1.5A, 280kHz, Boost Regulator
LM5171
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
inductor current is the product of output current and voltage gain (VOUT/VCC), assuming 100% energy transfer
efficiency. In continuous conduction mode, inductor ripple current is
Vcc(VOUT -VCC
)
IRIPPLE
=
(f)(L)(VOUT
)
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
of the inductor current ripple and the input capacitor’s effective series resistance (ESR) determine the VCC ripple.
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.
IIN
IL
CIN
+
-
VIN
RESR
Figure 25. Boost Input Voltage and Current Ripple
Waveforms
Figure 26. Boost Circuit Effective Input Filter
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
ceramic capacitor between VCC and ground as close as possible to the chip.
By examining the waveforms shown in Figure 27, we can see that the output voltage ripple comes from two
Dec. 2010 - Rev. 1.2.1
- 13 -
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
