LM3421 datasheet(19/65 Pages) TI1

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- OFF

TURN

24V

1.24V

23 PA

LM3421/23

ROV2

ROV1

OVLO

OVP

CMP

VCC

LED

CMP

0.9V

BYP

VCC

168

LM3421, LM3421-Q1

LM3423, LM3423-Q1

www.ti.com

SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011

START-UP REGULATOR

The LM3421/23 includes a high voltage, low dropout bias regulator. When power is applied, the regulator is

enabled and sources current into an external capacitor (CBYP) connected to the VCC pin. The recommended

bypass capacitance for the VCC regulator is 2.2 µF to 3.3 µF. The output of the VCC regulator is monitored by an

internal UVLO circuit that protects the device from attempting to operate with insufficient supply voltage and the

supply is also internally current limited. Figure 25 shows the typical start-up waveforms for the LM3421/23.

First, CBYP is charged to be above VCC UVLO threshold (~4.2V). The CVCC charging time (tVCC) can be estimated

as:

(22)

CCMP is then charged to 0.9V over the charging time (tCMP) which can be estimated as:

(23)

Once CCMP = 0.9V, the part starts switching to charge CO until the LED current is in regulation. The CO charging

time (tCO) can be roughly estimated as:

(24)

The system start-up time (tSU) is defined as:

(25)

In some configurations, the start-up waveform will overshoot the steady state COMP pin voltage. In this case, the

LED current and output voltage will overshoot also, which can trip the over-voltage or protection, causing a race

condition. The easiest way to prevent this is to use a larger compensation capacitor (CCMP), thereby slowing

down the control loop.

OVER-VOLTAGE LOCKOUT (OVLO)

Figure 26. Over-Voltage Protection Circuitry

The LM3421/23 can be configured to detect an output (or input) over-voltage condition via the OVP pin. The pin

features a precision 1.24V threshold with 23 µA (typical) of hysteresis current as shown in Figure 26. When the

OVLO threshold is exceeded, the GATE pin is immediately pulled low and a 23 µA current source provides

hysteresis to the lower threshold of the OVLO hysteretic band.

If the LEDs are referenced to a potential other than ground (floating), as in the buck-boost and buck

configuration, the output voltage (VO) should be sensed and translated to ground by using a single PNP as

shown in Figure 27.

The over-voltage turn-off threshold (VTURN-OFF) is defined:

Ground Referenced

(26)

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Product Folder Links: LM3421 LM3421-Q1 LM3423 LM3423-Q1

LM3421 Datasheet(PDF) 19 Page - Texas Instruments

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