RT8577 datasheet(9/12 Pages) RICHTEK

9 / 12 page

DS8577-00 November 2011

www.richtek.com

RT8577

≅

21.6 10

f100k +

SENSE

0.5V

Current Limit (A)

≅

Current Limit Protection

The RT8577 can sense the RSENSE voltage between the

SEN pin and GND to achieve over current protection. The

boost converter senses the inductor current during the on

period. The duty cycle depends on the current signal and

internal slope compensation compared with the error

signal. The external switch will be turned off when the

current signal is larger than the internal slope

compensation. In the off period, the inductor current will

decrease until the internal switch is turned on by the

oscillator. The current limit value can be calculated by the

following equation :

Brightness Control

The RT8577 features a digital dimming control scheme. A

very high contrast ratio true digital PWM dimming is

achieved by driving the PWM pin with a PWM signal. The

recommended PWM frequency is 200Hz to 10kHz, but

the LED current cannot be 100% proportional to duty cycle,

especially for high frequency and low duty ratio.

Over Temperature Protection

The RT8577 has over temperature protection function to

prevent the IC from overheating due to excessive power

dissipation. The IC will shut down and the STATUS pin

will be pulled high when junction temperature exceeds

150

°C. Main converter starts switching after junction

temperature cools down by approximately 20

°C.

Inductor Selection

The value of the inductance, L, can be approximated by

the following equation, where the transition is from

Discontinuous Conduction Mode (DCM) to Continuous

Conduction Mode (CCM) :

The duty cycle can be calculated as the following

equation :

where VOUT is the maximum output voltage, VIN is the

minimum input voltage, f is the operating frequency, and

IOUT is the sum of current from all LED strings.

The boost converter operates in DCM over the entire input

voltage range when the inductor value is less than this

value, L. With an inductance greater than L, the converter

operates in CCM at the minimum input voltage and may

be discontinuous at higher voltages.

The inductor must be selected with a saturated current

rating that is greater than the peak current as provided by

the following equation :

where

η is the efficiency of the power converter.

Diode Selection

Schottky diodes are recommended for most applications

because of their fast recovery time and low forward voltage.

Power dissipation, reverse voltage rating, and pulsating

peak current are important parameters for consideration

when making a Schottky diode selection. Make sure that

the diode's peak current rating exceeds IPEAK and reverse

voltage rating exceeds the maximum output voltage.

Capacitor Selection

The input capacitor reduces current spikes from the input

supply and minimizes noise injection to the converter. For

general applications, six 4.7

μF ceramic capacitors are

sufficient. A value higher or lower may be used depending

on the noise level from the input supply and the input

current to the converter.

It is recommended to choose a ceramic capacitor based

on the output voltage ripple requirements. The minimum

value of the output capacitor, COUT, can be calculated by

the following equation :

()

×−

××

OUT

D1 D

L =

2f I

−

OUT

D =

××

OUT

PEAK

VD T

η V2 L

Setting the Switching Frequency

The RT8577 switching frequency is programmable from

100kHz to 1MHz by adjusting the oscillator resistor, RRT.

The switching frequency can be calculated by the following

equation :

Δ×

OUT

where

ΔVOUT is the peak-to-peak ripple voltage at the

output.

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