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ISL97678IRZ-TK

8-Channel 45V 50mA LED Driver

Intersil 

ISL97678

OVP and VOUT Requirement

The Overvoltage Protection (OVP) pin has a function of

setting the overvoltage trip level as well as limiting the

VOUT regulation range.

The ISL97678 OVP threshold is set by RUPPER and

RLOWER as shown in Equation 1:

VOUT_OVP = 1.21V × (RUPPER + RLOWER) ⁄ RLOWER (EQ. 1)

VOUT can only regulate between 64% and 100% of the

VOUT_OVP such that:

Allowable VOUT = 64% to 100% of VOUT_OVP

For example, if 10 LEDs are used with the worst case

VOUT of 35V. If R1 and R2 are chosen such that the OVP

level is set at 40V, then the VOUT is allowed to operate

between 25.6V and 40V. If the requirement is changed to

a 6 LEDs 21V VOUT application, then the OVP level must

be reduced and users should follow VOUT = (64%

~100%)OVP requirement. Otherwise, the headroom

control will be disturbed such that the channel voltage

can be much higher than expected and sometimes it can

prevents the driver from operating properly.

The ratio of the OVP capacitors should be the inverse of

the OVP resistors. For example, if RUPPER/RLOWER=

33/1, then CUPPER/CLOWER=1/33 with CUPPER = 100pF

and CLOWER = 3.3nF.

Dimming Controls

The ISL97678 allows two ways of controlling the LED

current, and therefore, the brightness. They are:

1. DC current adjustment

2. PWM chopping of the LED current defined in Step 1.

There are various ways to achieve DC or PWM current

control, which will be described in the following.

In any dimming controls, the EN pin must be high. EN

is a high voltage pin that can be applied with a digital

signal or tied directly to VIN for enable function.

MAXIMUM DC CURRENT SETTING

The initial brightness should be set by choosing an

appropriate value for RSET. This should be chosen to fix

the maximum possible LED current:

ILEDmax

=

-7---0---7----.-9--

RSET

(EQ. 2)

Alternatively, the RSET can be replaced by a digital

potentiometer for adjustable current.

PWM CONTROL

The ISL97678 provides PWM dimming by PWM chopping

of the current in the LEDs for all 8 channels. To achieve

PWM dimming, the users need to apply a PWM signal at

the PWM pin. The PWM output will follow the PWM input

and the dimming frequency will be set by RPWM. During

the On periods, the LED current will be defined by the

value of RSET, as described in Equation 1.

PWM Dimming Frequency Adjustment

The dimming frequencies are set by an external

resistor at the FPWM pin as shown by Equation 3:

fPWM

=

6----.--6---6----×---1---0---7-

RPWM

(EQ. 3)

where fPWM is the desirable PWM dimming frequency

and RFPWM is the setting resistor.

Switching Frequency

The boost switching frequency can be adjusted by a

resistor as shown in Equation 4:

fSW

=

(---5----×---1---0---1---0----)

ROSC

(EQ. 4)

where fSW is the desirable boost switching frequency

and ROSC is the setting resistor.

5V and 2.3V Low Dropout Regulators

A 5V 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. The VDC pin can be used for a coarse

regulator or reference but do not pull more than few mA

from it.

Similarly, a 2.3V LDO regulator is present at the

VLOGIC pin to develop the necessary low voltage supply

for the chip’s internal logic control circuitry. A 1µF

bypass capacitor or more is needed for regulation. The

VLOGIC pin can be used as a coarse regulator or

reference but do not pull more than few mA from it.

Soft-Start

The ISL97678 uses a digital soft-start where the boost

current limit is stepped up in 8 steps. The initial current

limit level is set to one ninth of the full current limit, with

subsequent steps increasing this by a ninth every 2ms.

In the event that no LEDs have been conducting during

the interval since the last step (for example if the LEDs

are running at low duty cycle at low PWM frequency)

then the step will be delayed until the LEDs are

conducting. If the LEDs are disabled and re-enabled

again then soft start will be restarted when the LEDs are

enabled.

Fault Protection and Monitoring

The ISL97678 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

12

FN6998.1

November 5, 2009

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