LM3421 datasheet(12/24 Pages) NSC

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error amplifier’s internally programmed reference voltage

REF).

So, given the values selected, the final average LED current

can be calculated using the above equations:

If it is desirable to use the CSH pin as a low side current sense

input regulated to the 1.235V feedback voltage, simply tie

both HSP and HSN to ground to disable the high side sense

amplifier. An internal diode prevents reverse current flow to

the HSP and HSN pins.

CURRENT SENSE/CURRENT LIMIT

The LM3421/LM3423 devices provide current mode control

using a comparator that monitors the MOSFET transistor cur-

rent, comparing it with the COMP pin voltage. Further, in

incorporates a cycle-by-cycle over-current protection func-

tion. Current limit is accomplished by a redundant internal

current sense comparator. If the voltage at the current sense

comparator input (IS) exceeds 245mV (typical), the on cycle

is immediately terminated. The IS input pin has an internal N-

channel MOSFET which pulls it down at the conclusion of

every cycle. The discharge device remains on an additional

210ns (typical) after the beginning of a new cycle to blank the

leading edge spike on the current sense signal.

The R

DS(ON) of the main power MOSFET can be used as the

current sense resistor; the IS pin was designed to withstand

the high voltages present on the drain when the MOSFET is

in the off state. A sense resistor located in the source of the

MOSFET may be used for current sensing, but a low induc-

tance resistor is required. When designing with a current

sense resistor, all of the noise sensitive low power ground

connections should be connected together local to the con-

troller and a single connection should be made to the high

current PGND (sense resistor ground point).

OVER-VOLTAGE PROTECTION

30067358

FIGURE 3. Over-Voltage Lock-Out Circuitry

The LM3421/LM3423 devices can be configured to detect ei-

ther an input or an output over-voltage condition via the OVP

pin. The pin features a precision 1.24V threshold with 23µA

(typical) of hysteresis current. When the OVLO threshold is

exceeded, the over-voltage state is entered and the GATE pin

is immediately pulled low while the DDRV pin is pulled to the

LED off state to prevent damage to the LEDs. A current

source is turned on supplying 23µA of current out of the OVP

pin to allow a user programmed lower threshold of the OVP

hysteretic band (see Figure 3). To reduce the current con-

sumption of the OVP voltage divider when in shutdown, the

lower resistor may be tied to the RPD pin.

If the LEDs are referenced to a potential other than ground,

as in the V

IN referenced flyback configuration, the output volt-

age (V

LED) is best sensed and translated to ground in order

to use the OVLO function. This can be easily achieved using

a single PNP-type bipolar transistor as shown in Figure 4.

30067359

FIGURE 4. LED Forward Voltage OVP Sensing

Remember that the OVLO also protects the voltage on HSP

and HSN so the circuit in Figure 4 would not be appropriate

in cases where the total output voltage is greater than 75V

unless the sense resistor is imbedded within the LED string

at a voltage lower than 75V.

This OVLO feature can cause some interesting results if the

OVLO trip-point is set too close to the LED stack operating

voltage. At turn-on, the converter has a modest amount of

voltage overshoot before the control loop gains control of the

average current. If this overshoot exceeds the OVLO thresh-

old, the controller shuts down, but in doing so it opens the

dimming MOSFET. This isolates the LED load from the con-

verter and its output capacitors. With only the current flowing

into the HSP and HSN pins, the output voltage droops very

slowly and in approximately ½ second the output voltage

drops below the OVLO threshold and the converter turns back

on. An observer would see the LEDs blinking at about 2Hz.

This mode can often be escaped if the input voltage is re-

duced. This is because the maximum current limit on the IS

pin will limit the power intercepted by the converter at turn-on,

thus preventing any overshoot. A detailed description of the

turn-on overshoot and a simple solution are discussed in de-

tail in the section titled STARTUP INRUSH CURRENT.

OVER-CURRENT PROTECTION

The LM3421/LM3423 devices also feature over-current pro-

tection. Switching action is disabled whenever the current in

the LEDs is more than 30% above the regulation set point.

The dimming MOSFET switch driver (DDRV) is not disabled

however as this would immediately remove the fault condition

and cause oscillatory behavior.

THERMAL SHUTDOWN

Both devices include thermal shutdown. If the die temperature

reaches approximately 165°C the device will shut down until

it cools to a safe temperature at which point the device will

resume operation. If the adverse condition that is heating the

device is not removed, the device will continue to cycle on and

off to keep the die temperature below 165°C. Thermal shut-

down has approximately 25°C of hysteresis. When in thermal

shutdown, both the main regulator MOSFET (GATE) and the

dimming MOSFET switch driver (DDRV) are disabled.

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