LX1684 Ver la hoja de datos (PDF) - Microsemi Corporation
Número de pieza
componentes Descripción
Fabricante
LX1684 Datasheet PDF : 12 Pages
| |||
LX1684
TM
®
Voltage-Mode PWM Controller
PRODUCTION DATA SHEET
APPLICATION INFORMATION
OUTPUT INDUCTOR
The output inductor should be selected to meet the requirements
of the output voltage ripple in steady-state operation and the
inductor current slew-rate during transient. The peak-to-peak
output voltage ripple is:
VRIPPLE = ESR × IRIPPLE
where
( ) IRIPPLE =
VIN - VOUT
fSW × L
× VOUT
VIN
IRIPPLE is the inductor ripple current, L is the output inductor
value and ESR is the Effective Series Resistance of the output
capacitor.
IRIPPLE should typically be in the range of 20% to 40% of the
maximum output current. Higher inductance results in lower
output voltage ripple, allowing slightly higher ESR to satisfy the
transient specification. Higher inductance also slows the inductor
current slew rate in response to the load-current step change, ∆I,
resulting in more output-capacitor voltage droop. The inductor-
current rise and fall times are:
TRISE
=
L × ∆I
(VIN − VOUT)
and
TFALL = L × ∆I
VOUT
When using electrolytic capacitors, the capacitor voltage droop
is usually negligible, due to the large capacitance.
OUTPUT CAPACITOR
The output capacitor is sized to meet ripple and transient
performance specifications. Effective Series Resistance (ESR) is a
critical parameter. When a step load current occurs, the output
voltage will have a step that equals the product of the ESR and the
current step, ∆I. In an advanced microprocessor power supply, the
output capacitor is usually selected for ESR instead of capacitance
or RMS current capability. A capacitor that satisfies the ESR
requirement usually has a larger capacitance and current capability
than strictly needed. The allowed ESR can be found by:
ESR × (IRIPPLE + ∆I ) < VEX
where IRIPPLE is the inductor ripple current, ∆I is the maximum
load current step change, and VEX is the allowed output voltage
excursion in the transient.
OUTPUT CAPACITOR (continued)
Electrolytic capacitors can be used for the output capacitor, but
are less stable with age than tantalum capacitors. As they age, their
ESR degrades, reducing the system performance and increasing the
risk of failure. It is recommended that multiple parallel capacitors
be used, so that, as ESR increases with age, overall performance
will still meet the processor’s requirements.
There is frequently strong pressure to use the least expensive
components possible, however, this could lead to degraded long-
term reliability, especially in the case of filter capacitors.
Linfinity’s demonstration boards use Sanyo MV-GX filter
capacitors, which are aluminum electrolytic, and have
demonstrated reliability. The Oscon series from Sanyo generally
provides the very best performance in terms of long term ESR
stability and general reliability, but at a substantial cost penalty.
The MV-GX series provides excellent ESR performance at a
reasonable cost. Beware of off-brand, very low-cost filter
capacitors, which have been shown to degrade in both ESR and
general electrolytic characteristics over time.
INPUT CAPACITOR
The input capacitor and the input inductor are to filter the
pulsating current generated by the buck converter to reduce
interference to other circuits connected to the same 12V rail. In
addition, the input capacitor provides local de-coupling the buck
converter. The capacitor should be rated to handle the RMS current
requirement. The RMS current is:
IRMS = IL d (1− d )
where IL is the inductor current and the d is the duty cycle. The
maximum value, when d = 50%, IRMS = 0.5IL . For 12V input and
output in the range of 3V, the required RMS current is very close
to 0.43IL.
SOFT-START CAPACITOR
The value of the soft-start capacitor determines how fast the
output voltage rises and how large the inductor current is required
to charge the output capacitor. The output voltage will follow the
voltage at SS pin if the required inductor current does not exceed
the maximum current in the inductor. The SS pin voltage can be
expressed as:
VSS = VSET (1 − e−t / RSSCSS )
where VSET is the reference voltage. RSS and CSS are soft start
resistor and capacitor. The required inductor current for the output
capacitor to follow the SS-pin voltage equals the required capacitor
current plus the load current. The soft-start capacitor should be
selected so that the overall inductor current does not exceed its
maximum.
Copyright © 2000
Rev. 1.0c, 2005-03-08
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 6
Share Link: 