7

FN6454.1

November 2, 2007

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 2.0A, the current limit of the

ISL97519. 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 upon the input and

output voltages, the maximum output current, the inductor

value and the noise allowed to put back on 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 ISL97519 is powered from the VIN. A High frequency

0.1µF bypass cap is recommended to be close to the VIN

pin to reduce supply line noise and ensure stable operation.

Loop Compensation

The ISL97519 incorporates a transconductance amplifier in

its feedback path to allow the user some adjustment on the

transient response and better regulation. The ISL97519

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 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 2k to 7.5k and the compensation capacitor in

the range of 3nF to 10nF.

Soft-Start

The soft-start is provided by an internal 6µA current source

is limited by the voltage on the capacitor. This in turn controls

the rising rate of the output voltage. The regulator goes

through the start-up sequence as well, after the EN pin is

pulled to HI.

Frequency Selection

The ISL97519 switching frequency can be user selected to

operate at either constant 620kHz or 1.25MHz. Connecting

FSEL pin to ground sets the PWM switching frequency to

620kHz. When connecting FSEL high or VDD, the switching

frequency is set to 1.25MHz.

Shut-down Control

When the EN pin is pulled down, the ISL97519 is shut down

reducing the supply current to <1µA.

Maximum Output Current

The MOSFET current limit is nominally 2.0A and guaranteed

based on Equation 7:

where:

ΔI

D = MOSFET turn-on ratio:

switching frequency and 10µH inductor.

Cascaded MOSFET Application

An 25V N-Channel MOSFET is integrated in the boost

regulator. For the applications where the output voltage is

greater than 25V, an external cascaded MOSFET is needed

as shown in Figure 12. The voltage rating of the external

TABLE 1.

2.5

5

870

2.5

9

500

2.5

12

380

3.3

5

1150

3.3

9

655

3.3

12

500

5

9

990

5

12

750

I

L

I

L-AVG

12

⁄ΔI

L

×

()

+

=

(EQ. 7)

ΔI

L

V

IN

V

O

V

DIODE

+

() V

IN

–

[]

×

LV

O

(

V

f

S

×

+

×

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

=

(EQ. 8)

I

L-AVG

I

OUT

1D

–

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

=

D1

V

IN

V

OUT

V

DIODE

+

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

–

=

(EQ. 9)

ISL97519