LM3464 datasheet(15/22 Pages) NSC

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FIGURE 11. Thermal Foldback + PWM Dimming Control

LOW POWER STANDBY

The LM3464/64A will enter low power standby mode when

the EN pin is pulled to GND. The EN pin is internally biased

thus no external pull-up resistor or bias is required. Under

standby mode, all the output channels are cut-off and part of

the internal circuitries are disabled to maintain low power

consumption. Upon the EN pin is pulled low, the OutP pin

stopa sinking current from the feedback node of the primary

power stage. This causes the rail voltage fall back to V

RAIL

(nom) slowly as the output capacitors of the primary power

supply are being discharged by the LEDs. Pulling the EN pin

low will not disable the V

CC regulator. When the EN pin is

released (floating), the LM3464/64A exits low power standby

mode and the startup sequence begins as described in Figure

FAULT HANDLING and INDICATION

The LM3464/64A features a complete mechanism for fault

handling and indication. The LM3464/64A detects LED fail-

ures and raises fault indication signal at the Faultb pin upon

open or short circuits of LED strings, insufficient supply volt-

age and so on. In order to avoid false triggering the fault

detection circuitry, the LM3464/64A features a timer for fault

recognition. When a fault condition arises and sustains longer

than the time constant preset by the capacitor , C

FLT, a fault

is confirmed. The Faultb pin is then pulled low as an indica-

tion. The time constant for fault detection is defined by the

value of the capacitor connects across the FAULT_CAP pin

and GND, C

FLT. Normally, a 2.2 nF CFLT that set a 264 us

delay time is suitable for most application. For those applica-

tions with slow response primary power supply, the value of

FLT may need to increase accordingly. The time delay for

fault detection is governed by the following equation:

OPEN CIRCUIT OF LED STRINGS

Detection of LED open circuit is achieved by detecting the

voltages of the SEx pin and the internal gate control signal

being fed to the internal MOSFET gate driver. When a LED

string is open circuit, the V

SEx pin is pulled down to below

30mV by the current sensing resistor. As V

SEx falls below its

regulated level, the LM3464/64A increases the gate voltage

of the corresponding MOSFET (V

GDx) in order to maintain

current regulation. Thus, the requirement for LED open circuit

is V

SEx below 30mV and internal gate voltage reaches its

maximum (at V

GDx about 5V). When the requirement of LED

open fault is fulfilled, the LM3464/64A begins to charge up the

FLT. When the voltage of the FAULT_CAP reaches 3V, and

the condition of open fault retains, an open fault is confirmed.

After an open fault is confirmed, the failed channel(s) will be

disabled and excluded from DHC loop. To reactivate the dis-

abled channel(s), the EN pin can be pulled to GND for a soft

reset or re-power the primary power supply for a system reset.

Either reset methods results in a system restart with startup

sequence shows in Figure 3.

SHORT CIRCUIT OF LED STRINGS

If any LED string experiences partially short circuit after nor-

mal system startup, the drain voltage (DRx) of the corre-

sponding channel(s) will increase so as to maintain correct

current regulation. When drain voltage increases up to 8.4V

higher than the drain voltages of any other channels, the

shortened channel will be latched off and excluded from the

DHC loop to avoid further damages. Once a short fault is

confirmed, the Faultb pin will be pulled low no matter it is due

to failure of the power source or shortening of LED strings.

When a short circuit of LED sting is confirmed, the failed

channel(s) will be disabled and excluded from DHC loop. The

disabled channels can be reactivated by either pulling the EN

pin to GND or system re-powering.

DRx PIN OVER-VOLTAGE PROTECTION

The LM3464/64A features a over-voltage protection function

that prevents damaging of the external MOSFETs due to

short circuit of LED string(s). When the voltage of any DRx

pin reaches 19V typical, the fault detection timer is triggered

with the output current of the FAULT_CAP pin increases by

4 times (I

FAULT-CAP-OVP) and results in fault detection time 4

times shorter. If a over-voltage of any DRx pin is confirmed,

the particular channel will be latched off and excluded from

DHC loop until the EN pin is pulled low (soft reset) or system

re-powering is undertaken.

DRIVING LESS THAN FOUR LED STRINGS

The LM3464/64A allows users to disable the unused output

channels. Any output channel without a LED string connected

or with DRx and SEx pins floating will be disabled at system

startup. A disabled channel will be excluded from the DHC

loop and will not contribute headroom control signal to the

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