MAX) = (125°C − 25°C) / (53°C/W) = 1.88W for
WL-CSP-12B 1.65x1.95 (BSC) package
ꢀ Ensure all feedback network connections are short and
direct. Place the feedback network as close to the chip
as possible.
The maximum power dissipation depends on the operating
ambient temperature for fixed TJ(MAX) and thermal
resistance, θJA. The derating curve in Figure 2 allow the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
ꢀ The GND pin should be connected to a strong ground
plane for heat sinking and noise protection.
ꢀ An example of PCB layout guide is shown in Figure 3
for reference.
Copyright 2013 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
www.richtek.com
12
DS7271A-00 February 2013
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
