ISL97635A datasheet(25/29 Pages) RENESAS

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ISL97635A

FN6564 Rev.3.00

Page 25 of 29

Sep 26, 2017

Components Selections

According to the inductor Voltage-Second Balance principle, the

change of inductor current during the switching regulator On-

time is equal to the change of inductor current during the

switching regulator Off-time. Since the voltage across an inductor

is:

and

IL at On = IL at Off, therefore:

where D is the switching duty cycle defined by the turn-on time

over the switching period. VD is Schottky diode forward voltage

that can be neglected for approximation.

Rearranging the terms without accounting for VD gives the boost

ratio and duty cycle respectively as Equations 18 and 19:

Input Capacitor

Switching regulators require input capacitors to deliver peak

charging current and to reduce the impedance of the input

supply. This reduces interaction between the regulator and input

supply, improving system stability. The high switching frequency

of the loop causes almost all ripple current to flow in the input

capacitor, which must be rated accordingly.

A capacitor with low internal series resistance should be chosen

to minimize heating effects and improve system efficiency, such

as X5R or X7R ceramic capacitors, which offer small size and a

lower value of temperature and voltage coefficient compared to

other ceramic capacitors.

In boost mode, input current flows continuously into the inductor,

with an AC ripple component proportional to the rate of inductor

charging only and smaller value input capacitors may be used. It

is recommended that an input capacitor of at least 10µF be

used. Ensure the voltage rating of the input capacitor is suitable

to handle the full supply range.

Inductor

The selection of the inductor should be based on its maximum

current (ISAT) characteristics, power dissipation (DCR), EMI

susceptibility (shielded vs unshielded), and size. Inductor type

and value influence many key parameters, including ripple

current, current limit, efficiency, transient performance and

stability.

Its maximum current capability must be adequate to handle the

peak current at the worst case condition. If an inductor core is

chosen with too low a current rating, saturation in the core will

cause the effective inductor value to fall, leading to an increase

in peak to average current level, poor efficiency and overheating

in the core. The series resistance, DCR, within the inductor

causes conduction loss and heat dissipation. A shielded inductor

is usually more suitable for EMI susceptible applications, such as

LED backlighting.

The peak current can be derived from the fact that the voltage

across the inductor during the Off-period can be shown as

Equation 20:

The choice of 85% is just an average term for the efficiency

approximation. The first term is average current that is inversely

proportional to the input voltage. The second term is inductor

current change that is inversely proportional to L and fS. As a

result, for a given switching frequency and minimum input

voltage the system operates, the inductor ISAT must be chosen

carefully. At a given inductor size, usually the larger the

inductance, the higher the series resistance because of the extra

winding of the coil. Thus, the higher the inductance, the lower the

peak current capability. The ISL97635A current limit may also

have to be taken into account.

Output Capacitors

The output capacitor acts to smooth the output voltage and

supplies load current directly during the conduction phase of the

power switch. Output ripple voltage consists of the discharge of

the output capacitor for ILPEAK during FET On and the voltage

drop due to flowing through the ESR of the output capacitor. The

ripple voltage can be shown as Equation 21:

The conservation of charge principle in Equation 19 also brings

up a fact that during the boost switch off-period, the output

capacitor is charged with the inductor ripple current minus a

relatively small output current in boost topology. As a result, the

users need to select an output capacitor with low ESD and with a

enough input ripple current capability.

Output Ripple

VCo can be reduced by increasing CO or fS, or using small ESR

capacitors. In general, ceramic capacitors are the best choice for

output capacitors in small to medium sized LCD backlight

applications due to their cost, form factor, and low ESR.

A larger output capacitor will also ease the driver respond during

PWM dimming Off-period due to the longer sample and hold

effect of the output drooping. The driver does not need to boost

harder in the next On-period that minimizes transient current.

The output capacitor is also needed for compensation and in

general 2x4.7µF/50V ceramic capacitors are suitable for the

notebook display backlight applications.

Schottky Diode

A high speed rectifier diode is necessary to prevent excessive

voltage overshoot, especially in the boost configuration. Low

forward voltage and reverse leakage current will minimize

losses, making Schottky diodes the preferred choice. Although

the Schottky diode turns on only during the boost switch Off-

period, it carries the same peak current as the inductor’s, and

therefore, a suitable current rated Schottky diode must be used.

I

L t





(EQ. 16)



 L



DtS



VO VD VI

–







 t



–





–

(EQ. 17)

VO VI 11 D

–





(EQ. 18)

DVO



VI VO



–

(EQ. 19)

ILpeak



IO 85%



VI 12 VI VO



VI L



VO fS





–













(EQ. 20)

V



O CO

DfS



ESR











(EQ. 21)

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