RT7259 datasheet(13/15 Pages) RICHTEK

13 / 15 page

DS7259-00

January 2012

www.richtek.com

RT7259

is a registered trademark of Richtek Technology Corporation.

Table 4. Suggested Capacitors for CIN and COUT

Location

Component Supplier

Part No.

Capacitance (

μF)

Case Size

CIN

MURATA

GRM31CR61E106K

1206

CIN

TDK

C3225X5R1E106K

1206

CIN

TAIYO YUDEN

TMK316BJ106ML

1206

COUT

MURATA

GRM31CR60J476M

1206

COUT

TDK

C3225X5R0J476M

1210

COUT

MURATA

GRM32ER71C226M

1210

COUT

TDK

C3225X5R1C22M

1210

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 load change. When a

step load occurs, VOUT immediately shifts by an amount

equal to

ΔILOAD x ESR also begins to charge or discharge

COUT generating a feedback error signal for 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 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 RT7259, the maximum junction temperature is 125

°C.

The junction to ambient thermal resistance,

θJA, is layout

dependent. For WDFN-14L 4x3 package, the thermal

resistance,

θJA, is 60°C/W on a standard JEDEC 51-7

four-layer thermal test board.

The output ripple will be the highest at the 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 requirement.

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. 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. This ringing

can couple to the output and be mistaken. A sudden inrush

of current through the long wires can potentially cause a

voltage spike at VIN large enough to damage the part.

For the input capacitor, two 10

μF low ESR ceramic

capacitors are recommended. For the recommended

capacitor, please refer to Table 4 for more details.

The selection of COUT is determined by the required ESR

to minimize voltage ripple.

Moreover, the amount of bulk capacitance is also a key

for COUT selection to ensure that the control loop is stable.

Loop stability can be checked by viewing the load transient

response as described in a later section.

The output ripple,

ΔVOUT , is determined by :

OUT

ESR

8fC

⎡⎤

Δ≤ Δ

⎢⎥

⎣⎦

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