ISL97656

FN6439 Rev 6.00

Page 7 of 9

July 19, 2012

Inductor Selection

The inductor selection determines the output ripple voltage,

transient response, output current capability and efficiency. Its

selection depends on the input voltage, output voltage, switching

frequency and maximum output current. For most applications,

the inductance should be in the range of 2µH to 33µH. The

inductor maximum DC current specification must be greater than

the peak inductor current required by the regulator. The peak

inductor current can be calculated using Equation 5:

Output Capacitor

Low ESR capacitors should be used to minimize the output

voltage ripple. Multilayer ceramic capacitors (X5R and X7R) are

preferred for the output capacitors because of their lower ESR

and small packages. Tantalum capacitors with higher ESR can

also be used. The output ripple can be calculated using

Equation 6:

For noise sensitive applications, a 0.1µF placed in parallel with

the larger output capacitor is recommended to reduce the

switching noise coupled from the LX switching node.

Schottky Diode

In selecting the Schottky diode, the reverse break-down voltage,

forward current and forward voltage drop must be considered for

optimum converter performance. The diode must be rated to

handle 4.0A, the current limit of the ISL97656. The breakdown

voltage must exceed the maximum output voltage. Low forward

voltage drop, low leakage current, and fast reverse recovery will

help the converter to achieve the maximum efficiency.

Input Capacitor

The value of the input capacitor depends on the input and the

output voltages, maximum output current, inductor value and

maximum permissible noise fed back in the input line. For most

applications, a minimum 10µF is required. For applications that

run close to the maximum output current limit, an input

capacitor in the range of 22µF to 47µF is recommended.

The ISL97656 is powered from the VIN. A High frequency 0.1µF

bypass capacitor is recommended to be close to the VIN pin to

reduce supply line noise and ensure stable operation.

Loop Compensation

The ISL97656 incorporates a transconductance amplifier in its

feedback path to allow the user some adjustment on the

transient response and better regulation. The ISL97656 uses

current mode control architecture, which has a fast current sense

loop and a slow voltage feedback loop. The fast current feedback

loop does not require any compensation. The slow voltage loop

must be compensated for stable operation. The compensation

network is a series RC network from the COMP pin to ground. The

resistor sets the high frequency integrator gain for fast transient

response and the capacitor sets the integrator zero to ensure

loop stability. For most applications, the compensation resistor in

the range of 0k to 2.0k and the compensation capacitor in the

range of 3nF to 10nF.

Soft-Start

The regulator goes through the soft-start sequence after EN is

pulled high. The soft-start is provided by an internal 4.5µA

current source. This internal current source is used to charge the

increases, this results in ramping up of the current limit from 0A

to full scale. This in turn controls the rising rate of the output

voltage.

Frequency Selection

The ISL97656 switching frequency can be user selectable. The

ISL97656 operates at either constant 640KHz or 1.22MHz

switching frequency. Connecting the FREQ pin to ground sets the

PWM switching frequency to 640kHz. When connecting FREQ

Shutdown Control

When the EN pin is pulled low, the ISL97656 is in shutdown

mode, reducing the supply current to <1µA.

Maximum Output Current

The MOSFET current limit is nominally 4.0A and guaranteed

on Equation 7:

where:

D = MOSFET turn-on ratio:





------------------------------------ 12



–









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



+

=

(EQ. 5)

V

O







+

=

(EQ. 6)



12

I

L





+

=

(EQ. 7)

I

L

+

 V

IN

–









+



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

=

(EQ. 8)

1D

–

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

=

(EQ. 9)

D1

+

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

–

=

(EQ. 10)