ISL97672B

FN7995 Rev.3.00

Page 12 of 17

Sep 7, 2017

Component Selection

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. As shown in Equations 6 and 7,

since the voltage across an inductor is:

and

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

that can be neglected for approximation.

ratio and duty cycle, respectively, as shown in Equations 8 and 9:

TABLE 1. PROTECTIONS TABLE

CASE

FAILURE MODE

DETECTION MODE

FAILED CHANNEL ACTION

GOOD CHANNEL ACTION

REGULATED BY

1CHX short-circuit

Upper

Over-Temperature

Protection limit (OTP)

not triggered, and

VCHX < 7.5V

CHX ON and burns power.

Remaining channels normal

Highest VF of all

channels

2

CHX short-circuit

Upper OTP triggered,

but VCHX < 7.5V

All channels go off until chip cools,

and then come back on with current

reduced to 76%. Subsequent OTP

Same as CHX

Highest VF of

remaining

channels

3

CHX short-circuit

Upper OTP not

triggered, but

CHX > 7.5V

CHX disabled after six PWM cycle

time-outs.

Remaining channels normal

Highest VF of

remaining

channels

4

CHX open circuit with

infinite resistance

Upper OTP not

triggered, and

CHX < 7.5V

after six PWM cycles and switches

Remaining channels normal

Highest VF of

remaining

channels

5CHX LED open circuit

but has paralleled

Zener

Upper OTP not

triggered, and

CHX < 7.5V

CHX remains ON and has highest VF;

Remaining channels ON, remaining

channel FETs burn power

VF of CHX

6CHX LED open circuit

but has paralleled

Zener

Upper OTP triggered,

but CHX < 7.5V

All channels go off until chip cools,

and then come back on with current

reduced to 76%. Subsequent OTP

Same as CHX

VF of CHX

7CHX LED open circuit

but has paralleled

Zener

Upper OTP not

triggered, but

CHX > 7.5V

CHX remains ON and has highest VF;

OFF after six PWM cycles. This is an

unwanted shut off and can be

prevented by setting OVP at an

appropriate level.

VF of CHX

8

Channel-to-channel

VF too high

Lower OTP triggered,

but CHX < 7.5V

Any channel below the target current faults out after six PWM cycles.

Remaining channels are driven with normal current.

Highest VF of

remaining

channels

9

Channel-to-channel

VF too high

Upper OTP triggered,

but CHX < 7.5V

All channels go off until chip cools and then come back on with current

Boost switches off

10

Output LED stack

voltage too high

Any channel that is below the target current times out after six PWM cycles,

Highest VF of

remaining

channels

11

GND at start-up, or

operation

LX current and timing

monitored.

OVP pins monitored

for excursions below

20% of OVP threshold.

GND.

I

L

L

-------x

t

=

(EQ. 6)

V



I

0

 L





–

–



=



L1



D

 t

S



–





–

(EQ. 7)

–





=



(EQ. 8)





–

=

(EQ. 9)