9

FN7333.3

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

Schottky diode is forward biased, Figure 30. 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

during the second cycle is:

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.

FIGURE 28. BOOST CONVERTER

FIGURE 29. BOOST CONVERTER - CYCLE 1, POWER

SWITCH CLOSED

FIGURE 30. BOOST CONVERTER - CYCLE 2, POWER

SWITCH OPEN

Output Voltage

An external feedback resistor divider is required to divide the

output voltage down to the nominal 1.294V 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:

The nominal VFB voltage is 1.294V.

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:

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

as:

∆I

L

∆T2

–

L

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

×

=

∆T2

1D

–

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

=

∆I1 ∆I2

+

0

=

D

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

L

--------- 1D

–

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

–

L

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

×

+

×

0

=

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

1

1D

–

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

=

EL7516

LD

EL7516

L

EL7516

LD

1

-------

+

⎝⎠

⎜⎟

⎛⎞

×

=

()

×

------------------------------------ 12

⁄

–

()

×

×

×

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

×

+

=

∆V

O

×

×

×

+

=

EL7516