ISL8105, ISL8105A

FN6306 Rev 5.00

Page 13 of 16

April 15, 2010

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:

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

capacitors physically close to the MOSFETs and between the

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..

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

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

I =

--------------------------------

----------------



(EQ. 11)



-------------------------------

=



–

--------------------------------

=

(EQ. 12)





=



–



I

12

--------D

+

=

OR

D

VIN

----------

=

(EQ. 13)

FIGURE 11. INPUT-CAPACITOR CURRENT MULTIPLIER FOR

SINGLE-PHASE BUCK CONVERTER

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

DUTY CYCLE (D)

0.5Io

I = 0Io

0.25Io