1.5A, 280kHz, Boost Regulator

LM5171

Dec. 2010 - Rev. 1.2.1

- 13

-

HTC

Magnetic Component Selection

When choosing a magnetic component, one must consider factors such as peak current, core and ferrite

material, output voltage ripple, EMI, temperature range, physical size and cost. In boost circuits, the average

efficiency. In continuous conduction mode, inductor ripple current is

)

(f)(L)(V

)

V

-

(V

V

=

I

OUT

CC

OUT

cc

RIPPLE

where:

f = 280 kHz

The peak inductor current is equal to average current plus half of the ripple current, which should not cause

inductor saturation. The above equation can also be referenced when selecting the value of the inductor based

on the tolerance of the ripple current in the circuits. Small ripple current provides the benefits of small input

capacitors and greater output current capability. A core geometry like a rod or barrel is prone to generating

high magnetic field radiation, but is relatively cheap and small. Other core geometries, such as toroids, provide

a closed magnetic loop to prevent EMI.

Input Capacitor Selection

In boost circuits, the inductor becomes part of the input filter, as shown in Figure 26. In continuous mode, the

input current waveform is triangular and does not contain a large pulsed current, as shown in Figure 25. This

reduces the requirements imposed on the input capacitor selection. During continuous conduction mode, the

peak to peak inductor ripple current is given in the previous section. As we can see from Figure 25, the product

In most applications, input capacitors in the range of 10µF to 100 µF with an ESR less than 0.3Ω work well up to

a full 1.5A switch current.

+

-

Figure 25. Boost Input Voltage and Current Ripple

Figure 26. Boost Circuit Effective Input Filter

Waveforms

The situation is different in a flyback circuit. The input current is discontinuous and a significant pulsed

current is seen by the input capacitors. Therefore, there are two requirements for capacitors in a flyback

regulator: energy storage and filtering. To maintain a stable voltage supply to the chip, a storage capacitor

larger than 20 µF with low ESR is required. To reduce the noise generated by the inductor, insert a 1.0µF

By examining the waveforms shown in Figure 27, we can see that the output voltage ripple comes from two