LM3421 Datasheet, PDF (24/72 Pages) National Semiconductor (TI) – N-Channel Controllers for Constant Current LED Drivers

LM3421, LM3421-Q1, LM3423, LM3423-Q1
SNVS574F – JULY 2008 – REVISED JULY 2015
www.ti.com
Feature Description (continued)
7.3.13 LM3423 Only: DPOL, FLT, TIMR, and LRDY
The LM3423 has four additional pins: DPOL, FLT, TIMR, and LRDY. The DPOL pin is simply used to invert the
DDRV polarity . If DPOL is left open, then it is internally pulled high and the polarity is correct for driving a series
N-channel dimFET. If DPOL is pulled low then the polarity is correct for using a series P-channel dimFET in high-
side dimming applications. For a parallel P-channel dimFET, as used in the buck topology, leave DPOL open for
proper polarity.
Among the LM3423's other additional pins are TIMR and FLT which can be used in conjunction with an input
disconnect MOSFET switch as shown in Figure 28 to protect the module from various fault conditions.
A fault is detected and an 11.5 µA (typical) current is sourced from the TIMR pin whenever any of the following
conditions exist:
1. LED current is above regulation by more than 30%.
2. OVLO has engaged.
3. Thermal shutdown has engaged.
An external capacitor (C_TMR) from TIMR to AGND programs the fault filter time as follows:
C_TMR =
t_FLT x 11.5 2A
1.24V
(30)
When the voltage on the TIMR pin reaches 1.24 V, the device is latched off and the N-channel MOSFET open-
drain FLT pin transitions to a high impedance state. The TIMR pin will be immediately pulled to ground (reset) if
the fault condition is removed at any point during the filter period. Otherwise, if the timer expires, the fault will
remain latched until one of three things occurs:
1. The EN pin is pulled low long enough for the V_CC pin to drop below 4.1 V (approximately 200 ms).
2. The TIMR pin is pulled to ground.
3. A complete power cycle occurs.
When using the EN and OVP pins in conjunction with the RPD pulldown pin, a race condition exists when exiting
the disabled (EN low) state. When disabled, the OVP pin is pulled up to the output voltage because the RPD
pulldown is disabled, and this will appear to be a real OVLO condition. The timer pin will immediately rise and
latch the controller to the fault state. To protect against this behavior, a minimum timer capacitor (C_TMR = 220 pF)
should be used. If fault latching is not required, short the TMR pin to AGND, which will disable the FLT flag
function.
The LM3423 also includes an LED Ready (LRDY) flag to notify the system that the LEDs are in proper
regulation. The N-channel MOSFET open-drain LRDY pin is pulled low whenever any of the following conditions
are met:
1. V_CC UVLO has engaged.
2. LED current is below regulation by more than 20%.
3. LED current is above regulation by more than 30%.
4. Overvoltage protection has engaged
5. Thermal shutdown has engaged.
6. A fault has latched the device off.
The LRDY pin is pulled low during start-up of the device and remains low until the LED current is in regulation.
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