LM3402 datasheet(13/38 Pages) TI

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LM3402, LM3402HV

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SNVS450E – SEPTEMBER 2006 – REVISED MAY 2013

INTERNAL MOSFET AND DRIVER

The LM3402/02HV features an internal power MOSFET as well as a floating driver connected from the SW pin to

the BOOT pin. Both rise time and fall time are 20 ns each (typical) and the approximate gate charge is 3 nC. The

high-side rail for the driver circuitry uses a bootstrap circuit consisting of an internal high-voltage diode and an

external 10 nF capacitor, CB. VCC charges CB through the internal diode while the power MOSFET is off. When

the MOSFET turns on, the internal diode reverse biases. This creates a floating supply equal to the VCC voltage

minus the diode drop to drive the MOSFET when its source voltage is equal to VIN.

FAST SHUTDOWN FOR PWM DIMMING

The DIM pin of the LM3402/02HV is a TTL logic compatible input for low frequency PWM dimming of the LED. A

logic low (below 0.8V) at DIM will disable the internal MOSFET and shut off the current flow to the LED array.

While the DIM pin is in a logic low state the support circuitry (driver, bandgap, VCC) remains active in order to

minimize the time needed to turn the LED array back on when the DIM pin sees a logic high (above 2.2V). A 75

µA (typical) pull-up current ensures that the LM3402/02HV is on when DIM pin is open circuited, eliminating the

need for a pull-up resistor. Dimming frequency, fDIM, and duty cycle, DDIM, are limited by the LED current rise time

and fall time and the delay from activation of the DIM pin to the response of the internal power MOSFET. In

general, fDIM should be at least one order of magnitude lower than the steady state switching frequency in order

to prevent aliasing.

PEAK CURRENT LIMIT

The current limit comparator of the LM3402/02HV will engage whenever the power MOSFET current (equal to

the inductor current while the MOSFET is on) exceeds 735 mA (typical). The power MOSFET is disabled for a

cool-down time that is 10x the steady-state on-time. 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 LM3402/02HV 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 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 20

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 LM3402/02HV 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 below 0V. 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.

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