ISL97636 Просмотр технического описания (PDF) - Renesas Electronics

Номер в каталоге

Компоненты Описание

производитель

ISL97636

8-Channel LED Driver

Renesas Electronics 

ISL97636

For example, if the maximum required LED current

(ILED(max)) is 20mA, rearranging Equation 1

yields Equation 3:

RSET = 733  0.02 = 36.6k

(EQ. 2)

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 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 ISL97636 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 ISL97636 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

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

and L.

Fault Protection and Monitoring

The ISL97636 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 ISL97636 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 ISL97636 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 ISL97636 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.

FN6570 Rev 0.00

May 9, 2008

Page 11 of 18

Share Link: