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ISL97678IRZ

8-Channel 45V, 50mA LED Driver

Renesas Electronics 

ISL97678

TABLE 1. PROTECTIONS TABLE (Continued)

CASE FAILURE MODE

DETECTION MODE

FAILED CHANNEL ACTION

GOOD CHANNELS ACTION

VOUT REGULATED

BY

3 CH1 Short Circuit

Upper OTP not triggered CH1 disabled after 6 PWM cycles

but VIIN1 > VSC

time-out.

If 3 channels are already shut

down, all channels will be shut

down. Otherwise CH2-8 will

remain as normal

Highest VF of CH2

through CH8

4 CH1 Open Circuit

with infinite

resistance

Upper OTP not triggered

and VIIN1 < VSC

VOUT will ramp to OVP. CH1 will

time-out after 6 PWM cycles and

switch off. VOUT will drop to normal

level.

CH2 through CH8 Normal

Highest VF of CH2

through CH8

5 CH1 LED Open

Circuit but has

paralleled Zener

Upper OTP not triggered CH1 remains ON and has highest CH2 through CH8 ON, Q2 through VF of CH1

and VIIN1 < VSC

VF, thus VOUT increases

Q8 burn power

6 CH1 LED Open

Circuit but has

paralleled Zener

Upper OTP triggered but CH1 goes off

VIIN1 < VSC

Same as CH1

VF of CH1

7 CH1 LED Open

Circuit but has

paralleled Zener

Upper OTP not triggered CH1 OFF

but VIIN1 > VSC

Upper OTP not triggered CH1 remains ON and has highest

but VIINx > VSC

VF, thus VOUT increases.

CH2 through CH8 Normal

Highest VF of CH2

through CH8

VOUT increases then CH-X

switches OFF. This is an

unwanted shut off and can be

prevented by setting OVP and/or

VSC at an appropriate level.

VF of CH1

8 Channel-to-Channel Lower OTP triggered but Any channel at below the target current will fault out after 6 PWM

VF too high

VIINx < VSC

cycles.

Remaining channels driven with normal current.

Highest VF of CH1

through CH8

9 Channel-to-Channel Upper OTP triggered but All channels switched off

VF too high

VIINx < VSC

Highest VF of CH1

through CH8

10 Output LED string

voltage too high

VOUT > VOVP

11 VOUT/LX shorted to

GND

Driven with normal current. Any channel that is below the target current Highest VF of CH1

will time-out after 6 PWM cycles.

through CH8

LX will not switch

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. The voltage across an inductor is as

shown in Equation 6:

VL = L  IL  t

(EQ. 6)

and IL @ On = IL @ Off, therefore:

VI – 0   L  D  tS= VO – VD – VI  L  1 – D   tS

(EQ. 7)

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

over the switching periods. 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 8 and 9:

VO  VI = 1  1 – D

(EQ. 8)

D = VO – VI   VO

(EQ. 9)

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.

FN6998 Rev.3.00

Sep 8, 2017

Page 14 of 17

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