RT6318B datasheet(20/25 Pages) RICHTEK

20 / 25 page

RT6318B/C

DS6318B/C-01 December 2021

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Figure 7. Derating Curve of Maximum Power Dissipation

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

100

125

Ambient Temperature (°C)

Four-Layer PCB

Layout Considerations

Printed circuit board (PCB) layout design for switch-mode

power supply IC is critical and important. Improper PCB

layout brings lots of misbehaviors on power supply, such

as poor output voltage regulation, switching jitter, bad

thermal performance, excessively radiate noise and

alleviating component reliability. For avoiding those issues,

designers have to understand current trace and signal flow

in the switching power supply. The following design

concepts present design consideration of PCB layout for

switching power supply.



For suppressing phase ring and extra power losses that

affect device reliability, the input capacitor has to place

close to VIN pin to reduce the influence of parasitic

inductor.



For thermal stress and power consumption

considerations, the current paths of VIN and VOUT are

as short and wide as possible to decrease the trace

impedance.



Since the LX node voltage swings from VIN to GND with

very fast rising and falling times, switching power supply

suffers quite serious EMI issues. To eliminate EMI

problems, the inductor must put as close as possible

to IC to narrow the LX node area. Besides, the LX node

should arrange in the same plate to reduce coupling

noise path caused by parasitic capacitance.

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 :

PD(MAX) = (TJ(MAX)

− TA) / θJA

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 UQFN-

12HL 3x3 package, the thermal resistance,

θJA, 35.8°C/

W is measured in the natural convection at TA = 25

°C on

a four-layer Richtek evaluation board. The maximum power

dissipation at TA = 25

°C can be calculated as below :

PD(MAX) = (125

°C − 25°C) / (35.8°C/W) = 2.79W for a

UQFN-12HL 3x3 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 7 allows

the designer to see the effect of rising ambient temperature

on the maximum power dissipation.

Table 2. Dividing Resistors of RT6318B/C

RT6318B

RT6318C

90k



150k



20k



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