ISL97635IRZ datasheet(24/28 Pages) RENESAS

24 / 28 page

ISL97635

FN6434 Rev.3.00

Page 24 of 28

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 as shown in Equation 16:

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, thereby 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;

AC ripple component is only proportional to the rate of the

inductor charging, thus, 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.

The inductor’s maximum current capability must be adequate

enough 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 voltage across the

inductor during the Off-period, as expressed in Equation 20:

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

approximation. The first term is the average current, which is

inversely proportional to the input voltage. The second term is

the inductor current change, which is inversely proportional to L

and fSW. As a result, for a given switching frequency and

minimum input voltage on which 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 ISL97635

current limit should 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 also brings up the 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 user needs to

select an output capacitor with low ESD and enough input ripple

current capability.

Output Ripple

VCo, can be reduced by increasing Co or fSW, 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 response

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

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, 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 fSW





–













(EQ. 20)

V



O CO

DfS



ESR











(EQ. 21)

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