ISL6227CAZ-T Intersil, ISL6227CAZ-T Datasheet - Page 22

• Manufacturer Part Number: ISL6227CAZ-T
• Description: IC CONTROLLER DDR, DDR2 28QSOP
• Manufacturer: Intersil

Specifications of ISL6227CAZ-T

• Package / Case: 28-QSOP
• Mounting Type: Surface Mount
• Voltage - Output: 0.9 ~ 5.5 V
• Voltage - Input: 5 ~ 28 V
• Operating Temperature: -10°C ~ 100°C
• Applications: Controller, DDR, DDR2
• Number Of Outputs: 2
• Peak Reflow Compatible (260 C): Yes
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Based on the above description and functional block
diagram, the OC set resistor can be calculated as
Equation 18:
I
Since inductor peak current changes with input voltage, it is
better to use an oscilloscope when testing the overcurrent
setting point to monitor the inductor current, and to
determine when the OC occurs. To get consistent test results
on different boards, it is best to keep the MOSFET at a fixed
temperature.
The MOSFET will not heat-up when applying a very low
frequency and short load pulses with an electronic load to
the output.
As an example, assume the following:
• The maximum normal operation load current is 1
• The inductor peak current is 1.15x to 1.3x higher than the
• The r
I
maximum load current to avoid nuisance overcurrent trip.
Selection of the LC Filter
The duty cycle of a buck converter is a function of the input
voltage and output voltage. Once an output voltage is fixed,
it can be written as Equation 19:
The switching frequency, F
peak-to-peak ripple current going through the inductor can
be written as Equation 20:
As higher ripple current will result in higher switching loss
and higher output voltage ripple, the peak-to-peak current of
the inductor is generally designed with a 20% to 40%
peak-to-peak ripple of the nominal operation current. Based
on this assumption, the inductor value can be selected with
Equation 20. In addition to the mechanical dimension, a
shielded ferrite core inductor with a very low DC resistance,
DCR, is preferred for less core loss and copper loss. The DC
copper loss of the inductor can be estimated by Equation 21:
R
D V
I
P
OC
OC
pp
copper
set
(
load current, depending on the inductor value and the
input voltage
IN
=
is the inductor peak current and not the load current.
should set at least 1.8 to 2 times higher than the
=
)
V
---------------------------------------- -
DS(ON)
---------------------------------------------------
I
-------------------------------- -
o
=
OC
R
(
=
1 D V
CS
---------
V
F
V
I
r
–
IN
sw
load
DS ON
o
+
10.3V
L
(
(
140
has a 45% increase at higher temperature
o
2
DCR
IN
)
)
+
)
8μA
sw
22
, of ISL6227 is 300kHz. The
(EQ. 18)
(EQ. 19)
(EQ. 20)
(EQ. 21)
ISL6227
The inductor copper loss can be significant in the total
system power loss. Attention has to be given to the DCR
selection. Another factor to consider when choosing the
inductor is its saturation characteristics at elevated
temperature. Saturated inductors could result in nuisance
OC, or OV trip.
Output voltage ripple and the transient voltage deviation are
factors that have to be taken into consideration when
selecting an output capacitor. In addition to high frequency
noise related MOSFET turn-on and turn-off, the output
voltage ripple includes the capacitance voltage drop and
ESR voltage drop caused by the AC peak-to-peak current.
These two voltages can be represented by Equations 22
and 23:
These two components constitute a large portion of the total
output voltage ripple. Several capacitors have to be
paralleled in order to reduce the ESR and the voltage ripple.
If the output of the converter has to support another load
with high pulsating current, more capacitors are needed in
order to reduce the equivalent ESR and suppress the
voltage ripple to a tolerable level.
To support a load transient that is faster than the switching
frequency, more capacitors have to be used to reduce the
voltage excursion during load step change. Another aspect
of the capacitor selection is that the total AC current going
through the capacitors has to be less than the rated RMS
current specified on the capacitors, to prevent the capacitor
from over-heating.
Selection of the Input Capacitor
When the upper MOSFET is on, the current in the output
inductor will be seen by the input capacitor. Even though this
current has a triangular shape top, its RMS value can be
fairly approximated by Equation 24:
This RMS current includes both DC and AC components.
Since the DC component is the product of duty cycle and
load current, the AC component can be approximated by
Equation 25:
AC components will be provided from the input capacitor.
The input capacitor has to be able to handle this ripple
current without overheating and with tolerable voltage ripple.
In addition to the capacitance, a ceramic capacitor is
generally used between the drain terminal of the upper
lin
li
ΔV
ΔV
nac
rms
c
esr
(
V
=
(
V
IN
=
--------------------- -
8C
IN
)
I
I
pp
)
o
=
pp
F
=
ESR
sw
(
D V
D V
(
(
IN
IN
) D V
)*I
–
load
(
IN
)
2
)I
load
August 7, 2007
(EQ. 24)
(EQ. 25)
(EQ. 22)
(EQ. 23)
FN9094.5