ISL8105AIBZ-T Intersil, ISL8105AIBZ-T Datasheet - Page 13

• Manufacturer Part Number: ISL8105AIBZ-T
• Description: IC PWM CTRLR BUCK 1PHASE 8-SOIC
• Manufacturer: Intersil

Specifications of ISL8105AIBZ-T

• Pwm Type: Voltage Mode
• Number Of Outputs: 1
• Frequency - Max: 660kHz
• Duty Cycle: 100%
• Voltage - Supply: 6.5 V ~ 14.4 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 8-SOIC (3.9mm Width)
• Frequency-max: 660kHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

However, the equivalent series inductance (ESL) of these
capacitors increases with case size and can reduce the
usefulness of the capacitor to high slew-rate transient loading.
Unfortunately, ESL is not a specified parameter. Work with
your capacitor supplier and measure the capacitor’s
impedance with frequency to select a suitable component. In
most cases, multiple electrolytic capacitors of small case size
perform better than a single large case capacitor.
Output Inductor Selection
The output inductor is selected to meet the output voltage
ripple requirements and minimize the converter’s response
time to the load transient. The inductor value determines the
converter’s ripple current and the ripple voltage is a function
of the ripple current. The ripple voltage and current are
approximated by Equation 11:
Increasing the value of inductance reduces the ripple current
and voltage. However, the large inductance values reduce
the converter’s response time to a load transient.
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
ISL8105 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
t
ΔI =
TRAN
RISE
FALL
RISE
V
------------------------------- -
is the response time to the application of load
is the response time to the removal of load
=
IN
is the transient load current step
F
------------------------------- -
V
L
S
- V
O
IN
x L
×
OUT
–
I
TRAN
V
OUT
•
V
--------------- -
V
OUT
IN
t
FALL
13
ΔV
=
OUT
L
------------------------------ -
O
= ΔI x ESR
V
×
OUT
I
TRAN
ISL8105, ISL8105A
(EQ. 12)
(EQ. 11)
I
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 1.25x greater than the maximum input
voltage and a voltage rating of 1.5x is a conservative
guideline. The RMS current rating requirement for the input
capacitor of a buck regulator is approximately as shown in
Equation 13..
I
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 ISL8105 requires 2 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
IN RMS
IN RMS
FIGURE 11. INPUT-CAPACITOR CURRENT MULTIPLIER FOR
,
,
0.60
0.50
0.40
0.30
0.20
0.10
0.00
OR
0
=
=
K
0.1
I
ICM
O
2
SINGLE-PHASE BUCK CONVERTER
(
D D
•
0.2
–
I
O
2
)
0.3
+
I Δ
------- - D
12
DUTY CYCLE (D)
2
1
0.4
and the source of Q
0.5
DS(ON)
0.6
, gate supply
1
D
0.7
turns on. Place the
0.25Io
=
----------
VIN
V
0.8
O
Δ I = 0Io
2
.
April 15, 2010
0.9
(EQ. 13)
0.5Io
FN6306.5
1.0