LM2743MTCSLASHNOPB datasheet(16/44 Pages) TI2

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BOOT

VIN

VCC

LM2743

SNVS276I – APRIL 2004 – REVISED FEBRUARY 2019

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Feature Description (continued)

Figure 27. Charge Pump with Added Gate Drive

All the gate drive circuits shown in Figure 25 through Figure 27 typically use 100-nF ceramic capacitors in the

bootstrap locations.

7.3.8 Power Good Signal

The open drain output on the Power Good pin needs a pull-up resistor to a low voltage source. The pull-up

resistor should be chosen so that the current going into the Power Good pin is less than 1 mA. A 100-k

Ω resistor

is recommended for most applications.

The Power Good signal is an OR-gated flag which takes into account both output over-voltage and under-voltage

conditions. If the feedback pin (FB) voltage is 18% above its nominal value (118% x VFB = 0.708V) or falls 28%

below that value (72 %x VFB = 0.42V) the Power Good flag goes low. The Power Good flag can be used to signal

other circuits that the output voltage has fallen out of regulation, however the switching of the LM2743 continues

regardless of the state of the Power Good signal. The Power Good flag will return to logic high whenever the

feedback pin voltage is between 72% and 118% of 0.6V.

7.3.9 UVLO

The 2.76V turn-on threshold on VCC has a built in hysteresis of about 300 mV. If VCC drops below 2.42V, the chip

enters UVLO mode. UVLO consists of turning off the top and bottom MOSFETS and remaining in that condition

until VCC rises above 2.76V. As with shutdown, the soft-start capacitor is discharged through an internal

MOSFET, ensuring that the next start-up will be controlled by the soft-start circuitry.

7.3.10 Current Limit

Current limit is realized by sensing the voltage across the low-side MOSFET while it is on. The RDS(ON) of the

MOSFET is a known value; hence the current through the MOSFET can be determined as:

VDS = IOUT x RDS(ON)

(8)

The current through the low-side MOSFET while it is on is also the falling portion of the inductor current. The

current limit threshold is determined by an external resistor, RCS, connected between the switching node and the

ISEN pin. A constant current of 40 µA is forced through RCS, causing a fixed voltage drop. This fixed voltage is

compared against VDS and if the latter is higher, the current limit of the chip has been reached. To obtain a more

accurate value for RCS you must consider the operating values of RDS(ON) and ISEN-TH at their operating

temperatures in your application and the effect of slight parameter differences from part to part. RCS can be found

by using the following equation using the RDS(ON) value of the low side MOSFET at it's expected hot temperature

and the absolute minimum value expected over the full temperature range for the for the ISEN-TH which is 25 µA:

RCS = RDSON-HOT x ILIM / 40 µA

(9)

LM2743MTC/NOPB Datasheet(PDF) 16 Page - Texas Instruments

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