ISL97635IRZ datasheet(14/28 Pages) RENESAS

14 / 28 page

ISL97635

FN6434 Rev.3.00

Page 14 of 28

Sep 26, 2017

Short-Circuit Protection (SCP)

The short-circuit detection circuit monitors the voltage on each

channel and disables faulty channels which are detected above

the programmed short-circuit threshold. There are two selectable

levels of short-circuit threshold (3.1V and 8.0V) that can be

programmed through the Configuration Register 0x08 Bit 0.

When an LED becomes shorted, the action taken is described in

Table 1. The default short-circuit threshold is 8V. The detection of

this failure mode can be disabled via Register 0x08 bit 1 if

required.

Open Circuit Protection (OCP)

When one of the LEDs becomes open circuit, it can behave as

either an infinite resistance or a gradually increasing finite

resistance. The ISL97635 monitors the current in each channel

such that any string which reaches at least 75% of the intended

output current is considered “good”. Should the current

subsequently fall below 50% of the target, the channel will be

considered an “open circuit”. Furthermore, should the boost

output of the ISL97635 reach the OVP limit or should the lower

over-temperature threshold be reached, all channels which are

not “good” will immediately be considered as “open circuit”.

Detection of an “open circuit” channel will result in a time-out

before disabling of the affected channel. This time-out is sped up

when the device is above the lower over-temperature threshold in

an attempt to prevent the upper over-temperature trip point from

being reached.

Some users employ some special types of LEDs that have zener

diode structure in parallel with the LED for ESD enhancement, thus

enabling open circuit operation. When this type of LED goes open

circuit, the effect is as if the LED forward voltage has increased,

but no lighting. Any affected string will not be disabled, unless the

failure results in the boost OVP limit being reached, allowing all

other LEDs in the string to remain functional. Care should be taken

in this case that the boost OVP limit and SCP limit are set properly,

so as to make sure that multiple failures on one string do not

cause all other good channels to be faulted out. This is due to the

increased forward voltage of the faulty channel making all other

channel look as if they have LED shorts. Refer to Table 1 for details

for responses to fault conditions.

Overvoltage Protection (OVP)

The integrated OVP circuit monitors the output voltage and keeps

the voltage at a safe level. The OVP threshold is set as

Equation 14:

These resistors should be large to minimize the power loss. For

example, a 1MΩ RUPPER and 39kΩ RLOWER sets OVP to 32.2V.

Large OVP resistors also allow COUT discharges slowly during the

PWM Off-time.

Undervoltage Lockout

If the input voltage falls below the UVLO level of 2.45V, the device

will stop switching and be reset. Operation will restart when the

voltage comes back into the operating range.

Input Overcurrent Protection

During normal switching operation, the current through the

internal boost power FET is monitored. If the current exceeds the

current limit, the internal switch will be turned off. This

monitoring happens on a cycle by cycle basis in a self protecting

way.

Additionally, the ISL97635 monitors the voltage at the LX and

OVP pins. At start-up, a fixed current is injected out of the LX pins

and into the output capacitor. The device will not start-up unless

the voltage at LX exceeds 1.2V. Furthermore, should the voltage

at LX not rise above this threshold during any subsequent period

where the power FET is not switched on, it will immediately

disable the input protection FET. The OVP pin is also monitored

such that if it rises above and subsequently falls below 20% of

the target OVP level, the input protection FET will also be

switched off.

Over-Temperature Protection (OTP)

The ISL97635 includes two over-temperature thresholds. The lower

threshold is set to +130°C. When this threshold is reached, any

channel which is outputting current at a level significantly below the

regulation target will be treated as “open circuit” and disabled after

a time-out period. This time-out period is also reduced to 800µs

when it is above the lower threshold. The intention of the lower

threshold is to allow bad channels to be isolated and disabled before

they cause enough power dissipation (as a result of other channels

having large voltages across them) to hit the upper temperature

threshold.

The upper threshold is set to +150°C. Each time this is reached,

the boost will stop switching and the output current sources will

be switched off. Once the device has cooled to approximately

+100°C, the device will restart with the DC LED current level

reduced to 77% of the initial setting. If the dissipation problem

persists, subsequent hitting of the limit will cause identical

behavior, with the current reduced in steps to 53% and finally

30%. Hitting of the upper threshold will also set the thermal fault

bit of the Fault/Status register 0x02. Unless disabled via the EN

pin, the device stays in an active state throughout, allows the

external processor to interrogate the fault condition.

For the extensive fault protection conditions, please refer to

Figure 19 and Table 1 for details.

OVP

1.21V

RUPPER RLOWER

 R

LOWER





(EQ. 14)

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