RT7278

11

DS7278-00 January 2013

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Copyright 2013 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

OUT

OUT

L

IN

VV

I =

1

fL

V

⎡⎤ ⎡

⎤

Δ× −

⎢⎥ ⎢

⎥

×

⎣⎦ ⎣

⎦

Having a lower ripple current reduces not only the ESR

losses in the output capacitors but also the output voltage

ripple. High frequency with small ripple current can achieve

highest efficiency operation. However, it requires a large

OUT

OUT

L(MAX)

IN(MAX)

VV

L =

1

fI

V

⎡⎤ ⎡

⎤

×−

⎢⎥ ⎢

⎥

×Δ

⎣⎦ ⎣

⎦

Input and Output Capacitors Selection

trapezoidal current at the Source of the high side MOSFET.

A low ESR input capacitor with larger ripple current rating

should be used for the maximum RMS current. The RMS

current is given by :

OUT

IN

RMS

OUT(MAX)

IN

OUT

V

V

I

= I

1

VV

−

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, two 10

μF and 0.1μF low ESR ceramic

capacitors are recommended.

to minimize voltage ripple.

Moreover, the amount of bulk capacitance is also a key

The output ripple,

Output Voltage Setting

The resistive divider allows the FB pin to sense the output

voltage as shown in Figure 4.

Under Voltage Lockout Protection

The RT7278 has Under Voltage Lockout Protection (UVLO)

voltage is lower than UVLO threshold voltage, the RT7278

will be turned off in this state. This is non-latch protection.

Over Temperature Protection

The RT7278 equips an Over Temperature Protection (OTP)

circuitry to prevent overheating due to excessive power

dissipation. The OTP will shut down switching operation

when junction temperature exceeds 150

°C. Once the

junction temperature cools down by approximately 20

°C

the main converter will resume operation. To keep operating

at maximum, the junction temperature should be prevented

from rising above 150

°C.

Inductor Selection

The inductor value and operating frequency determine the

ripple current according to a specific input and an output

voltage. The ripple current

and decreases with higher inductance.

Figure 4. Output Voltage Setting

The output voltage is set by an external resistive divider

according to the following equation. It is recommended to

use 1% tolerance or better divider resistors.

)

OUT

R1

V

= 0.765 (1

R2

×+

inductor to achieve this goal. For the ripple current

selection, the value of

starting point. The largest ripple current occurs at the

below the specified maximum, the inductor value should

be chosen according to the following equation :

OUT

L

OUT

1

VI ESR

8fC

⎡⎤

Δ≤ Δ

+

⎢⎥

⎣⎦

The output ripple will be highest at the maximum input

voltage since

capacitors placed in parallel may need to meet the ESR

and RMS current handling requirements.

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

GND

FB

R1

R2

RT7278