RT8048 datasheet(10/11 Pages) RICHTEK

10 / 11 page

RT8048

DS8048-01

June 2011

www.richtek.com

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 :

PD(MAX) = (TJ(MAX)

− TA) / θJA

where TJ(MAX) is the maximum junction temperature, TA is

the ambient temperature, and

θJAis the junction to ambient

thermal resistance.

For recommended operating condition specifications of

the RT8048, the maximum junction temperature is 125

°C

and TA is the ambient temperature. The junction to ambient

thermal resistance,

θJA, is layout dependent. For WDFN-

6L 2x2 packages, the thermal resistance,

θJA, is 120°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 formula :

PD(MAX) = (125

°C − 25°C) / (120°C/W) = 0.833W for

WDFN-6L 2x2 package

The maximum power dissipation depends on the operating

ambient temperature for fixed TJ(MAX) and thermal

resistance,

θJA. For the RT8048 packages, the derating

curve in Figure 3 allows the designer to see the effect of

rising ambient temperature on the maximum power

dissipation.

Figure 3. Derating Curve for the RT8048 Packages

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

100

125

Ambient Temperature (°C)

Four-Layer PCB

polymer, aluminum electrolytic and ceramic capacitors are

all available in surface mount packages. Special polymer

capacitors offer very low ESR, but have lower capacitance

density than other types. Tantalum capacitors have the

highest capacitance density, but it is important to only

use types that have been surge tested for use in switching

power supplies. Aluminum electrolytic capacitors have

significantly higher ESR but can be used in cost sensitive

applications provided that consideration is given to ripple

current ratings and long term reliability. Ceramic capacitors

have excellent low ESR characteristics, but can have a

high voltage coefficient and audible piezoelectric effects.

The high Q of ceramic capacitors with trace inductance

can also lead to significant ringing.

Using Ceramic Input and Output Capacitors

Higher values, lower cost ceramic capacitors are now

becoming available in smaller case sizes. Their high ripple

current, high voltage rating and low ESR make them ideal

for switching regulator applications. However, care must

be taken when these capacitors are used at the input and

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. At best, this ringing can couple to the output

and be mistaken as loop instability. At worst, a sudden

inrush of current through the long wires can potentially

cause a voltage spike at VIN large enough to damage the

part.

Checking Transient Response

The regulator loop response can be checked by looking

at the load transient response. Switching regulators take

several cycles to respond to a step in load current. When

a load step occurs, VOUT immediately shifts by an amount

equal to

ΔILOAD (ESR), where ESR is the effective series

resistance of COUT.

ΔILOAD also begins to charge or

discharge COUT, generating a feedback error signal used

by the regulator to return VOUT to its steady-state value.

During this recovery time, VOUT can be monitored for

overshoot or ringing, which would indicate a stability

problem.

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