ISL976787IBZ 查看數據表（PDF） - Renesas Electronics

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ISL976787IBZ

4-Channel LED Driver with Phase Shift Control and 10-Bit Dimming Resolution

Renesas Electronics 

ISL97687

This is not a problem when the LED off times are short and the

duty cycle is running at a high duty cycle, or the output

capacitance is large. However, it presents two problems. First, for

low duty cycles at low frequencies, VOUT can droop between

on-times, resulting in under-regulation of the current when the

LEDs are next switched on. Second, at high PWM frequencies or

very low duty cycles, LED on-times can be shorter than the

minimum number of boost cycles needed to ramp up the

inductor current to the required level to support the load. For

example, a 1% on-time while running at 20kHz PWM dimming

frequency is only 500ns. If the boost switching frequency is set at

500kHz, this only represents a quarter of a switching cycle per

LED on-time, which may not be sufficient to ramp the inductor

current to the required level.

The ISL97687 incorporates an additional PFM switching

mechanism that allows the boost stage to continue to switch at

low current levels in order to replace the energy lost from the

output capacitor due to the OVP stack resistance and capacitor

self discharge. For very short pulses, this also means that the

charge delivered to the LEDs in the on-times is provided entirely

by the output capacitor, kept at the correct voltage by the PFM

mode in the off-times. This allows the output to always remain

very close to the required level, so that when the LEDs are

re-enabled, the boost output is already at the correct level. This

dramatically improves LED PWM performance, providing industry

leading linearity down to sub 1% levels, and reduces the

overshoot in the boost inductor current, caused by transient

switching when the LEDs are switched on, to a minimum level.

The system will continue to maintain VOUT at the target level for

120ms after the last time the LEDs were on. If all LEDs are off for

a longer period than this, the converter will stop switching and go

into a sleep mode, allowing VOUT to decay, in order to save power

during long backlight-off periods.

Switching Frequency

The boost switching frequency can be adjusted by the resistor on

the OSC pin, which must be connected to AGND, and follows

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 2.4V Low Dropout Regulators

A 5V LDO regulator is used to provide the low voltage supply

needed to drive internal circuits. The output of this LDO is the VDC

pin. A decoupling capacitor of 1µF or more is required between

this pin and AGND for correct operation. Similarly, a 2.4V LDO

regulator is present at the VLOGIC pin, and also requires a 1µF

decoupling capacitor. Both pins can be used as a coarse voltage

reference, or as a supply for other circuits, but can only support a

load of up to ~10mA and should not be used to power noisy

circuits that can feed significant noise onto their supply.

Soft-Start and Boost Current Limit

The boost current limit should be set by using a resistor from CS

to PGND. The typical current limit can be calculated as:

ILIMIT

=

0----.-1----7--

RCS

(EQ. 5)

The CS resistor should be chosen based on the maximum load

that needs to be driven. Typically, a limit of 30~40% more than is

required under DC conditions is sufficient to allow for necessary

overshoots during load transients. Values of 20~100mΩ are

supported.

It is important that PGND pin 14 (QFN)/18 (SOIC) is connected

directly to the base of the sense resistor, with no other

connection to the ground system, except via this path. This is

because this pin is used as a ground reference for the CS pin.

Connecting it here gives the maximum noise immunity and the

best stability characteristics.

The ISL97687 uses a digital current limit based soft start. The

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

eight subsequent steps increasing this by a ninth of the final

value every 2ms until it reaches the full limit. 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), the step will be delayed until the LEDs are

conducting again.

If the LEDs are off for more than 120ms, making the converter

go into sleep mode, soft-start will be restarted when the LEDs are

re-enabled.

Fault Protection and Monitoring

The ISL97687 features extensive protection functions to cover all

perceivable failure conditions. The failure mode of an LED can be

either open or short circuit. The behavior of an open circuit LED

can additionally take the form of either infinite or very high

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 LED failure 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 significant step changes in LED

duty cycle, or a change in LED current caused by CSEL switching)

can transiently look like LED fault modes. The ISL97687 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 strings to fault out.

See Figure 26 and Table 2 for more details.

Short Circuit Protection (SCP)

The short circuit detection circuit monitors the voltage on each

channel and disables faulty channels which are detected to be

more than the short circuit threshold, 8V above the lowest CH

pin, following a timeout period.

FN7714 Rev.3.00

Sep 13, 2017

Page 14 of 24

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