LM3421 datasheet(10/24 Pages) NSC

10 / 24 page

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 VCC to

within several hundred millivolts of V

IN. If during normal op-

eration V

CC falls below the VCC 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

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.

LM3421 датащи(PDF) 10 Page - National Semiconductor (TI)

LM3421 датащи(HTML) 10 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

датащи скачать

ссылки URL