LX1918CLD MICROSEMI [Microsemi Corporation], LX1918CLD Datasheet - Page 6

• Manufacturer Part Number: LX1918CLD
• Description: 1.8A Step-Down Synchronous Converter
• Manufacturer: MICROSEMI [Microsemi Corporation]

Copyright © 2004
Rev. 1.1, 2005-07-05
LX1918 is synchronous step down converter with input
voltage range from 2.7V to 5.5V; and outputs ranging from
0.6V to 80% of V
output current up to 1.8A. The LX1918 has two selectable
operating modes for light loads: high efficiency PFM
(MODE=GND) or low noise forced PWM (MODE=VCC).
Under heavy load, LX1918 always operates in PWM mode
and switches at fixed internal 1MHz frequency or can be
synchronized with an external oscillator clock with a
frequency ranging from 500 KHz to 1.25MHz.
Switching frequency
The E/S pin has dual function. If E/S pin pulls HIGH, the
1MHz internal oscillator is enabled. If E/S pin is held
LOW, the LX1918 will enter Shut Down mode. Designers
can apply an external oscillator signal, 500 KHz to
1.25MHz, to E/S pin to synchronize the switching
frequency to a system clock. When applying external
oscillator signal, the MODE pin must pull up to VCC
activating the forced PWM mode.
High Efficiency Operation: PFM, PWM, and EMI
force PWM modes
The LX1918 has two selectable operating modes for light
loads: high efficiency PFM (MODE=GND) or low noise
forced PWM(MODE=VCC). Only one mode can be active
at any time.
Under light load operation, defined as I
MODE pin = VCC, the continuous inductor current mode is
selected. This forces fixed frequency PWM and
synchronous rectification regardless of loading. The
inductor ripple current and duty cycle remain the same, the
synchronous rectifier allows the inductor current to go
positive or negative preventing system from going into
discontinuous conduction and preventing possible EMI
associated with inductor ringing.
TM
IN
. The LX1918 can deliver a maximum
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
OUT
T H E O R Y O F O P E R A T I O N
<300mA, if the
Integrated Products Division
®
Microsemi
1.8A Step-Down Synchronous Converter
If MODE pin is LOW, discontinuous current is selected.
The PWM frequency slows down as the load decreases and
this reduces switching losses. In this mode, the NMOS
switch turns off to prevent negative current flow and
allows the inductor current to reduce to zero (which is
known as discontinuous conduction mode).
Comparing the efficiency of the two modes of operation
under light loading, when I
efficiency is 80% (MODE=GND) and 68%
(MODE=VCC).
Under heavy load (I
in PWM mode, with an efficiency up to 92%.
Protection:
LX1918 provides thermal shutdown, UVLO, and current
limit protection:
Thermal shutdown: if the die temperature reaches
150ºC.
UVLO: an under voltage lock out, LX1918 will shut
down when V
Current limiting protection:
o
o
o
P
Switch current limit: The PMOS switch is
limited to a peak current of 2.7A. The PMOS
“switch on” cycle is terminated when the
switch current limit is reached.
Soft start: Upon start up, the switch current
is limited to a gradual rise of 0 to 2.7A in
100us. The synchronous rectifier remains off
during soft start preventing reverse load
current.
Hiccup mode: If the PMOS switch is current
limited when it attempts to turns on
(indicating an output short circuit) , the
converter will enter a partial soft start cycle
mode to prevent a runaway output current.
RODUCTION
IN
OUT
< 2.7V.
>500mA), LX1918 always operates
D
OUT
ATA
= 25mA, LX1918
S
HEET
LX1918
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