ISL97672B

FN7995 Rev.3.00

Page 13 of 17

Sep 7, 2017

Input Capacitor

Switching regulators require input capacitors to deliver peak

charging current and to reduce the impedance of the input

supply. The capacitors reduce interaction between the regulator

and input supply, thus improving system stability. The high

switching frequency of the loop causes almost all ripple current

to flow into the input capacitor, which must be rated accordingly.

A capacitor with low internal series resistance should be chosen

to minimize heating effects and to improve system efficiency.

The X5R and X7R ceramic capacitors offer small size and a lower

value for temperature and voltage coefficient compared to other

ceramic capacitors.

An input capacitor of 10µF is recommended. Ensure that the

voltage rating of the input capacitor is able to handle the full

supply range.

Inductor

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.

Inductor maximum current capability must be adequate to

handle the peak current in the worst-case condition. If an

inductor core with too low a current rating is chosen, 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 shown in Equation 10:

The value of 85% is an average term for the efficiency

approximation. The first term is the average current that is

inversely proportional to the input voltage. The second term is

the inductor current change that is inversely proportional to L and

must be chosen carefully.

Output Capacitors

The output capacitor smooths the output voltage and supplies

load current directly during the conduction phase of the power

switch. Output ripple voltage consists of discharge and charge of

the output capacitor during FET On and OFF time and the voltage

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

ripple voltage can be shown in Equation 11:

The conservation of charge principle shown in Equation 9 also

indicates 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 must select an output capacitor with low ESR and adequate

input ripple current capability.

Note: Capacitors have a voltage coefficient that makes their

effective capacitance drop as the voltage across them increases.

a particular voltage and not the manufacturer’s stated value,

measured at 0V.

The value of

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 also eases driver response during the

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. Schottky diodes are recommended because

of their fast recovery time, low forward voltage and reverse

leakage current, which minimize losses. The reverse voltage

rating of the selected Schottky diode must be higher than the

maximum output voltage. Also the average/peak current rating

of the Schottky diode must meet the output current and peak

inductor current requirements.

Applications

High-Current Applications

Each channel of the ISL97672B can support up to 30mA

multiple channels can be grouped to achieve the desired current

(Figure 22). For example, the cathode of the last LED can be

connected to CH0 through CH2; this configuration can be treated

as a single string with 90mA current driving capability.















–







+







=

(EQ. 10)

V

CO

I



I



O

ESR





+







=

(EQ. 11)

FIGURE 22. GROUPING MULTIPLE CHANNELS FOR HIGH CURRENT

APPLICATIONS

CH0

CH1

CH2