11

FN9228.0

April 11, 2006

Inductor

The boost inductor is a critical part which influences the

output voltage ripple, transient response, and efficiency.

Values of 3.3µH to 10µH are used to match the internal

slope compensation. The inductor must be able to handle

the following average and peak current:

Some inductors are recommended in Table 3.

Rectifier Diode

A high-speed diode is necessary due to the high switching

frequency. Schottky diodes are recommended because of

their fast recovery time and low forward voltage. The reverse

voltage rating of this diode should be higher than the

maximum output voltage. The rectifier diode must meet the

output current and peak inductor current requirements. The

following table is some recommendations for boost converter

diode.

Output Capacitor

The output capacitor supplies the load directly and reduces

the ripple voltage at the output. Output ripple voltage

consists of two components: the voltage drop due to the

inductor ripple current flowing through the ESR of output

capacitor, and the charging and discharging of the output

capacitor.

For low ESR ceramic capacitors, the output ripple is

dominated by the charging and discharging of the output

capacitor. The voltage rating of the output capacitor should

be greater than the maximum output voltage.

Note: Capacitors have a voltage coefficient that makes their

effective capacitance drop as the voltage across then

effective value of the capacitor at a particular voltage and not

the manufacturer’s stated value, measured at zero volts.

The following table shows some selections of output

capacitors.

Compensation

The boost converter of ISL97644 can be compensated by a

RC network connected from CM1 pin to ground. 4.7nF and

10k RC network is used in the demo board. The larger value

resistor and lower value capacitor can lower the transient

overshoot, however, at the expense of stability of the loop.

Cascaded MOSFET Application

An 20V N-channel MOSFET is integrated in the boost

regulator. For the applications where the output voltage is

greater than 20V, an external cascaded MOSFET is needed

as shown in Figure 15. The voltage rating of the external

Linear-Regulator (LDO)

The ISL97644 includes a LDO with adjustable output, and it

can supply current up to 350mA. The output voltage is

adjusted by connection of ADJ pin. When ADJ pin is

connected to ground, the output voltage is set to 3.3V; when

ADJ pin is floating, the output voltage is set to 2.85V, and

when ADJ pin is connected to LDO_OUT pin, the output

voltage is set to 2.5V.

TABLE 3. BOOST INDUCTOR RECOMMENDATION

INDUCTOR

DIMENSIONS

(mm)

MFG

PART NUMBER

7.3x6.8x3.2

TDK

RLF7030T-6R8N3R0

8.3X8.3X4.5

Sumida

CDR8D43-100NC

Cooper

Bussmann

CD1-5R2

TABLE 4. BOOST CONVERTER RECTIFIER DIODE

RECOMMENDATION

DIODE

PACKAGE

MFG

SS23

30V/2A

SMB

Fairchild

Semiconductor

MBRS340

40V/3A

SMC

International

Rectifier

SL23

30V/2A

SMB

Vishay

Semiconductor

1D

–

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

=

∆I

L

2

--------

+

=

–

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

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

1

----

×

×

+

×

=

TABLE 5. BOOST OUTPUT CAPACITOR RECOMMENDATION

CAPACITOR

SIZE

MFG

PART NUMBER

10µF/25V

1210

TDK

C3225X7R1E106M

10µF/25V

1210

Murata

GRM32DR61E106K

FIGURE 15. CASCADED MOSFET TOPOLOGY FOR HIGH

OUTPUT VOLTAGE APPLICATIONS

Intersil

ISL97644

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ISL97644