LM3421_09 Datasheet, PDF (12/24 Pages) National Semiconductor (TI) – N-Channel Controllers for Constant Current LED Drivers

error amplifier’s internally programmed reference voltage
(V_REF).
So, given the values selected, the final average LED current
can be calculated using the above equations:
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.
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.
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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