RT7252B datasheet(12/14 Pages) RICHTEK

12 / 14 page

RT7252A/B

DS7252A/B-01 April 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

CIN and COUT Selection

The input capacitance, CIN, is needed to filter the

trapezoidal current at the source of the high side MOSFET.

To prevent large ripple current, a low ESR input capacitor

sized for the maximum RMS current should be used. The

RMS current is given by :

OUT

RMS

OUT(MAX)

OUT

= I

−

This formula has a maximum at VIN = 2VOUT, where IRMS =

IOUT/2. This simple worst-case condition is commonly used

for design because even significant deviations do not offer

much relief. Choose a capacitor rated at a higher

temperature than required. Several capacitors may also

be paralleled to meet size or height requirements in the

design. For the input capacitor, a 10

μF low ESR ceramic

capacitor is recommended. For the recommended

capacitor, please refer to table 3 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

⎢⎥

⎣⎦

The output ripple will be 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. Dry tantalum,

special polymer, aluminum electrolytic and ceramic

capacitors are all available in surface mount packages.

Special polymer capacitors offer very low ESR value.

However, it provides lower capacitance density than other

types. Although Tantalum capacitors have the highest

capacitance density, it is important to only use types that

pass the surge test for use in switching power supplies.

Aluminum electrolytic capacitors have significantly higher

ESR. However, it can be used in cost-sensitive applications

for ripple current rating and long term reliability

considerations. 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.

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 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) 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.

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