isl6236 Intersil Corporation, isl6236 Datasheet - Page 32

• Manufacturer Part Number: isl6236
• Description: High-efficiency, Quad-output, Main Power Supply Controllers For Notebook Computers
• Manufacturer: Intersil Corporation

Inductor Selection
The switching frequency (on-time) and operating point (%
ripple or LIR) determine the inductor value as follows:
Example: I
f = 200kHz, 35% ripple current or LIR = 0.35:
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite cores
are often the best choice. The core must be large enough
not to saturate at the peak inductor current (IPEAK):
The inductor ripple current also impacts transient response
performance, especially at low V
inductor values allow the inductor current to slew faster,
replenishing charge removed from the output filter capacitors
by a sudden load step. The peak amplitude of the output
transient (VSAG) is also a function of the maximum duty
factor, which can be calculated from the on-time and
minimum off-time:
where minimum off-time = 0.35µs (max) and K is from
Table 2.
Determining the Current Limit
The minimum current-limit threshold must be great enough
to support the maximum load current when the current limit
is at the minimum tolerance value. The valley of the inductor
current occurs at I
current; therefore:
where: I
voltage divided by the r
Use the worst-case maximum value for r
MOSFET Q2/Q4 data sheet and add some margin for the
rise in r
allow 0.2% additional resistance for each °C of temperature
rise.
Examining the 5A circuit example with a maximum
r
following:
L
IPEAK
VSAG
I
DS(ON)
L
LIMIT LOW
=
=
-----------------------------------------------------------------
12V 200kHz 0.35 5A
-------------------------------------------------------------------- -
V
(
IN
V
DS(ON)
=
=
LIMIT(LOW)
OUT_
= 5mΩ at room temperature. At +125°C reveals the
⋅
⋅ ⋅
5V 12V 5V
(
--------------------------------------------------------------------------------------------------------------------------- -
2 C
f LIR I
I
ΔI
LOAD(MAX)
LOAD MAX
⋅
)
(
>
LOAD MAX
(
OUT
I
V
LOAD MAX
with temperature. A good general rule is to
IN
⋅
(
–
LOAD MAX
⋅
(
+
LOAD(MAX)
⋅
V
V
= minimum current-limit threshold
(
OUT
)
OUT_
)
+
= 5A, V
)
(
⋅
DS(ON)
)
[
2
(
)
LIR 2 ⁄
⋅
K
–
)
L K
⎛
⎜
⎝
[
=
)
⎛
⎜
⎝
V
------------------------------- -
(
32
LIR 2 ⁄
IN
IN
8.3μH
minus half of the ripple
⎛
⎜
⎝
) I
of Q2/Q4.
V
------------------ -
V
⋅
–
IN
= 12V, V
OUT_
V
IN
V
LOAD MAX
IN
) I
OUT
- V
⋅
LOAD MAX
OUT
+
⎞
⎟
⎠
(
t
-
DS(ON)
OFF MIN
t
OUT2
OFF MIN
differences. Low
(
)
(
]
(
= 5V,
)
from the
]
)
⎞
⎟
⎠
)
⎞
⎟
⎠
(EQ. 10)
(EQ. 12)
(EQ. 11)
(EQ. 8)
(EQ. 9)
ISL6236
4.17A is greater than the valley current of 4.12A, so the
circuit can easily deliver the full-rated 5A using the 30mV
nominal current-limit threshold voltage.
Output Capacitor Selection
The output filter capacitor must have low enough equivalent
series resistance (ESR) to meet output ripple and
load-transient requirements, yet have high enough ESR to
satisfy stability requirements. The output capacitance must
also be high enough to absorb the inductor energy while
transitioning from full-load to no-load conditions without
tripping the overvoltage fault latch. In applications where the
output is subject to large load transients, the output
capacitor's size depends on how much ESR is needed to
prevent the output from dipping too low under a load
transient. Ignoring the sag due to finite capacitance:
where V
In non-CPU applications, the output capacitor's size
depends on how much ESR is needed to maintain an
acceptable level of output voltage ripple:
where V
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 tantalum, OS-CON, and
other electrolytic-type capacitors).
When using low-capacity filter capacitors such as polymer
types, capacitor size is usually determined by the capacity
required to prevent V
undervoltage and overvoltage fault latches during load
transients in ultrasonic mode.
For low input-to-output voltage differentials (V
additional output capacitance is required to maintain stability
and good efficiency in ultrasonic mode. The amount of
overshoot due to stored inductor energy can be calculated
as:
where I
R
R
I
4.17A 4.12A
V
LIMIT LOW
ESR
SER
SOAR
(
≤
>
≤
PEAK
-----------------------------------------------
L
--------------------------------- -
I
DIP
P-P
=
LOAD MAX
IR
----------------------------------------------- -
2 C
)
V
⋅
is the maximum-tolerable transient voltage drop.
I
is the peak-to-peak output voltage ripple. The
⋅
=
DIP
V
LOAD MAX
is the peak inductor current.
(
I
P P
PEAK
OUT
(
25mV
–
(
)
⋅
2
V
SAG
⋅
)
OUT_
L
⁄
)
(
(
5mΩ
and V
×
SOAR
1.2
)
>
5A
from tripping the
–
(
0.35 2 ⁄
IN
/ V
)5A
OUT
June 28, 2007
(EQ. 16)
(EQ. 13)
(EQ. 14)
(EQ. 15)
(EQ. 17)
)
FN6373.3
< 2),