RT8494GS datasheet(12/17 Pages) RICHTEK

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RT8494

DS8494-01 November 2015

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

SW, Buck-Boost

OUT

V0.08V

) I





 





For Boost application, select a resistor according to :

SW, Boost

OUT

V0.08V





 





For Buck-Boost application, select a resistor according

to :

The placement of RSW should be close to the source of

the N-MOSFET and GND of the RT8494. The ISW pin

input to RT8494 should be a Kelvin connection to the

positive terminal of RSW.

Output Over-Voltage Setting

The RT8494 is equipped with an Over-Voltage Protection

(OVP) function. When the voltage at OVP pin exceeds a

threshold of approximately 1.18V, the power switch is

turned off. The power switch can be turned on again once

the voltage at OVP pin drops below 1.18V. For the Boost

and Buck-Boost application, the output voltage could be

clamped at a certain voltage level. The OVP voltage can

be set by the following equation :

OUT, OVP

V1.18



 





Where,

R1 and R2 are the voltage divider from VOUT to GND with

the divider center node connected to OVP pin.

fOSC (kHZ)

RRSET (k)

1000

8.34

800

11.41

600

16.68

500

20.9

300

38.04

200

60.35

100

130

Table 1. Switching Frequency vs. RRSET Value (1%

Resistors)

Figure 5. Switching Frequency vs. RRSET

The ACTL pin can also be used in conjunction with a

thermistor to provide over temperature protection for the

LED load, or with a voltage divider to VIN to reduce output

power and switching current when VIN is low. The presence

of a time varying differential voltage signal (ripple) across

ISP and ISN at the switching frequency is expected.

The amplitude of this signal is increased by high LED load

current, low switching frequency and/or a smaller value

output filter capacitor. The compensation capacitor on the

VC pin filters the signal so the average difference between

ISP and ISN is regulated on the user-programmed value.

Programmable Switching Frequency

The RSET frequency adjust pin allows the user to program

the switching frequency from 100kHz to 1MHz for optimized

efficiency and performance or external component size.

Higher frequency operation allows for smaller component

size but increases switching losses and gate driving

current, and may not allow sufficiently high or low duty

cycle operation. Lower frequency operation gives better

performance but with larger external component size. For

an appropriate RRSET resistor value see Table 1 or Figure

5. An external resistor from the RSET pin to GND is

required and do not leave this pin open.

Frequency vs. RRSET

100

200

300

400

500

600

700

800

900

1000

153045607590

105 120 135

RRSET (kΩ)

RRSET (k

Ω)

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