isl6341a Intersil Corporation, isl6341a Datasheet - Page 14
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isl6341a
Manufacturer Part Number
isl6341a
Description
5v Or 12v Single Synchronous Buck Pulse-width Modulation Pwm Controller
Manufacturer
Intersil Corporation
Datasheet
1.ISL6341A.pdf
(17 pages)
Available stocks
Company
Part Number
Manufacturer
Quantity
Price
Part Number:
isl6341aCRZ
Manufacturer:
INTERSIL
Quantity:
20 000
Part Number:
isl6341aIRZ-T
Manufacturer:
INTERSIL
Quantity:
20 000
A stable control loop has a gain crossing with close to a
-20dB/decade slope and a phase margin greater than 45°.
Include worst case component variations when determining
phase margin. The mathematical model presented makes a
number of approximations and is generally not accurate at
frequencies approaching or exceeding half the switching
frequency. When designing compensation networks, select
target crossover frequencies in the range of 10% to 30% of
the switching frequency, f
This is just one method to calculate compensation components;
there are variations of the compensation break frequency
equations. The error amp is similar to that on other Intersil
regulators, so existing tools can be used here as well. Special
consideration is needed if the size of a ceramic output
capacitance in parallel with bulk capacitors gets too large; the
calculation needs to model them both separately (attempting to
combine two different capacitors types into one composite
component model may not work properly; a special tool may be
needed; contact your local Intersil person for assistance).
Component Selection Guidelines
Output Capacitor Selection
An output capacitor is required to filter the output and supply
the load transient current. The filtering requirements are a
function of the switching frequency and the ripple current.
The load transient requirements are a function of the slew
rate (di/dt) and the magnitude of the transient load current.
These requirements are generally met with a mix of
capacitors and careful layout.
Modern components and loads are capable of producing
transient load rates above 1A/ns. High frequency capacitors
initially supply the transient and slow the current load rate
seen by the bulk capacitors. The bulk filter capacitor values
are generally determined by the ESR (Effective Series
Resistance) and voltage rating requirements rather than
actual capacitance requirements.
High frequency decoupling capacitors should be placed as
close to the power pins of the load as physically possible. Be
FIGURE 14. ASYMPTOTIC BODE PLOT OF CONVERTER GAIN
0
LOG
20
log
⎛
⎜
⎜
⎝
R
------- -
R
2
1
⎞
⎟
⎟
⎠
F
Z1
F
F
LC
Z2
SW
F
F
.
14
CE
P1
F
0
F
20
P2
log
G
CL
COMPENSATION GAIN
d
---------------------------------
OPEN LOOP E/A GAIN
CLOSED LOOP GAIN
MAX V
G
V OSC
MOD
MODULATOR GAIN
ISL6341, ISL6341A, ISL6341B
FREQUENCY
⋅
IN
G
FB
careful not to add inductance in the circuit board wiring that
could cancel the usefulness of these low inductance
components. Consult with the manufacturer of the load on
specific decoupling requirements.
Use only specialized low-ESR capacitors intended for
switching-regulator applications for the bulk capacitors. The
bulk capacitor’s ESR will determine the output ripple voltage
and the initial voltage drop after a high slew-rate transient. An
aluminum electrolytic capacitor’s ESR value is related to the
case size with lower ESR available in larger case sizes.
However, the Equivalent Series Inductance (ESL) of these
capacitors increases with case size and can reduce the
usefulness of the capacitor to high slew-rate transient loading.
Unfortunately, ESL is not a specified parameter. Work with
your capacitor supplier and measure the capacitor’s
impedance with frequency to select a suitable component. In
most cases, multiple electrolytic capacitors of small case size
perform better than a single large case capacitor.
Output Inductor Selection
The output inductor is selected to meet the output voltage
ripple requirements and minimize the converter’s response
time to the load transient. The inductor value determines the
converter’s ripple current and the ripple voltage is a function
of the ripple current. The ripple voltage and current are
approximated by Equation 10:
Increasing the value of inductance reduces the ripple current
and voltage. However, the large inductance values reduce
the converter’s response time to a load transient.
One of the parameters limiting the converter’s response to
a load transient is the time required to change the inductor
current. Given a sufficiently fast control loop design, the
ISL6341, ISL6341A, ISL6341B will provide either 0% or
100% duty cycle in response to a load transient. The
response time is the time required to slew the inductor
current from an initial current value to the transient current
level. During this interval, the difference between the
inductor current and the transient current level must be
supplied by the output capacitor. Minimizing the response
time can minimize the output capacitance required.
The response time to a transient is different for the
application of load and the removal of load. Equation 11
gives the approximate response time interval for application
and removal of a transient load:
where: I
response time to the application of load, and t
response time to the removal of load. The worst case
response time can be either at the application or removal of
load. Be sure to check Equation 11 at the minimum and
maximum output levels for the worst case response time.
ΔI =
t
RISE
=
V
IN
Fsw x L
TRAN
V
- V
L x I
IN
OUT
- V
is the transient load current step, t
TRAN
OUT
x
V
V
OUT
IN
t
FALL
ΔV
OUT
=
L x I
= ΔI x ESR
V
OUT
TRAN
FALL
RISE
August 20, 2007
is the
(EQ. 10)
(EQ. 11)
FN6538.0
is the