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SNVS574E – JULY 2008 – REVISED MAY 2013
1. The EN pin is pulled low long enough for the V CC pin to drop below 4.1V (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 pull-down 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 pull-down 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 =
220pF) 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. Over-voltage protection has engaged
5. Thermal shutdown has engaged.
6. A fault has latched the device off.
Note that the LRDY pin is pulled low during startup of the device and remains low until the LED current is in
regulation.
V IN
V SW
LM3421/23
FLT
V IN
High = LED in regulation
LRDY
TIMR
Figure 33. Fault Detection and LED Status Circuit
Design Considerations
This section describes the application level considerations when designing with the LM3421/23. For
corresponding calculations, refer to the Design Guide section.
INDUCTOR
The inductor (L1) is the main energy storage device in a switching regulator. Depending on the topology, energy
is stored in the inductor and transfered to the load in different ways (as an example, buck-boost operation is
detailed in the CURRENT REGULATORS section). The size of the inductor, the voltage across it, and the length
of the switching subinterval (t ON or t OFF ) determines the inductor current ripple ( Δ i L-PP ). In the design process, L1
is chosen to provide a desired Δ i L-PP . For a buck regulator the inductor has a direct connection to the load, which
is good for a current regulator. This requires little to no output capacitance therefore Δ i L-PP is basically equal to
the LED ripple current Δ i LED-PP . However, for boost and buck-boost regulators, there is always an output
capacitor which reduces Δ i LED-PP , therefore the inductor ripple can be larger than in the buck regulator case
where output capacitance is minimal or completely absent.
24
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