CS5127 datasheet(8/24 Pages) CHERRY

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Applications Information: continued

microseconds to return to a steady-state. V2ª control uses

the ripple voltage from the output capacitor and a ÒfastÓ

control loop to respond to load transients, with the result

that the transient response of the CS5127 is very close to

the theoretical limit. Response times are defined below.

tRESPONSE(INCREASING) =

tRESPONSE(DECREASING) =

Note that the response time to a load decrease is limited

only by the inductor value.

Inductor current rating is an important consideration. If

the regulated output is subject to short circuit or overcur-

rent conditions, the inductor must be sized to handle the

fault without damage. Sizing the inductor to handle fault

conditions within the maximum DC current rating helps to

ensure the coil doesnÕt overheat. Not only does this pre-

vent damage to the inductor, but it reduces unwanted heat

generated by the system and makes thermal management

easier.

Selecting an open core inductor will minimize cost, but

EMI/EMC performance may be degraded. This is a tough

choice, since there are no guidelines to ensure these com-

ponents will not prove troublesome.

Core materials influence the saturation current and satura-

tion characteristics of the inductor. For example, a slightly

undersized inductor with a powdered iron core may pro-

vide satisfactory operation because powdered iron cores

have a ÒsoftÓ saturation curve compared to other core

materials.

Small physical size, low core losses and high temperature

operation will also increase cost. Finally, consider whether

an alternate supplier is an important consideration. All of

these factors can increase the cost of the inductor.

For light load designs, the CS5127 will operate in discon-

tinuous current mode (DCM). In this regime, external

components can be smaller, since high power dissipation is

not an issue. In discontinuous mode, maximum output

current is defined as:

IOUT(MAX) =

where IPK is the maximum current allowed in the switch

FET.

Output capacitors are chosen primarily on the value of

equivalent series resistance, because this is what deter-

mines how much output ripple voltage will be present.

Most polarized capacitors appear resistive at the typical

oscillator frequencies of the CS5127. As a rule of thumb,

physically larger capacitors have lower ESR. The capaci-

torÕs value in µF is not of great importance, and values

from a few tens of µF to several hundreds of µF will work

well. Tantalum capacitors serve very well as output capaci-

tors, despite their bad reputation for spectacular failure

due to excessive inrush current. This is not usually an issue

for output capacitors, because the failure is not associated

with discharge surges. Ripple current in the output capaci-

tor is usually small enough that the ripple current rating is

not an issue. The ripple current waveform is triangular,

and the formula to calculate the ripple current value is:

IRIPPLE =

and output ripple voltage due to inductor ripple current is

given by:

VRIPPLE(ESR) =

A load step will produce an instantaneous change in

output voltage defined by the magnitude of the load step,

capacitor ESR and ESL.

DVO = (DIO ´ ESD) +

ESL

A good practice is to first choose the output capacitor to

accommodate voltage transient requirements and then to

choose the inductor value to provide an adequate ripple

voltage.

Increasing a capacitorÕs value typically reduces its ESR, but

there is a limit to how much improvement can be had. In

most applications, placing several smaller capacitors in

parallel will result in acceptable ESR while maintaining a

small PC board footprint. A warning is necessary at this

point. The V2ª topology relies on the presence of some

amount of output ripple voltage being present to provide

the input signal for the ÒfastÓ control loop, and it is impor-

tant that some ripple voltage be present at the lightest load

condition in normal operation to avoid subharmonic oscil-

lation. Externally generated slope compensation can be

added to ensure proper operation.

The VFFB lead is tied to the PWM comparatorÕs non-invert-

ing input, and provides the connection for the

externally-generated artificial ramp signal that is required

whenever duty cycle is greater than 50%.

Selecting the VFFB Lead Components

(VIN - VOUT) ´ VOUT ´ ESR

´ L ´ VIN

(VIN - VOUT)VOUT

´ L ´ VIN

Selecting the Output Capacitor

(IPK)2 f ´ L(VIN)

2VOUT ´ (VIN(MAX) - VOUT)

Operating in Discontinuous Current Mode

Other Inductor Selection Concerns

L(ÆIOUT)

VOUT

L(ÆIOUT)

(VIN - VOUT) ´ 0.85

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