ISL97636AIRZ Просмотр технического описания (PDF) - Renesas Electronics

Номер в каталоге

Компоненты Описание

производитель

ISL97636AIRZ

6-Channel LED Driver

Renesas Electronics 

ISL97636A

PWM CURRENT CONTROL

The average LED current of each channel can be controlled by an

external PWMI signal as shown in Equation 3:

ILEDave = ILED  PWMI

(EQ. 3)

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 prevents high input

current for systems that have only a low to medium output

current requirement.

For systems with no master fault protection FET, the inrush

current will flow towards COUT when VIN is applied and it is

FN6566 Rev 0.00

May 9, 2008

determined by the ramp rate of VIN and the values of COUT

and L.

Fault Protection and Monitoring

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 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.

Some users employ some special types of LEDs that have

zener diode structure in parallel with the LED for ESD

enhancement and enabling open circuit operation. When this

type of LED is open circuited, 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

Page 11 of 18

Share Link: