lx1660 Microsemi Corporation, lx1660 Datasheet - Page 10

• Manufacturer Part Number: lx1660
• Description: Advanced Pwm Controller
• Manufacturer: Microsemi Corporation

LX1660/1661
10
The LX1660/61 devices are very easy to design with, requiring
only a few simple calculations to implement a given design. The
following procedures and considerations should provide effective
operation for virtually all applications. Refer to the Application
Information section for component reference designators.
SELECTING BETWEEN THE LX1660 AND THE LX1661
In order to provide maximum user versatility, the Advanced PWM
Controller is offered in two versions: the LX1660 and the LX1661.
the current sense resistor voltage drop. This allows optimal
transient response for high-speed systems, such as Pentium Pro
processor power supplies. Overall system design could be more
economical with the LX1661, since output capacitance require-
ments could be eased.
40mV offset is not included in the device. This device is good for
critical DC applications, such as core power in slower micropro-
cessors and related systems.
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.
value and ESR is the Effective Series Resistance of the output
capacitor.
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 slew rates at rise and fall edges are:
droop is usually negligible, due to the large capacitance.
The LX1661 has a 40mV offset built-in which compensates for
The LX1660 provides a very accurate DC set-point, since the
See "Theory Of Operation" section earlier in this data sheet.
The peak-to-peak output voltage ripple is:
where,
I
I
and,
When using electrolytic capacitors, the capacitor voltage
RIPPLE
RIPPLE
V
I
T
T
RIPPLE
RISE
FALL
RIPPLE
is the inductor ripple current, L is the output inductor
should typically be in the range of 20% to 40% of the
= L * I / (V
= L * I / V
=
= ESR * I
V
IN
f
SW
- V
* L
RIPPLE
OUT
OUT
IN
- V
*
OUT
V
V
)
OUT
IN
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
U S I N G T H E L X 1 6 6 0 / 6 1 D E V I C E S
P
A
R O D U C T I O N
D V A N C E D
P W M C
D
INPUT INDUCTOR
In order to supply faster transient load changes, a smaller output
inductor is needed. However, reducing the size of the output
inductor will result in a higher ripple voltage on the input supply.
This noise on the 5V rail can affect other system components, such
as graphic cards. In this case, it is recommended that a 1 - 1.5µH
inductor is used on the input to the regulator, to filter the ripple
on the 5V supply. Ensure that this inductor has the same current
rating as the output inductor.
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 advance 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 capabil-
ity than strictly needed. The allowed ESR can be found by:
load current step change, and V
excursion in the transient. Adaptive voltage positioning increases
the value of V
of the output capacitor.
and the transient tolerance is 100mV, resulting in a V
(See Figure 3). The LX1660 does not have the positioning voltage
offset, so V
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.
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 character-
istics over time.
A T A
where I
Typically, the positioning voltage is 40mV, using the LX1661,
Electrolytic capacitors can be used for the output capacitor, but
There is frequently strong pressure to use the least expensive
O N T R O L L E R
ESR * (I
S
RIPPLE
EX
H E E T
RIPPLE
EX
is 100mV maximum.
, allowing a higher ESR value and reducing the cost
is the inductor ripple current, I is the maximum
+ I) < V
EX
EX
is the allowed output voltage
EX
Copyright © 1998
of 140mV
Rev. 1.1
7/98