RT7278
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. A sudden inrush of current through the long
wires can potentially cause a voltage spike at VIN large
enough to damage the part.
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 :
External Bootstrap Diode
Connect a 0.1μF low ESR ceramic capacitor between the
BOOT and SW pins. This capacitor provides the gate driver
voltage for the high side MOSFET. It is recommended to
add an external bootstrap diode between an external 5V
and the BOOT pin for efficiency improvement when input
voltage is lower than 5.5V or duty ratio is higher than 65%.
The bootstrap diode can be a low cost one, such as
1N4148 or BAT54. The external 5V can be a 5V fixed
input from system or a 5V output of the RT7278. Note
that the external boot voltage must be lower than 5.5V
PD(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, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
SOP-8 (Exposed Pad) package, the thermal resistance,
θJA, is 49°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 :
5V
PD(MAX) = (125°C − 25°C) / (49°C/W) = 2.041W for
BOOT
SOP-8 (Exposed Pad) package
RT7278
SW
0.1µF
The maximum power dissipation depends on the operating
ambient temperature for fixed TJ(MAX) and thermal
resistance, θJA. The derating curve in Figure 6 allow the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
Figure 5. External Bootstrap Diode
PVCC Capacitor Selection
2.5
Decouple with a 1μF ceramic capacitor. X7R or X5R grade
dielectric ceramic capacitors are recommended for their
stable temperature characteristics.
Four-Layer PCB
2.0
1.5
1.0
0.5
0.0
Over Current Protection
When the output shorts to ground, the inductor current
decays very slowly during a single switching cycle. An
over current detector is used to monitor inductor current
to prevent current runaway. The over current detector
monitors the voltage between SW and GND during the
low side MOSFET turn-on state. This is cycle-by-cycle
protection. The over current detector also supports
temperature compensated.
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 6. Derating Curve of Maximum PowerDissipation
Copyright 2013 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
12
DS7278-00 January 2013
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
