DS8813D datasheet(21/23 Pages) RICHTEK

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RT8813D

DS8813D-00

September 2016

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Output Capacitor Selection

The output filter capacitor must have ESR low enough to

meet output ripple and load transient requirement, yet have

high enough ESR to satisfy stability requirements. Also,

the capacitance must be high enough to absorb the inductor

energy going from a full load to no load condition without

tripping the OVP circuit. Organic semiconductor

capacitor(s) or special polymer capacitor(s) are

recommended.

MOSFET Selection

The majority of power loss in the step-down power

conversion is due to the loss in the power MOSFETs. For

low voltage high current applications, the duty cycle of

the high side MOSFET is small. Therefore, the switching

loss of the high side MOSFET is of concern. Power

MOSFETs with lower total gate charge are preferred in

such kind of application.

However, the small duty cycle means the low side MOSFET

is on for most of the switching cycle. Therefore, the

conduction loss tends to dominate the total power loss of

the converter. To improve the overall efficiency, the

MOSFETs with low RDS(ON) are preferred in the circuit

design. In some cases, more than one MOSFET are

connected in parallel to further decrease the on-state

resistance. However, this depends on the low side

MOSFET driver capability and the budget.

Figure 14. Derating Curve of Maximum Power

Dissipation

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

100

125

Ambient Temperature (°C)

Four-Layer PCB

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, TAis

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

WQFN-24L 4x4 package, the thermal resistance,

θJA, is

°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) / (28°C/W) = 3.57W for a

WQFN-24L 4x4 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 14 allows

the designer to see the effect of rising ambient temperature

on the maximum power dissipation.

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