RT8474A datasheet(10/13 Pages) RICHTEK

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RT8474A

DS8474A-05 April 2015

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

OUT

LIN(MAX)

L =



 





 

Application Information

The RT8474A can be used in multiple topologies. In Buck

converter applications, an OVP pin is specially designed

to be used for an over voltage protection function.

RT8474A uses a fixed frequency, current-mode control

scheme to provide excellent line and load regulation. The

control loop has a current sense amplifier which senses

the voltage between the ISP and ISN pins and provides an

output voltage at the VC pin. A PWM comparator then

turns off the internal power switch when the sensed power

switch current exceeds the compensated VC pin voltage.

The power switch will not be reset by the oscillator clock

in each cycle. If the comparator does not turn off the

switch in a cycle, the power switch will be on for more

than a full switching period until the comparator is tripped.

In this manner, the programmed voltage across the sense

resistor is regulated by the control loop.

Frequency Compensation

The RT8474A has an external compensation pin, allowing

the loop response to be optimized for specific applications.

An external resistor in series with a capacitor is connected

from the VC pin to GND to provide a pole and a zero for

proper loop compensation. The typical value for the

RT8474A is 10k and 3.3nF.

LED Current Setting

The LED current can be calculated by the following

equation :





ISP

ISN

LED(MAX)

SENSE

where (VISP

− VISN) is the voltage between the ISP and

ISN pins (100mV typ. if CTL dimming is not applied) and

the RSENSE is the resister between the ISP and ISN pins.

Current Limit

The RT8474A can limit the peak switch current with its

internal over-current protection feature. In normal operation,

the power switch is turned off when the switch current

hits the loop-set value. The over-current protection function

will turn off the power switch independent of the loop control

when the peak switch current reaches around 2A.

Output Over-Voltage Setting

The RT8474A OVP pin provides the Over-Voltage

Protection (OVP) function for buck topology only. The

OVP sense threshold is referenced to the top side of the

LED string with hysteresis. When the voltage difference

between the ISP pin and the OVP pin exceeds a threshold

of approximately 1.2V, the power switch will be turned off.

The power switch can be turned on again once the voltage

difference between the ISP pin and OVP pin drops below

1V. The OVP protection voltage level can be set by the

resistor divider R3 and R4 across the output capacitor C4

between ISP pin and the bottom end of the LED string,

with the center node of the resistor divider tied to the

OVP pin. Typically, set R4 = 10k

Ω˜is suggested.

Over-Temperature Protection

The RT8474A has Over-Temperature Protection (OTP)

function to prevent the excessive power dissipation from

overheating. The OTP function will shut down switching

operation when the die junction temperature exceeds

150

°C. The chip will automatically start to switch again

when the die junction temperature cools off.

Inductor Selection

Choose an inductor that can handle the necessary peak

current without saturating and ensure that the inductor

has a low DCR (copper-wire resistance) to minimize I

power losses. A 4.7

μH to 22μH inductor will meet the

demand of most of the RT8474A applications. Inductor

manufacturers specify the maximum current rating as the

current where the inductance falls to certain percentage

of its nominal value, typically 65%. In Multiple-Topology

application where the transition between discontinuous

and continuous modes occurs, the value of the required

output inductor, L, can be approximated by the following

equation :

For Buck application :

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