ISL97656

FN6439 Rev 6.00

Page 6 of 9

July 19, 2012

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 delivers higher

output voltage. The input voltage range is 2.2V to 6.0V and the

output voltage range is 5V to 25V. The switching frequency can be

selected between 640kHz and 1.22MHz. The higher switching

frequency allows use of 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 can be

user to shut down the device.

Boost Converter Operations

Figure 13 shows a boost converter with all the key components.

In steady state and continuous conduction mode, the boost

converter operates in two cycles. During the first cycle, as shown

in Figure 14, 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.

inductor 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 15). The energy

stored in the inductor is supplied to the output. This energy is

used to charge the output capacitor and supply output current. In

diode drop across Schottky diode). The change in inductor

current during the second cycle is shown in Equation 2:

In steady state operation, the change in the inductor current

must be equal as shown in Equation 3.

.

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.

I

L1

T1

L

---------



=

T1

D

----------

=

D

Duty Cycle

=

V

O

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

T

1



=

(EQ. 1)

I

L

T2

–

L

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



=

T2

1D

–

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

=

(EQ. 2)

I1 I2

+

0

=

D

----------

L

---------

1D

–

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

–

L

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



+



0

=

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

1

1D

–

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

=

(EQ. 3)

ISL97656

LD

FIGURE 13. BOOST CONVERTER

ISL97656

L

FIGURE 14. BOOST CONVERTER - CYCLE 1, POWER SWITCH CLOSED

ISL97656

LD

FIGURE 15. BOOST CONVERTER - CYCLE 2, POWER SWITCH OPEN

1

-------

+









=

(EQ. 4)