CS5172 datasheet(13/22 Pages) ONSEMI

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CS5171, CS5172, CS5173, CS5174

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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

IRIPPLE +

VCC(VOUT * VCC)

(f)(L)(VOUT)

where:

f = 280 kHz for CS5171/2 and 560 kHz for CS5173/4.

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 35. In continuous mode, the input

current waveform is triangular and does not contain a large

pulsed current, as shown in Figure 34. 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 34, 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 10

mF to 100 mF

with an ESR less than 0.3

W work well up to a full 1.5 A

switch current.

VCC ripple

Figure 34. Boost Input Voltage and Current

Ripple Waveforms

IIN

−

Figure 35. Boost Circuit Effective Input Filter

VCC

CIN

RESR

IIN

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

mF with low ESR

is required. To reduce the noise generated by the inductor,

insert a 1.0

mF ceramic capacitor between VCC and ground

as close as possible to the chip.

Output Capacitor Selection

Figure 36. Typical Output Voltage Ripple

VOUT ripple

By examining the waveforms shown in Figure 36, we can

see that the output voltage ripple comes from two major

sources,

namely

capacitor

ESR

and

the

charging/discharging of the output capacitor. In boost

circuits, when the power switch turns off, IL flows into the

output capacitor causing an instant

DV = IIN × ESR. At the

same time, current IL − IOUT charges the capacitor and

increases the output voltage gradually. When the power

switch is turned on, IL is shunted to ground and IOUT

discharges the output capacitor. When the IL ripple is small

enough, IL can be treated as a constant and is equal to input

current IIN.

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