RT8035 datasheet(8/11 Pages) RICHTEK

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RT8035

DS8035-05 October 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

The output ripple is highest at maximum input voltage

since

ΔIL increases with input voltage. Multiple capacitors

placed in parallel may be needed to meet the ESR and

RMS current handling requirements. Dry tantalum, special

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.

Output Voltage Setting

The resistive divider allows the FB pin to sense a fraction

of the output voltage as shown in Figure 1.

)

REF

OUT

Figure 1. Setting Output Voltage

where VREF is the internal reference voltage (0.6V typ.)

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 that would indicate a stability problem.

Thermal Considerations

For continuous operation, do not exceed absolute

maximum operation junction temperature. The maximum

power dissipation depends on the thermal resistance of

IC package, PCB layout, the rate of surroundings airflow

and temperature difference between junction to ambient.

The maximum power dissipation can be calculated by

following formula :

PD(MAX) = (TJ(MAX)

− TA) / θJA

Where TJ(MAX) is the maximum operation junction

temperature, TA is the ambient temperature and the

θJAis

the junction to ambient thermal resistance.

For recommended operating conditions specification, the

maximum junction temperature is 125

°C. The junction to

ambient thermal resistance

θJA is layout dependent. For

WDFN-10L 3x3 package, the thermal resistance

θJA is

°C/W on the standard JEDEC 51-7 four layers thermal

test board. The maximum power dissipation at TA = 25

°C

can be calculated by following formula :

PD(MAX) = (125

°C − 25°C) / (68°C/W) = 1.471W for

WDFN-10L 3x3

For adjustable voltage mode, the output voltage is set by

an external resistive divider according to the following

equation :

RT8035

GND

VOUT

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