be connected very close to pin 6 to effectively filter high

frequency noise. When operating at 1.25 MHz switching

frequency, a minimum bypass capacitance of 0.22 µF is

recommended.

Output Capacitor

The output capacitor in a boost converter provides all the

output current when the switch is closed and the inductor is

charging. As a result, it sees very large ripple currents. The

output capacitor should be capable of handling the maxi-

mum RMS current. The RMS current in the output capacitor

is:

where,

and

D=(V

The ESR and ESL of the output capacitor directly control the

output ripple. Use capacitors with low ESR and ESL at the

output for high efficiency and low ripple voltage. Surface

mount tantalums, surface mount polymer electrolytic, and

polymer tantalum, Sanyo OS-CON, or multi-layer ceramic

capacitors are recommended at the output.

Compensation

This section presents a step-by-step procedure to design the

compensation network at pin 1 (V

design methods will produce a conservative and stable con-

trol loop.

There is a minimum inductance requirement in any current

mode converter. This is a function of V

switching frequency, among other things. The graphs below

plot the recommended inductance range vs. duty cycle for

V

bounds of the recommended inductance range. The simpli-

fied compensation procedure that follows assumes that the

inductance never drops below the Q = 5 line.

Figure 4 plots

the equation:

(1)

where,

R

and Q = 0.5 and 5

Use Q = 5 to calculate the minimum inductance recom-

mended for a stable design. Choosing an inductor between

the Q = 0.5 and Q = 5 values provides a good tradeoff

between size and stability. Note that as V

5V, R

Characteristics section (R

case R

tance. To view plots for different Vout, multiply the Y axis by

a factor of V

output voltage.

20012654

20012653

FIGURE 4. Minimum Inductance Requirements for (a) f

www.national.com

10