isl6237 Intersil Corporation, isl6237 Datasheet - Page 30

• Manufacturer Part Number: isl6237
• 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 (V
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.
L
L
IPEAK
V
FIGURE 72. SETTING V
SAG
=
=
-------------------------------------------------------------------- -
V
ISL88732
ISL88732
ISL88733
ISL6237
ISL88733
ISL6237
ISL88734
ISL88734
-----------------------------------------------------------------
12V 200kHz 0.35 5A
UGATE_
UGATE2
UGATE_
IN
V
LGATE_
LGATE2
LGATE_
REFIN2
VOUT_
VOUT_
=
OUT _
=
˜ ˜
UGATE2
LGATE2
˜
--------------------------------------------------------------------------------------------------------------------------- -
2 C
OUT2
f LIR I
5V 12V 5V
REFIN2
'I
I
FB_
FB_
LOAD(MAX)
LOAD MAX
˜
OUT2
SAG
LOAD MAX
TRACKING
OUT
V
IN
˜
) is also a function of the maximum duty
LOAD MAX
+
–
˜
˜
V
V
OUT
OUT _
R
+
4
= 5A, V
˜
2
>
OUT2
˜
K
LIR 2 e
L K
§
¨
©
§
¨
©
V
------------------------------- -
=
30
IN
WITH A VOLTAGE DIVIDER FOR
V
§
¨
©
Q
IN
8.3PH
IN
Q2
Q2
Q2
Q
V
------------------ -
I
1
–
V
IN
2
OUT _
˜
V
= 12V, V
IN
V
R
IN
LOAD MAX
OUT
- V
3
OUT
+
·
¸
¹
t
-
OFF MIN
t
OFF MIN
OUT2
_ differences. Low
VR
VR
@
= 5V,
·
¸
¹
·
¸
¹
(EQ. 10)
(EQ. 9)
(EQ. 7)
(EQ. 8)
OUT2
ISL6237
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:
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
R
R
I
I
4.17A 4.12A
DS(ON)
LIMIT LOW
LIMIT LOW
ESR
SER
d
!
d
DS(ON)
-----------------------------------------------
L
LIMIT(LOW)
--------------------------------- -
I
DIP
P-P
= 5m: at room temperature. At +125°C reve als the
LOAD MAX
IR
˜
V
is the maximum-tolerable transient voltage drop.
I
is the peak-to-peak output voltage ripple. The
!
=
DIP
LOAD MAX
V
I
P P
LOAD MAX
with temperature. A good general rule is to
25mV
–
LOAD(MAX)
= minimum current-limit threshold
e
DS(ON)
5m:
–
>
LIR 2 e
minus half of the ripple
u
of Q2/Q4.
1.2
I
˜
!
LOAD MAX
5A
DS(ON)
–
0.35 2 e
from the
@
March 16, 2007
5A
(EQ. 12)
(EQ. 13)
(EQ. 14)
(EQ. 15)
(EQ. 11)
FN6418.1