ISL97682, ISL97683, ISL97684

FN7689 Rev.2.00

Page 15 of 18

Sep 19, 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 10:

and

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

voltage that can be neglected for approximation.

ratio and duty cycle as Equations 12 and 13:

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.

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

(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.

Additionally 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, expressed in Equation 14:

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

Applications

Low Voltage Operations

The ISL97682, ISL97683, ISL97684 VIN pin can be separately

biased from the LEDs power input to allow low voltage operation.

For systems that have only single supply, VOUT can be tied to the

driver VIN pin to allow initial start-up; see Figure 26. The circuit

works as follows; when the input voltage is available and the

start-up once the part is enabled. Once the driver starts up with

loss on VIN is acceptable, this configuration can be used for input

voltage as low as 3.0V. For systems where a single input supply

of 4V to 5.5V is available, the VIN pin can be shorted to VDC,

allowing a slight gain in efficiency due to bypassing the internal

LDO.

For systems that have dual supplies, the VIN pin can be biased

from 5V to 12V. The input voltage can be as low as 2.7V without

the limitations previously mentioned; see Figure 27.

L





=

(EQ. 10)

V





–

–



=



L1



D



–





–

(EQ. 11)

–





=



(EQ. 12)





–

=

(EQ. 13)















–







+







=

(EQ. 14)

FIGURE 26. SINGLE SUPPLY 3V OPERATION

VIN

VDC

EN

PWMI

FSW

RSET

COMP

LX

OVP

PGND

CH1

CH2

CH3

CH4

AGND

ISL97684

20mA