Ver: 1.1

Nov 17, 2005

TEL: 886-3-5788833

http://www.gmt.com.tw

8

G5121

Global Mixed-mode Technology Inc.

Pin Description

PIN

NAME

FUNCTION

1

SW

Switch Pin. The drain of the internal NMOS power switch. Connect this pin to inductor.

2

GND

Ground Pin.

3

FB

Feedback Pin. Connect current setting resistor Rs from this pin to ground. The LED current is set as

0.254V/ Rs.

4

SHDN

Active Low Shutdown Pin.

5

OVP

Over Voltage Protection Sense Pin.

6

VCC

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

Function Description

Normal Operation

The G5121 uses a constant frequency control scheme

to provide excellent line and load regulation. Operation

can be best understood by referring to the block dia-

gram. At the start of each oscillator cycle, the SR latch

is set, which turns on the power switch M1. An artificial

ramp is generated to the positive terminal of the PWM

comparator A2. When this voltage exceeds the level at

the negative input of A2, the SR latch is reset turning

off the power switch. The level at the negative input of

A2 is set by the error amplifier A1, and is simply an

amplified version of the difference between the feed-

back voltage and the reference voltage of 0.254V. In

this manner, the error amplifier sets the correct peak

current level to keep the output in regulation. If the

error amplifier’s output increases, more current is de-

livered to the output, if it decreases, less current is

delivered.

Over Voltage Protection

Over voltage protection function is designed to prevent

the damage of internal NMOS switch in case the in-

creased impedance of the LED load (include the LED

opened). Once the device detects over voltage (typical

19.5V) at the output, the internal NMOS switch is kept

off until the output voltage drops below 15V.

Application Information

Inductor Selection

A 4.7µF inductor is recommended for G5121 applica-

tions. Small size and high efficiency are the major

concerns for most G5121 applications. Inductor with

low core losses and small DCR (cooper wire resis-

tance) at 1MHz are good choice for G5121 applica-

tions.

Since the G5121 is designed to operate in discontinu-

ous mode, the inductor current reaches zero during

discharge phase. After the inductor current reaches

zero, the switch pin exhibits ringing due to the LC tank

circuit formed by the inductor in combination with

switch and diode capacitance. This ringing is not

harmful; far less spectral energy is contained in the

ringing than in the switch transitions. The ringing can

be damped by application of a 300

Ω resistor across

the inductor, although this will degrade efficiency.

Capacitor Selection

The small size of ceramic capacitors makes them suit-

able for G5121 applications. X5R and X7R types are

recommended because they retain their capacitance

over wider voltage and temperature ranges than other

types such as Y5V or Z5U. A minimum 1µF capacitor

for output is required for most applications. Larger in-

put/output capacitor minimizes input/output ripple.

Diode Selection

Schottky diodes, with their low forward voltage drop

and fast reverse recovery, are the ideal choices for

G5121 applications. The forward voltage drop of a

Schottky diode represents the conduction losses in the

sents the switching losses. For diode selection, both

forward voltage drop and diode capacitance need to

be considered. Schottky diodes with higher current

ratings usually have lower forward voltage drop and

larger diode capacitance, which can cause significant

switching losses at the 1MHz switching frequency of

the G5121. A Schottky diode rated at 500mA is suffi-

cient for most G5121 applications.