LM3421_09 Datasheet, PDF (10/24 Pages) National Semiconductor (TI) – N-Channel Controllers for Constant Current LED Drivers

Functional Description
ENABLE
The LM3421/LM3423 devices impliment “zero-current” shut-
down via the EN and RPD pins. When pulled low, the EN pin
places the devices into a near-zero current draw state in
which only leakage currents will be observed flowing into the
pins of the LM3421/LM3423.
The RPD pin connects to an open drain N-channel MOSFET
that is only enabled when the device is enabled. Tying the
bottom resistor of external resistor dividers, namely V_IN Un-
der-Voltage Lock-Out (UVLO) and Over-Voltage Lock-Out
(OVLO), allows them to float during shutdown, thus removing
their current paths. In this way, the LED module can be de-
signed to draw zero current from the V_IN input supply line
when disabled. All other internal pin functions are also dis-
abled and draw zero current.
The EN pin should be tied to V_IN if the low current disable
function is not desired. This pin, being a micro-power enable,
is not a precision comparator input and is not appropriate for
implementing UVLO. The nDIM pin may be used for an ac-
curate V_IN UVLO function, as discussed in detail below in the
section titled External Under-Voltage Protection.
STARTUP REGULATOR (V_CC LDO)
The LM3421/LM3423 devices include a high voltage, low
dropout (LDO) bias regulator. When power is applied and the
EN pin is high, the regulator is enabled and sources current
into an external capacitor connected to the V_CC pin. The out-
put voltage is 6.9V nominally and the supply is internally
current limited to 20mA minimum. The recommended bypass
capacitance range for the V_CC regulator is 2.2µF to 3.3µF.
The output of the V_CC regulator is monitored by an internal
UVLO circuit. The purpose of V_CC UVLO is to protect the de-
vice during startup, normal operation, and shutdown from
attempting to operate with insufficient supply voltage. During
startup, the V_CC UVLO circuitry ensures that the device does
not begin switching until the V_CC voltage exceeds the upper
threshold in the hysteretic band of the V_CC UVLO threshold.
When V_IN is low, the low dropout regulator will drive V_CC to
within several hundred millivolts of V_IN. If during normal op-
eration V_CC falls below the V_CC UVLO threshold for any rea-
son, the V_CC UVLO circuitry will disable the device. In this
case, the device will not resume operation until the V_CC UVLO
release threshold voltage is exceeded. On-chip filtering pre-
vents intermittent transient dips that are common in high
speed switching regulators from triggering V_CC UVLO.
EXTERNAL UNDER-VOLTAGE PROTECTION
The nDIM pin is a dual-function input that features an accurate
1.24V threshold with programmable hysteresis. This pin func-
tions as both the PWM input for fast dimming of the LEDs and
as a V_IN UVLO. When the pin voltage rises and exceeds the
1.24V threshold, 23µA (typical) of current is driven out of the
nDIM pin into the resistor divider providing programmable
hysteresis. To calculate the amount of V_IN hysteresis
achieved, simply multiply the top resistor in the divider (R1 in
Figure 1) by 23µA (for a two resistor system) or the Thevenin
resistance by 23µA for any other network. Note that if the
Thevenin resistance is used in the calculation the result is the
amount of voltage hysteresis observed at the nDIM pin. This
quantity must be gained up by the appropriate resistor divider
attenuation factor to calculate the actual V_IN hysteresis ob-
served.
30067356
FIGURE 1. Under-Voltage Lock-Out Circuitry
Cycling the EN Pin Causes Escape from UVLO
When the EN and RPD pins are used together to implement
the “zero-current” shutdown function, they allow the resistor
divider (R1 and R2) on the nDIM to pull the pin up to V_IN. This
will appear as a legal operating voltage (nDIM > 1.24V). This
condition is removed as soon as the EN pin is taken back to
a high state. If the input voltage is inside the UVLO threshold
hysteretic window and the controller is off, cycling the EN pin
low and then high will start the controller even though the UV-
LO turn-on threshold has not been reached.
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