ISL8105BEVAL1Z Intersil, ISL8105BEVAL1Z Datasheet - Page 13

• 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: -

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
ISL8105B 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 12
gives the approximate response time interval for application
and removal of a transient load:
where:
I
t
t
With a lower input source such as 1.8V or 3.3V, the worst
case response time can be either at the application or
removal of load and dependent upon the output voltage
setting. Be sure to check both of these equations at the
minimum and maximum output levels for the worst case
response time.
Input Capacitor Selection
Use a mix of input bypass capacitors to control the voltage
overshoot across the MOSFETs. Use small ceramic
capacitors for high frequency decoupling and bulk capacitors
to supply the current needed each time Q
small ceramic capacitors physically close to the MOSFETs
and between the drain of Q
The important parameters for the bulk input capacitor are the
voltage rating and the RMS current rating. For reliable
operation, select the bulk capacitor with voltage and current
ratings above the maximum input voltage and largest RMS
current required by the circuit. The capacitor voltage rating
should be at least 1.25 times greater than the maximum
input voltage and a voltage rating of 1.5 times is a
conservative guideline. The RMS current rating requirement
for the input capacitor of a buck regulator is approximately
as shown in Equation 13.
t
I
TRAN
RISE
FALL
RISE
IN RMS
I
IN RMS
,
,
is the response time to the application of load
is the response time to the removal of load
=
is the transient load current step
OR
------------------------------- -
V
L
=
=
O
IN
×
K
–
I
I
O
ICM
TRAN
V
2
OUT
(
D D
•
–
I
O
2
)
+
t
I Δ
------- - D
FALL
12
1
2
and the source of Q
13
=
L
------------------------------ -
O
V
×
OUT
I
TRAN
1
turns on. Place the
D
=
----------
VIN
V
2
O
.
(EQ. 12)
(EQ. 13)
ISL8105B
For a through-hole design, several electrolytic capacitors
(Panasonic HFQ series or Nichicon PL series or Sanyo
MV-GX or equivalent) may be needed. For surface mount
designs, solid tantalum capacitors can be used, but caution
must be exercised with regard to the capacitor surge current
rating. These capacitors must be capable of handling the
surge-current at power-up. The TPS series, available from
AVX, and the 593D, available series from Sprague, are both
surge current tested.
MOSFET Selection/Considerations
The ISL8105B requires two N-Channel power MOSFETs.
These should be selected based upon r
requirements, and thermal management requirements.
In high-current applications, the MOSFET power dissipation,
package selection and heatsink are the dominant design
factors. The power dissipation includes two loss
components: conduction loss and switching loss. The
conduction losses are the largest component of power
dissipation for both the top and the bottom-side MOSFETs.
These losses are distributed between the two MOSFETs
according to duty factor. The switching losses seen when
sourcing current will be different from the switching losses
seen when sinking current. When sourcing current, the
top-side MOSFET realizes most of the switching losses. The
bottom-side switch realizes most of the switching losses
when the converter is sinking current (see Equation 14).
These equations assume linear voltage current transitions
and do not adequately model power loss due to the reverse
recovery of the upper and lower MOSFET’s body diode. The
gate-charge losses are dissipated by the ISL8105B and do
not heat the MOSFETs. However, large gate charge
increases the switching interval, t
MOSFET switching losses. Ensure that both MOSFETs are
within their maximum junction temperature at high ambient
temperature by calculating the temperature rise according to
package thermal-resistance specifications. A separate
FIGURE 11. INPUT-CAPACITOR CURRENT MULTIPLIER FOR
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
0.1
SINGLE-PHASE BUCK CONVERTER
0.2
0.3
DUTY CYCLE (D)
0.4
0.5
SW
, which increases the
0.6
DS(ON)
0.7
0.25Io
, gate supply
0.8
Δ I = 0Io
April 15, 2010
0.9
0.5Io
FN6447.2
1.0