Ver: 1.1

Sep 20, 2004

TEL: 886-3-5788833

http://www.gmt.com.tw

6

G5115/G5117/G5119

Global Mixed-mode Technology Inc.

Pin Description

PIN

G5115

G5117

G5119

SOT-23-5

SOT-23-5

MSOP-8

NAME

FUNCTION

1

1

8

SW

Switch Pin. The drain of the internal NMOS power switch. Connect

this pin to inductor.

2

PGND Power Ground Pin.

2

2

7

AGND Analog Ground Pin.

3

3

6

FB

Feedback Pin.

4

5

SHDN

Active Low Shutdown Pin. Tie this pin to logical high to enable

the device or tied it to logical low to turn this device off. Internal

1.5M

Ω pulled high.

4

4

OVP

Over Voltage Protection Sense Pin.

5

5

1

VCC

Input Supply Pin. Bypass this pin with a capacitor as close to the

device as possible.

Function Description

Operation

The G5115/G5117/G5119 are boost converters with

NMOS switch embedded. They operate in a PFM

scheme with constant peak current control. The opera-

tion frequency is up to 1MHz and is determined by the

current limit, inductor value, input voltage and minimum

off time. The boost cycle is started when FB pin voltage

drop below 0.25V as the NMOS switch turns on. During

the switch on period, the inductor current ramps up until

250mA current limit is reached. Then turns the switch

off, while the inductor current flows through external

schottky diode, and ramps down to zero. During the

switch off period, the inductor current provides for load

current and also charges output capacitor. It makes the

LED current higher and results in larger voltage drop on

sense resistor Rs. The cycle stop when FB pin voltage

is above 0.25V.

The current limit function acts as an inherent soft start

by controlling the inrush current.

PWM Dimming

To control the brightness of the LEDs, use a low fre-

quency PWM waveform to turn G5117/G5119 on for

duty 0%~100%. How bright the LEDs at 100% duty are

determined by sense resistor Rs.

Overvoltage Protection (OVP)

OVP is designed to prevent the damage of internal

NMOS switch in case the increased impedance of the

LED load (include the LED opened). Once the device

detects over voltage at the output, the internal NMOS

switch is kept off until the output voltage drops below

14V.

Applications Information

Inductor Selection

The PFM peak current control scheme of the

G5115/G5117/G5119 is inherently stable. The inductor

value does not affect the stability of the regulator. The

selected inductor must have a saturation current that

meets the maximum peak current of the converter. An-

other important inductor parameter is the DC resistance.

The lower DC resistance has the higher the efficiency

of the converter.

Table 1. Recommended Inductors

PART

VALUE(µH) MAX DCR (

Ω) VENDOR

LQH32CN100K1

1

10

0.39

MURATA

972AS-100M

10

0.48

TOKO

960AW-100M

10

0.18

TOKO

Output Capacitor Selection

For better output voltage filtering, a low ESR output

capacitor is recommended. Ceramic capacitors have a

low ESR value, but depending on the application, tan-

talum capacitors can be used. The selection of the

output capacitor value directly influences the output

voltage ripple of the converter which also influences

line regulation. The larger output voltage ripple, the

larger line regulation, which means that the LED cur-

rent changes if the input voltage changes. If a certain

change in LED current gives a noticeable change in

LED brightness, depends on the LED manufacturer and

on the application. Applications requiring good line

regulation

±1%/V (TYP) must use output capacitor val-

ues

±1µF.