5

FN6439.4

April 2, 2010

Applications Information

The ISL97656 is a high frequency, high efficiency boost

regulator operated at constant frequency PWM mode. The

boost converter stores energy from an input voltage source

and deliver it to a higher output voltage. The input voltage

range is 2.3V to 6.0V and output voltage range is 5V to 25V.

The switching frequency is selectable between 640kHz and

1.2MHz allowing smaller inductors and faster transient

response. An external compensation pin gives the user

greater flexibility in setting output transient response and

tighter load regulation. The converter soft-start characteristic

allows the user to completely shutdown the device.

Boost Converter Operations

Figure 8 shows a boost converter with all the key

components. In steady state operating and continuous

conduction mode where the inductor current is continuous,

the boost converter operates in two cycles. During the first

cycle, as shown in Figure 9, the internal power FET turns on

and the Schottky diode is reverse biased and cuts off the

current flow to the output. The output current is supplied

from the output capacitor. The voltage across the inductor is

the inductance. The inductance is magnetized and energy is

stored in the inductor. The change in inductor current is

shown in Equation 1:

During the second cycle, the power FET turns off and the

Schottky diode is forward biased, (see Figure 10). The

energy stored in the inductor is pumped to the output

supplying output current and charging the output capacitor.

The Schottky diode side of the inductor is clamp to a Schottky

diode above the output voltage. So the voltage drop across the

the second cycle is shown in Equation 2:

For stable operation, the same amount of energy stored in

the inductor must be taken out. The change in inductor

current during the two cycles must be the same as shown in

Equation 3.

FIGURE 7. TRANSIENT RESPONSE

ΔI

L1

ΔT1

L

---------

×

=

ΔT1

D

----------

=

D

Duty Cycle

=

ΔV

O

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

ΔT

1

×

=

(EQ. 1)

ΔI

L

ΔT2

–

L

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

×

=

ΔT2

1D

–

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

=

(EQ. 2)

ΔI1 ΔI2

+

0

=

D

----------

L

---------

1D

–

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

–

L

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

×

+

×

0

=

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

1

1D

–

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

=

(EQ. 3)

ISL97656

LD

FIGURE 8. BOOST CONVERTER

ISL97656

L

FIGURE 9. BOOST CONVERTER - CYCLE 1, POWER

SWITCH CLOSED

ISL97656