RT6255A/B
For output voltage accuracy, use divider resistors with 1%
or better tolerance.
Thermal Considerations
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 :
External BOOT Bootstrap Diode
When the input voltage is lower than 5.5V it is
recommended to add an external bootstrap diode between
VIN and the BOOT pin to improve enhancement of the
internal MOSFET switch and improve efficiency. The
bootstrap diode can be a low cost one such as 1N4148 or
BAT54.
PD(MAX) = (TJ(MAX) − TA) / θJA
External BOOT Capacitor Series Resistance
The internal power MOSFET switch gate driver is
optimized to turn the switch on fast enough for low power
loss and good efficiency, but also slow enough to reduce
EMI. Switch turn-on is when most EMI occurs since VLX
rises rapidly. During switch turn-off, LX is discharged
relatively slowly by the inductor current during the dead
time between high-side and low-side switch on-times. In
some cases it is desirable to reduce EMI further, at the
expense of some additional power dissipation. The switch
turn-on can be slowed by placing a small (<47Ω)
resistance between BOOT and the external bootstrap
capacitor. This will slow the high-side switch turn-on and
VLX's rise. To remove the resistor from the capacitor
charging path (avoiding poor enhancement due to
undercharging the BOOT capacitor), use the external diode
shown in Figure 6 to charge the BOOT capacitor and place
the resistance between BOOT and the capacitor/diode
connection.
where TJ(MAX) is the maximum junction temperature, TA is
the ambient temperature, and θJA is the junction-to-ambient
thermal resistance.
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
TSOT-23-6 (FC) package, the thermal resistance, θJA, is
52°C/W on a standard JEDEC 51-7 high effective-thermal-
conductivity four-layer test board. For a TSOT-23-8 (FC)
package, the thermal resistance, θJA, is 52°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 :
PD(MAX) = (125°C − 25°C) / (52°C/W) = 1.923W for a
TSOT-23-6 (FC) package.
PD(MAX) = (125°C − 25°C) / (52°C/W) = 1.923W for a
TSOT-23-8 (FC) package.
5V
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 7 allows
the designer to see the effect of rising ambient temperature
on the maximum power dissipation.
BOOT
RT6255A/B
LX
0.1µF
Figure 6. External Bootstrap Diode
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16
DS6255A/B-02 March 2017
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
