LM3406_09 Datasheet, PDF (10/26 Pages) National Semiconductor (TI) – 1.5A Constant Current Buck Regulator for Driving High Power LEDs

FIGURE 3. Dimming with a Parallel FET
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PEAK CURRENT LIMIT
The current limit comparator of the LM3406/06HV 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 ap-
proximately 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
LM3406/06HV and the external circuit components.
OVER-VOLTAGE/OVER-CURRENT COMPARATOR
The CS pin includes an output over-voltage/over-current
comparator that will disable the power MOSFET whenever
V_SNS exceeds 300 mV. This threshold provides a hard limit
for the output current. Output current overshoot is limited to
300 mV / R_SNS by this comparator during transients. The OVP/
OCP comparator limits the maximum ripple voltage at the CS
pin to 200 mV_P-P.
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 volt-
age can only rise as high as the input voltage, and the
minimum off-time requirement ensures that V_O(MAX) is slightly
less than V_IN. Figure 4 shows a method using a zener diode,
Z1, and zener limiting resistor, R_Z, to limit output voltage to
the reverse breakdown voltage of Z1 plus 200 mV. The zener
diode reverse breakdown voltage, V_Z, must be greater than
the maximum combined V_F of all LEDs in the array. The max-
imum recommended value for R_Z is 1 kΩ.
The output stage (SW and VOUT pins) of the LM3406/06HV
is capable of withstanding V_O(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 mag-
netic field in the output inductor can exceed V_IN and can
damage the LM3406/06HV IC. As an alternative to the zener
clamp method described previously, a diode can be connect-
ed from the output to the input of the regulator circuit that will
clamp the inductive surge to one V_D above V_IN.
Regardless of which protection method is used a resistance
in series with the VOUT pin, R_OUT, 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 frequent-
ly if the lead lengths to the LEDs are long and the inductance
is significant. A resistor between 1 kΩ and 10 kΩ is recom-
mended.
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