LM3402HV datasheet(13/39 Pages) TI1

13 / 39 page

GND

DIM

BOOT

RON

LM3402/02HV

VIN

RSNS

RON

CIN

VIN

VCC

LM3402, LM3402HV

www.ti.com

SNVS450F – SEPTEMBER 2006 – REVISED OCTOBER 2015

Feature Description (continued)

7.3.9 Overvoltage and Overcurrent Comparator

The CS pin includes an output overvoltage/overcurrent 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 can also be used to prevent the output voltage from rising to VO(MAX) in the event of

an output open-circuit. This is the most common failure mode for LEDs, due to breaking of the bond wires. In a

current regulator an output open circuit causes VSNS to fall to zero, commanding maximum duty cycle. Figure 18

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.

As discussed in the Maximum Output Voltage section, there is a limit to how high VO can rise during an output

open-circuit that is always less than VIN. If no output capacitor is used, the output stage of the LM3402/HV is

capable of withstanding VO(MAX) indefinitely, however the voltage at the output end of the inductor will oscillate

and can go above VIN or less than 0 V. A small (typically 10 nF) capacitor across the LED array dampens this

oscillation. For circuits that use an output capacitor, the system can still withstand VO(MAX) indefinitely as long as

CO is rated to handle VIN. The high current paths are blocked in output open-circuit and the risk of thermal stress

is minimal, hence the user may opt to allow the output voltage to rise in the case of an open-circuit LED failure.

Figure 18. Output Open Circuit Protection

7.4 Device Functional Modes

7.4.1 Low Power Shutdown

The LM3402/HV can be switched to a low power state (IIN-SD = 90 µA) by grounding the RON pin with a signal-

level MOSFET as shown in Figure 19. Low power MOSFETs like the 2N7000, 2N3904, or equivalent are

recommended devices for putting the LM3402/HV into low power shutdown. Logic gates can also be used to shut

down the LM3402/HV as long as the logic low voltage is below the over temperature minimum threshold of 0.3V.

Noise filter circuitry on the RON pin can cause a few pulses with a longer ON-time than normal after RON is

grounded or released. In these cases the OVP/OCP comparator will ensure that the peak inductor or LED current

does not exceed 300 mV / RSNS.

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