LM3424 datasheet(21/69 Pages) TI1

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HYSO

- OFF

TURN

24V

OV1

OV2

- OFF

TURN

24V

CMP

VCC

CMP

VCC

20 k

CMP

CMP - SS

0.70V

LED

LM3424, LM3424-Q1

www.ti.com

SNVS603C – AUGUST 2009 – REVISED AUGUST 2015

Feature Description (continued)

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:

(23)

If CSS is greater than 40% of CCMP, the compensation capacitor will only charge to 0.7V over a smaller CCMP

charging time (tCMP-SS) which can be estimated as:

(24)

Then COMP will clamp to SS, forcing COMP to rise (the last 200 mV before switching begins) according to the

CSS charging time (tSS) which can be estimated as:

(25)

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

CSS < 0.4 x CCMP

(26)

CSS > 0.4 x CCMP

(27)

As a general rule of thumb, standard smooth startup operation can be achieved with CSS = CCMP.

7.3.9 Overvoltage Lockout (OVLO)

The LM3424 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 20 µA (typical) of hysteresis current as shown in Figure 28. When the

OVLO threshold is exceeded, the GATE pin is immediately pulled low and a 20 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 29.

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

Ground Referenced

(28)

Floating

(29)

In the ground referenced configuration, the voltage across ROV2 is VO - 1.24 V whereas in the floating

configuration it is VO - 620 mV where 620 mV approximates VBE of the PNP.

The over-voltage hysteresis (VHYSO) is defined:

(30)

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