ISL976787IBZ 查看數據表（PDF） - Renesas Electronics

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ISL976787IBZ

4-Channel LED Driver with Phase Shift Control and 10-Bit Dimming Resolution

Renesas Electronics 

ISL97687

TABLE 2. PROTECTIONS TABLE

CASE FAILURE MODE

DETECTION MODE

FAILED CHANNEL ACTION

GOOD CHANNELS ACTION

VOUT

REGULATED BY

1 CH1 Short Circuit

Over-Temperature

CH1 ON and burns power

Protection limit (OTP) not

triggered and VCH1 < VSC

CH2 through CH4 Normal

Highest VF of CH2

through CH4

2 CH1 Short Circuit

OTP not triggered but

VCH1 > VSC

CH1 disabled after 6 PWM cycles

time-out.

(Note: Time-out can be longer than 6

PWM cycles in direct PWM mode)

If 3 channels are already shut

down, all channels will be shut

down. Otherwise CH2-4 will

remain as normal

Highest VF of CH2

through CH4

3 CH1 Open Circuit with OTP not triggered and

infinite resistance VCH1 < VSC

VOUT will ramp to OVP. CH1 will time-out CH2 through CH4 Normal

after 6 PWM cycles and switch off. VOUT

will drop to normal level.

Highest VF of CH2

through CH4

4 CH1 Open Circuit with OTP triggered and

infinite resistance VCH1 < VSC

during operation

All IC shut down

VOUT disabled

5 CH1 LED Open Circuit OTP not triggered and

but has paralleled VCH1 < VSC

Zener

CH1 remains ON and has highest VF,

thus VOUT increases

CH2 through CH4 ON, Q2 through VF of CH1

Q4 burn power. CH2-4 will fault

out if they reach VSC as a result of

VOUT increase due to increase VF

in CH1

6 CH1 LED Open Circuit OTP not triggered but

but has paralleled VCHx > VSC

Zener

CH1 remains ON and has highest VF,

thus VOUT increases.

VOUT increases then CH-X

VF of CH1

switches OFF. This is an unwanted

shut off and can be prevented by

setting OVP at an appropriate

level.

7 Channel-to-Channel OTP triggered but

VF too high

VCHx < VSC

All channels switched off

VOUT disabled

8 Output LED string

voltage too high

VOUT reaches OVP and not Driven with normal current. Any channel that is below the target current VOUT disabled

sufficient to regulate LED will time-out after 6 PWM cycles.

current

(Note: Time-out can be longer than 6 PWM cycles in case direct PWM

mode)

9 VOUT/SW shorted to

GND

SW will not switch if started up in this condition. VOUT shorted to ground

during operation will also cause the converter to shut down

Component Selections

According to the inductor Voltage-Second Balance principle, the

change of inductor current during the power MOSFET switching

on-time is equal to the change of inductor current during the

power MOSFET switching off-time under steady state operation.

The voltage across an inductor is shown in Equation 6:

VL = L  IL  t

(EQ. 6)

and IL @ tON = IL @ tOFF, therefore:

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

(EQ. 7)

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

over the switching period. VD is a Schottky diode forward voltage,

which can be neglected for approximation. tsw is the switching

period where tsw = 1/fsw, and the fsw is the switching frequency

of the boost converter.

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.

During the normal continuous conduction mode of the boost

converter, its 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.

FN7714 Rev.3.00

Sep 13, 2017

Page 16 of 24

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