ISL97635IRZ【PDF 24/28 页】IC LED DRVR WHT/RGB BCKLGT 24QFN

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ISL97635

REGISTER 0x09

OUTPUT CHANNEL REGISTER

CH7

Bit 7 (R/W)

CH6

Bit 6 (R/W)

CH5

Bit 5 (R/W)

CH4

Bit 4 (R/W)

CH3

Bit 3 (R/W)

CH2

Bit 2 (R/W)

CH1

Bit 1 (R/W)

CH0

Bit 0 (R/W)

BIT ASSIGNMENT

CH[7..0]

BIT FIELD DEFINITIONS

CH7 = Channel 7, CH6 = Channel 6 and so on

FIGURE 29. OUTPUT CHANNEL REGISTER

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:

Inductor

The selection of the inductor should be based on its

maximum current (I SAT ) 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

V L = L × Δ I L ? Δ t

and Δ I L @ On = Δ I L @ Off, therefore:

( V I – 0 ) ? L × D × t S = ( V O – V D – V I ) ? L × ( 1 – D ) × t S

(EQ. 16)

(EQ. 17)

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

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

time over the switching period. V D is Schottky diode forward

voltage that can be neglected for approximation.

Rearranging the terms without accounting for V D gives the

boost ratio and duty cycle respectively as Equations 18

and 19:

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

V O ? V I = 1 ? ( 1 – D )

D = ( V O – V I ) ? V O

(EQ. 18)

(EQ. 19)

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

IL peak = ( V O × I O ) ? ( 85% × V I ) + 1 ? 2 [ V I × ( V O – V I ) ? ( L × V O × f SW )

(EQ. 20)

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.

24

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 f SW . As a result, for a given switching

frequency and minimum input voltage on which the system

operates, the inductor I SAT 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 I LPEAK during FET On

and the voltage drop due to flowing through the ESR of the

FN6434.2

December 22, 2008

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