ISL97636AIRZ【PDF 11/18 页】IC LED DRIVR WHITE BCKLGT 24-QFN

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ISL97636A

For example, if the maximum required LED current (I LED(max) )

is 20mA, rearranging Equation 1 yields Equation 3:

prevents high input current for systems that have only a low

to medium output current requirement.

R SET = 733 ? 0.02 = 36.6k Ω

(EQ. 2)

For systems with no master fault protection FET, the inrush

current will flow towards C OUT when VIN is applied and it is

PWM CURRENT CONTROL

The average LED current of each channel can be controlled by

an external PWMI signal as shown in Equation 3:

determined by the ramp rate of VIN and the values of C OUT

and L.

Fault Protection and Monitoring

I LED ( ave ) = I LED × PWMI

(EQ. 3)

The ISL97636A features extensive protection functions to

cover all the perceivable failure conditions. The failure mode

of a LED can be either open circuit or as a short. The

The PWM dimming frequency can be, for example, 20kHz,

but there are a minimum on and off time requirements such

that the dimming will be in the range of 10% to 99.5%. If the

dimming frequency is below 5kHz, the dimming range can

be 1% to 99.5%.

The PWM dimming off time cannot be longer than 28ms or

else the driver will enter shutdown.

5V Low Dropout Regulator

A 5.2V LDO regulator is present at the VDC pin to develop

the necessary low voltage supply which is used by the chips

internal control circuitry. Because VDC is an LDO pin, it

requires a bypass capacitor of 1μF or more for the

regulation. For applications with an input voltage ≤ 5.5V, the

VIN and VDC pins can be connected together. The VDC pin

can be used as a coarse reference with few mA sourcing

capability.

Inrush Control and Soft-start

The ISL97636A has separately built-in independent inrush

control and soft-start functions. The inrush control function is

built around the short circuit protection FET, and is only

available in applications, which include this device. At start-

up, the fault protection FET is turned on slowly due to a

30μA pull-down current output from the FAULT pin. This

discharges the fault FET's gate-source capacitance, turning

on the FET in a controlled fashion. As this happens, the

output capacitor is charged slowly through the weakly turned

on FET before it becomes fully enhanced. This results in a

low inrush current. This current can be further reduced by

adding a capacitor (in the 1nF to 5nF range) across the gate

source terminals of the FET.

Once the chip detects that the fault protection FET is turned

on hard, it is assumed that inrush is complete. At this point,

the boost regulator will begin to switch and the current in the

inductor will ramp-up. The current in the boost power switch

is monitored and the switching terminated in any cycle

where the current exceeds the current limit. The ISL97636A

includes a soft-start feature where this current limit starts at a

low value (375mA). This is stepped up to the final 3A current

limit in seven further steps of 375mA. These steps will

happen over a 1ms total time, such that after 1ms the final

limit will be reached. This allows the output capacitor to be

charged to the required value at a low current limit and

11

behavior of an open circuited LED can additionally take the

form of either infinite resistance or, for some LEDs, a zener

diode, which is integrated into the device in parallel with the

now opened LED.

For basic LEDs (which do not have built-in zener diodes), an

open circuit failure of an LED will only result in the loss of

one channel of LEDs without affecting other channels.

Similarly, a short circuit condition on a channel that results in

that channel being turned off does not affect other channels

unless a similar fault is occurring.

Due to the lag in boost response to any load change at its

output, certain transient events (such as LED current steps

or significant step changes in LED duty cycle) can transiently

look like LED fault modes. The ISL97636A uses feedback

from the LEDs to determine when it is in a stable operating

region and prevents apparent faults during these transient

events from allowing any of the LED stacks to fault out. See

Table 1 for more details.

A fault condition that results in an input current that exceeds

the devices electrical limits will result in a shutdown of all

output channels.

Short Circuit Protection (SCP)

The short circuit detection circuit monitors the voltage on

each channel and disables faulty channels which are

detected above 8V (the action taken is described in Table 1.)

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 ISL97636A 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 ISL97636A 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.

FN6566.0

May 9, 2008

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