Global Mixed-mode Technology Inc.
Pin Description
G5115
SOT-23-5
1
2
3
G5115/G5117/G5119
FUNCTION
PIN
G5117
SOT-23-5
1
2
3
4
G5119
MSOP-8
8
2
7
6
5
4
NAME
SW
PGND
AGND
FB
SHDN
4
5
5
OVP
VCC
1
Switch Pin.
The drain of the internal NMOS power switch. Connect
this pin to inductor.
Power Ground Pin.
Analog Ground Pin.
Feedback Pin.
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
1.5MΩ pulled high.
Over Voltage Protection Sense Pin.
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
LQH32CN100K1
1
972AS-100M
960AW-100M
10
10
0.48
0.18
TOKO
TOKO
VALUE(µH) MAX DCR (Ω) VENDOR
10
0.39
MURATA
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.
Ver: 1.1
Sep 20, 2004
TEL: 886-3-5788833
http://www.gmt.com.tw
6
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
