6

FN6439.4

April 2, 2010

Output Voltage

An external feedback resistor divider is required to divide the

output voltage down to the nominal 1.24V reference voltage.

The current drawn by the resistor network should be limited

to maintain the overall converter efficiency. The maximum

value of the resistor network is limited by the feedback input

bias current and the potential for noise being coupled into

the feedback pin. A resistor network less than 100k is

recommended. The boost converter output voltage is

determined by the relationship as shown in Equation 4. The

nominal VFB voltage is 1.24V..

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 minimized 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 application, 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 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, 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 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 soft-start is provided by an internal 4.5µA current source

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 ISL97656 switching frequency can be user selected to

operate at either constant 640kHz or 1.25MHz. Connecting

FREQ pin to ground sets the PWM switching frequency to

640kHz. When connecting FREQ high or IN, the switching

frequency is set to 1.2MHz.

Shutdown Control

When the EN pin is pulled down, the ISL97656 is shutdown

reducing the supply current to <1µA.

ISL97656

LD

FIGURE 10. BOOST CONVERTER - CYCLE 2, POWER

SWITCH OPEN

1

-------

+

⎝⎠

⎜⎟

⎛⎞

×

=

(EQ. 4)

()

×

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

12

⁄

–

()

×

×

×

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

×

+

=

(EQ. 5)

ΔV

O

×

×

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

×

+

=

(EQ. 6)

ISL97656