LM2742 datasheet(13/33 Pages) TI

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BOOT

LM2742

CBOOT

VCC

LM2742

www.ti.com

SNVS266C – MARCH 2004 – REVISED MARCH 2013

Figure 29. BOOT Supplied by Charge Pump

In a system without a separate, higher voltage, a charge pump (bootstrap) can be built using a diode and small

capacitor, Figure 29. The capacitor serves to maintain enough voltage between the top FET gate and source to

control the device even when the top FET is on and its source has risen up to the input voltage level.

The LM2742 gate drives use a BiCMOS design. Unlike some other bipolar control ICs, the gate drivers have rail-

to-rail swing, ensuring no spurious turn-on due to capacitive coupling.

POWER GOOD SIGNAL

The power good signal is the or-gated flag representing over-voltage and under-voltage protection. If the output

voltage is 18% over it's nominal value, VFB = 0.7V, or falls 30% below that value, VFB = 0.41V, the power good

flag goes low. It will return to a logic high whenever the feedback pin voltage is between 70% and 118% of 0.6V.

The power good pin is an open drain output that can be pulled up to logic voltages of 5V or less with a 10k

Ω

resistor.

UVLO

The 4.2V turn-on threshold on VCC has a built in hysteresis of 0.6V. Therefore, if VCC drops below 3.6V, the chip

enters UVLO mode. UVLO consists of turning off the top FET, turning off the bottom FET, and remaining in that

condition until VCC rises above 4.2V. As with shutdown, the soft start capacitor is discharged through a FET,

ensuring that the next start-up will be smooth.

CURRENT LIMIT

Current limit is realized by sensing the voltage across the low side FET while it is on. The RDSON of the FET is a

known value, hence the current through the FET can be determined as:

VDS = I * RDSON

(4)

The current through the low side FET while it is on is also the falling portion of the triangle wave inductor current.

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

the ISEN pin. A constant current of 50 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. RCS can be found

by using the following equation:

RCS = RDSON(LOW) * ILIM/50µA

(5)

For example, a conservative 15A current limit in a 10A design with a minimum RDSON of 10mΩ would require a

3.3k

Ω resistor. Because current sensing is done across the low side FET, no minimum high side on-time is

necessary. In the current limit mode the LM2727/37 will turn the high side off and the keep low side on for as

long as necessary. The LM2727/37 enters current limit mode if the inductor current exceeds the current limit

threshold at the point where the high side FET turns off and the low side FET turns on. (The point of peak

inductor current. See Figure 30.) Note that in normal operation mode the high side FET always turns on at the

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