ISL8105BEVAL1Z Intersil, ISL8105BEVAL1Z Datasheet - Page 2

• Manufacturer Part Number: ISL8105BEVAL1Z
• Description: EVAL BOARD ISL8105B
• Manufacturer: Intersil

Specifications of ISL8105BEVAL1Z

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.8V
• Current - Output: 15A
• Voltage - Input: 9.6 ~ 14.4V
• Regulator Topology: Buck
• Frequency - Switching: 300kHz
• Board Type: Fully Populated
• Utilized Ic / Part: ISL8105B
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

The total power loss in MOSFET consists of conduction loss
and switching loss, as shown in Equation 5:
In this relatively small duty cycle design, the low-side
MOSFET conducts current most of the time. To optimize the
converter efficiency, select the high-side MOSFET with low
gate charge for fast switching transition and low-side
MOSFET with low r
To achieve the target efficiency, the budget power losses in
high-side and low-side MOSFETs are 0.5W and 1W,
respectively.
LOW-SIDE MOSFET SELECTION
The low-side MOSFET’s RMS current is approximated in
Equation 6:
Therefore, the ON-resistance of the low-side MOSFET must
be less than 5mΩ. Infineon’s BSC030N03LS is employed in
the ISL8105BEVAL1Z, ISL8105BEVAL2Z evaluation board.
The conduction loss in the low-side MOSFET is calculated
using Equation 7:
The switching loss in the low-side MOSFET is dominated by
the loss in body diode which can be calculated using
Equation 8:
Where t
(~60µs) and V
body diode.
The total power dissipation in the low-side MOSFET is
calculated using Equation 9:
HIGH-SIDE MOSFET SELECTION
For the high-side MOSFET selection, first we assume that
the conduction loss and the switching loss contribute evenly
to the total power dissipation.
The high-side MOSFET’s RMS current is approximated
using Equation 10:
P
P
P
I
I
P
H rms
L RMS
MOSFET TOT
LFET cond
diode
LFET TOT
(
(
)
(
(
)
=
D
=
=
I
(
is the total dead time in each switching period
I
O
OUT
I
)
OUT
)
⋅
=
=
t
D
F
)
0.88W
⋅
I
⋅
2
L RMS
=
⋅
is the forward voltage drop of MOSFET’s
V
(
D
F
P
1 D
DS(ON)
cond
⋅
⋅
–
F
SW
1
)
⋅
⋅
+
r
+
DS ON
----- -
12
=
1
P
1
.
sw
+
(
0.3W
⋅
⎛
⎜
⎝
----- -
12
2
1
------------ -
I
OUT
ΔI
)
⋅
LFET
L
⎛
⎜
⎝
-------------
I
OUT
⎞
⎟
⎠
ΔI
2
L
≈
=
⎞
⎟
⎠
5.85A
2
0.58W
≈
13.9A
Application Note 1288
(EQ. 10)
(EQ. 7)
(EQ. 8)
(EQ. 9)
(EQ. 5)
(EQ. 6)
Hence, the required ON-resistance of the high-side MOSFET is
7.3mΩ. Infineon’s BSC080N03LS is selected. The conduction
loss in the high-side MOSFET is calculated using Equation 11:
The switching loss in the high-side MOSFET can be
approximated using Equation 12:
where t
times.
The total power dissipation in high-side MOSFET is shown in
Equation 13:
Overcurrent Protection Setting
The overcurrent function protects the converter from a shorted
output by using the low-side MOSFET’s r
the current. A resistor, R
level. If overcurrent is detected, the output immediately shuts
off, it cycles the soft-start function in a hiccup mode (2 dummy
soft-start time-outs, then up to one real one) to provide fault
protection. If the shorted condition is not removed, this cycle
will continue indefinitely.
The overcurrent function will trip at a inductor current (I
determined using Equation 14:
where I
source.
The OC trip point varies mainly due to the MOSFET’s r
variations. To avoid overcurrent tripping in the normal operating
load range, calculate the R
using:
Determine I
output inductor ripple current.
I
P
P
P
trip
1. The maximum r
2. The minimum I
HFET cond
HFET SW
HFET TOT
datasheet.
=
2 I •
-------------------------------------------------------- -
(
(
(
tr
OCSET
is the combined ON and OFF MOSFET transition
OCSET
)
)
trip
)
=
r
=
DS ON
=
1
-- - I
2
is the internal 21.5µA (typ.) OCSET current
for I
0.44W
I
(
2
H RMS
⋅
•
(
O
OCSET
R
DS(ON)
trip
)
⋅
BSOC
V
IN
> I
)
=
⋅
BSOC
⋅
r
OUT(MAX)
DS ON
0.17W
BSOC
t
tr
from the specification table of the
at the highest junction temperature.
⋅
(
F
, programs the overcurrent trip
SW
resistor from Equation 14
)
HFET
+
1
-- - C
2
+ (ΔI)/2, where ΔI is the
⋅
=
OSS
DS(ON)
0.27W
⋅
V
2
IN
to monitor
October 30, 2008
⋅
F
SW
(EQ. 12)
(EQ. 13)
(EQ. 11)
(EQ. 14)
DS(ON)
AN1288.1
trip
) is