ISL88550AIRZ Intersil, ISL88550AIRZ Datasheet - Page 19

• Manufacturer Part Number: ISL88550AIRZ
• Description: IC PWM CONTROLLER 28TQFN
• Manufacturer: Intersil

Specifications of ISL88550AIRZ

• Applications: PWM Controller
• Voltage - Input: 2 ~ 25 V
• Current - Supply: 25µA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-TQFN
• Rohs Compliant: YES
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Supply: -

Most inductor manufacturers provide inductors in standard
values, such as 1.0µH, 1.5µH, 2.2µH, 3.3µH, etc. Also look
for nonstandard values, which can provide a better
compromise in LIR across the input voltage range. If using a
swinging inductor (where the no-load inductance decreases
linearly with increasing current), evaluate the LIR with
properly scaled inductance values.
Input Capacitor Selection (Buck)
The input capacitor must meet the ripple current requirement
(I
For most applications, non-tantalum chemistry capacitors
(ceramic, aluminum, or OSCON) are preferred due to their
resistance to power-up surge currents typical of systems
with a mechanical switch or connector in series with the
input. If the ISL88550A are operated as the second stage of
a two-stage power conversion system, tantalum input
capacitors are acceptable. In either configuration, choose a
capacitor that has less than +10°C temperature rise at the
RMS input current for optimal reliability and lifetime.
Output Capacitor Selection (Buck)
The output filter capacitor must have low enough equivalent
series resistance (R
transient requirements, yet have high enough ESR to satisfy
stability requirements. For processor core voltage converters
and other applications in which the output is subject to
violent load transients, the output capacitor’s size depends
on how much R
dipping too low under a load transient. Ignoring the sag due
to finite capacitance in Equation 14:
In applications without large and fast load transients, the
output capacitor’s size often depends on how much R
needed to maintain an acceptable level of output voltage
ripple. The output ripple voltage of a step-down controller is
approximately equal to the total inductor ripple current
multiplied by the output capacitor’s R
maximum R
shown in Equation 15:
The actual capacitance value required relates to the physical
size needed to achieve low ESR, as well as to the chemistry
of the capacitor technology. Thus, the capacitor is usually
selected by ESR and voltage rating rather than by
capacitance value (this is true of tantalums, OSCONs,
polymers, and other electrolytics).
I
R
R
RMS
RMS
ESR
ESR
=
≤
≤
I
) imposed by the switching currents in Equation 13:
LOAD
I
Δ
LOAD
I
LOAD
V
V
STEP
(
RIPPLE
MAX
V
V
(
ESR
OUT
MAX
IN
)
×
)
⎛
⎜
⎜
⎝
LIR
ESR
1
required to meet ripple specifications is
−
V
V
ESR
OUT
is needed to prevent the output from
IN
⎞
⎟
⎟
⎠
) to meet output ripple and load
19
ESR
. Therefore, the
(EQ. 13)
(EQ. 14)
(EQ. 15)
ESR
ISL88550A
is
When using low-capacity filter capacitors, such as ceramic
capacitors, size is usually determined by the capacity
needed to prevent V
during load transients. Generally, once enough capacitance
is added to meet the overshoot requirement, undershoot at
the rising load edge is no longer a problem (see the V
and V
page 22).
VTT Output Capacitor Selection (LDO)
Place 2µFx10µF 0805 ceramic capacitor as close to VTT
output as possible for optimum performance of output
loading up to +2.5A/-2.0A. In most applications, it is not
necessary to add more capacitance. However, optional
additional capacitances can be added further away (>1.5”)
from VTT output.
VTTR Output Capacitor Selection (LDO)
The VTTR buffer is a scaled down version of the VTT
regulator with much smaller output transconductance. Its
compensation capacitor can therefore be smaller, and its
ESR larger than what is required for its larger counterpart.
For typical applications requiring load current up to ±20mA, a
ceramic capacitor with a minimum value of 1µF is
recommended (ESR <0.3Ω). Tie this capacitor between
VTTR and analog ground plane.
VTTI Input Capacitor Selection (LDO)
Both the VTT and VTTR output stages are powered from the
same VTTI input. Their output voltages are referenced to the
same REFIN input. The value of the VTTI bypass capacitor
is chosen to limit the amount of ripple/noise at VTTI, or the
amount of voltage dip during a load transient. Typically, a
ceramic capacitor of at least 10µF should be used. This
value is to be increased with larger load current, or if the
trace from the VTTI pin to the power source is long and has
significant impedance. Furthermore, to prevent undesirable
VTTI bounce from coupling back to the REFIN input and
possibly causing instability in the loop, the REFIN pin should
ideally tap its signal from a separate low impedance DC
source rather than directly to the VTTI input. If the latter is
unavoidable, increase the amount of bypass at the VTTI
input and add additional bypass at the REFIN pin.
MOSFET Selection (Buck)
The ISL88550A drive external, logic-level, N-Channel
MOSFETs as the circuit-switch elements. The key selection
parameters are as follows:
Maximum Drain-To-Source Voltage (V
least 20% higher than input supply rail at the high side
MOSFET’s drain.
Choose the MOSFETs with rated r
a good compromise between efficiency and cost, choose the
high-side MOSFET that has a conduction loss equal to
switching loss at nominal input voltage and maximum output
current. For low-side MOSFET, make sure that it does not
SOAR
equations in “Transient Response (Buck)” on
SAG
and V
SOAR
DS(ON)
from causing problems
DSS
at V
): Should be at
GS
= 4.5V. For
April 23, 2008
FN6168.3
SAG