GND

BOOT

SW

VOUT

RON

LM3406/6HV

VIN,VINS

D1

L1

CB

RSNS

CF

RON

CIN

VIN

IF

VCC

CS

COMP

CC

Z1

RZ

D2

DIM

ROUT

LM3406, LM3406HV, LM3406HV-Q1

www.ti.com

SNVS512E – SEPTEMBER 2008 – REVISED MARCH 2014

8.13 PEAK CURRENT LIMIT

The current limit comparator of the LM3406 family will engage whenever the power MOSFET current (equal to

the inductor current while the MOSFET is on) exceeds 2.1A (typical). The power MOSFET is disabled for a cool-

down time that of approximately 100 µs. At the conclusion of this cool-down time the system re-starts. If the

current limit condition persists the cycle of cool-down time and restarting will continue, creating a low-power

hiccup mode, minimizing thermal stress on the LM3406 family and the external circuit components.

8.14 OVER-VOLTAGE/OVER-CURRENT COMPARATOR

The CS pin includes an output over-voltage/over-current comparator that will disable the power MOSFET

whenever VSNS exceeds 300 mV. This threshold provides a hard limit for the output current. Output current

overshoot is limited to 300 mV / RSNS by this comparator during transients. The OVP/OCP comparator limits the

maximum ripple voltage at the CS pin to 200 mVP-P.

8.15 OUTPUT OPEN-CIRCUIT

The most common failure mode for power LEDs is a broken bond wire, and the result is an output open-circuit.

When this happens the feedback path is disconnected, and the output voltage will attempt to rise. In buck

converters the output voltage can only rise as high as the input voltage, and the minimum off-time requirement

ensures that VO(MAX) is slightly less than VIN. shows a method using a zener diode, Z1, and zener limiting

resistor, RZ, to limit output voltage to the reverse breakdown voltage of Z1 plus 200 mV. The zener diode reverse

breakdown voltage, VZ, must be greater than the maximum combined VF of all LEDs in the array. The maximum

recommended value for RZ is 1 kΩ.

The output stage (SW and VOUT pins) of the LM3406 family is capable of withstanding VO(MAX) indefinitely as

long as the output capacitor is rated to handle the full input voltage. When an LED fails open-circuit and there is

no output capacitor present the surge in output voltage due to the collapsing magnetic field in the output inductor

can exceed VIN and can damage the LM3406 family IC. As an alternative to the zener clamp method described

previously, a diode can be connected from the output to the input of the regulator circuit that will clamp the

inductive surge to one VD above VIN.

Regardless of which protection method is used a resistance in series with the VOUT pin, ROUT, is recommended

to limit the current in the event the VOUT pin is pulled below ground when the LED circuit is reconnected. This

can occur frequently if the lead lengths to the LEDs are long and the inductance is significant. A resistor between

1 k

Ω and 10 kΩ is recommended.

Two Methods of Output Open Circuit Protection

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