lx1912 Microsemi Corporation, lx1912 Datasheet - Page 5

• Manufacturer Part Number: lx1912
• Description: Vref @ 800mv, 1.0a, 1.2mhz Pwm Current Mode Pwm Buck Regulator
• Manufacturer: Microsemi Corporation

Copyright © 2005
Rev. 1.0, 2006-09-06
O
O
F
compensation.
limit on a pulse by pulse basis and with thermal shutdown.
Thermal shutdown is activated with a junction temperature of
160°C (typical) and has 20°C of hysteresis.
100µs to ensure all circuitry is operating before enabling the
Switch output.
recovery from either thermal shutdown or short circuit. The Soft
start control block generates a voltage ramp that clamps the error
amplifier non-inverting reference voltage. As this clamp voltage
rises, the duty cycle is gradually increased, thus limiting the peak
inrush currents.
current condition. The PFM mode increases system efficiency by
reducing the switching frequency thus switching losses. During
light loading, I
the switching frequency begins to decrease, the frequency change
occurs over a continuous range, decreasing further as I
decreases.
a voltage that is 15% higher than the internal referenced VREF.
Once the FB pin voltage goes 15% above the internal reference,
the internal PMOS control switch is turned off, which allows the
output voltage to decrease toward regulation.
impedance of both feedback resistors should not exceed 50K Ohms
to ensure optimal frequency stability. The value of R1 can be
determined using the following equation, note V
to as V
forward voltage. Efficiency is determined mostly by the diode’s
forward voltage. The diode conducts 1-D%, for V
becomes 76% in a 5V system.
optimal performance of the converter circuit for the intended
application. A primary consideration requires the selection of an
inductor that will not saturate at the peak current level. EMI,
output voltage ripple, and overall circuit efficiency affect inductor
choice.
application’s requirements and some experimentation with actual
devices in-circuit is typically necessary to make the most effective
choice.
D
I
NDUCTOR
UNCTIONAL
UTPUT
UTPUT
IODE
Resistors R1 and R2 program the output voltage. The total
A Schottky diode is required for switching speed and low
Selecting the appropriate inductor type and value ensures
The LX1912 is a Current Mode PWM regulator with internal
The internal PMOS high side switch is protected with current
The regulator has an internal Power On Reset delay of 50-
Soft Start is activated upon initial power-on, or following
PWM / PFM mode of operation is determined by the load
The over voltage comparator compares the FB pin voltage to
R1
FBT
S
TM
O
V
ELECTION
=
.
OLTAGE
VER
S
The inductor that works best depends upon the
R2
ELECTION
D
OUT
⎡
⎢
⎢
⎣
ESCRIPTION
V
⎛
⎜
⎜
⎝
OLTAGE
V
V
< 200mA typically, PFM mode becomes active,
OUT
P
REF
ROGRAMMING
⎞
⎟
⎟
⎠
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
−
P
1
ROTECTION
⎤
⎥
⎥
⎦
REF
A P P L I C A T I O N N O T E
OUT
is also referred
Integrated Products Division
=1.2V this
®
Microsemi
OUT
inductor value of 2.7µH ±20%. The benefit of a larger inductor
value can increase efficiency at the lower output currents and
reduces output voltage ripple, thus output capacitance related to
ripple filtering. Smaller inductors typically provide smaller package
size (critical in many portable applications) at the expense of
increasing output ripple current. Regardless of inductor value,
selecting a device manufactured with a ferrite-core produces lower
losses at higher switching frequencies and thus better overall
performance. Larger inductors may lead to diminished Step-Load
response.
resistance (ESR) are recommended. Multi-layer ceramic capacitors
with X5R or X7R dielectric make an effective choice because they
feature small size, very low ESR, a temperature stable dielectric,
and can be connected in parallel to increase capacitance. Typical
output capacitance values of 20 to 60µF have proven effective.
Other low ESR capacitors such as solid tantalum, specialty polymer,
or organic semiconductor, make effective choices provided that the
capacitor is properly rated for the output voltage and ripple current.
Finally, choose an input capacitor of sufficient size to effectively
decouple the input voltage source impedance (e.g., C
DC/DC converter applications require careful attention to device
layout for optimal performance. Basic design rules include: (1)
maintaining wide traces for power components (e.g., width >
50mils); (2) place CIN, COUT, the Schottky diode, and the inductor
close to the LX1912; (3) minimizing trace capacitance by reducing
the etch area connecting the SW pin to the inductor; and (4)
minimizing the etch length to the FB pin to reduce noise coupling
into this high impedance sense input. Other considerations include
placing a 0.1uF capacitor between the LX1912 VOUT pin and GND
pin to reduce high frequency noise and decoupling the VCC, VPWR
pins using a 0.1µF capacitor.
I
C
L
NDUCTOR
AYOUT
APACITOR
The LX1912 stability performance is optimized by using an
To minimize ripple voltage, output capacitors with a low series
The high peak currents and switching frequencies present in
VREF @ 800mV, 1.0A, 1.2MHz PWM
C
ONSIDERATIONS
S
ELECTION
S
ELECTION
P
RODUCTION
,
CONT
.
D
ATA SHEET
IN
> 4.7μF).
LX1912
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