ISL97673IRZ 查看數據表（PDF） - Renesas Electronics

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ISL97673IRZ

6-Channel SMBus or PWM Dimming LED Driver with Phase Shift Control

Renesas Electronics 

ISL97673

ILED0

ILED1

ILED2

ILED3

ILED4

ILED5

tFPWM

tON

tPD

tPD

tPD

tPD

tPD

tOFF

FIGURE 23. PHASE SHIFT WITH 7-BIT

PROGRAMMABLE DELAY

The ISL97673 allows the user to program the amount of

phase shift degree in 7-bit resolution, as shown in

Figure 24. To enable programmable phase shifting, the

user must write to the Phase Shift Control register with

EqualPhase = 0 and the desirable phase shift value of

PhaseShift[6:0]. The delay between CH5 and the

repeated CH0 is the rest of the PWM cycle.

Switching Frequency

There are 2 levels of switching frequencies enable for the

boost regulator’s control of the LX pin: 600kHz or

1.2MHz. Each can be programmed in the Configuration

Register 0x08 bit 2. The default switching frequency is at

600kHz.

5V Low Dropout Regulator

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. Low input voltage also allows only lower

output voltage applications only with the maximum

boost ratio defined in “Components Selections” on

page 24. The VDC pin can be used as a coarse reference

with a few mA sourcing capability.

In-rush Control and Soft-start

The ISL97673 has separately built in independent in-rush

control and soft-start functions. The in-rush 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 15µA 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 in-rush 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 in-rush has

completed. At this point, the boost regulator will begin to

switch and the current in the inductor will ramp-up. The

FN7633 Rev.3.00

Sep 19, 2017

current in the boost power switch is monitored and the

switching is terminated in any cycle where the current

exceeds the current limit. The ISL97673 includes a

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

value (275mA). This is stepped up to the final 2.2A

current limit in 7 further steps of 275mA. These steps will

happen over at least 8ms, and will be extended at low

LED PWM frequencies if the LED duty cycle is low. 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 ISL97673 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. LED

faults are reported via the SMBus/I2C interface to

Register 0x02 (Fault/Status register). The controller is

able to determine which channels have failed via Register

0x09 (Output Masking register). The controller can also

choose to use Register 0x09 to disable faulty channels at

start-up, resulting in only further faulty channels being

reported by Register 0x02.

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 ISL97673 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 2 for more details.

A fault condition that results in high input current due to

a short on VOUT will result in a shutdown of all output

channels. The control device logic will remain functional

such that the Fault/Status Register can be interrogated

by the system. The root cause of the failure will be

loaded to the volatile Fault/Status Register so that the

host processor can interrogate the data for failure

monitoring.

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