LM2746 datasheet(13/24 Pages) NSC

13 / 24 page

Application Information (Continued)

Figure 9 shows a second possibility for bootstrapping the

MOSFET drives using a doubler. This circuit provides an

equal voltage drive of V

CC -3VD +VIN to both the high-side

and low-side MOSFET drives. This method should only be

used in circuits that use 3.3V for both V

CC and VIN. Even with

IN =VCC = 3.0V (10% lower tolerance on 3.3V) and VD =

0.5V both high-side and low-side gates will have at least

4.5V of drive. The power dissipation of the gate drive cir-

cuitry is directly proportional to gate drive voltage, hence the

thermal limits of the LM2746 IC will quickly be reached if this

circuit is used with V

CC or VIN voltages over 5V.

All the gate drive circuits shown in the above figures typically

use 100nF ceramic capacitors in the bootstrap locations.

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 recom-

mended for most applications.

The Power Good signal is an OR-gated flag which takes into

account both output overvoltage and undervoltage condi-

tions. If the feedback pin (FB) voltage is 18% above its

nominal value (118% x V

FB = 0.708V) or falls 28% below that

value (72 %x V

FB = 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 LM2746 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.

UVLO

The 2.76V turn-on threshold on V

CC has a built in hysteresis

of about 300mV. If V

CC drops below 2.42V, the chip enters

UVLO mode. UVLO consists of turning off the top and bot-

tom MOSFETS and remaining in that condition until V

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.

CURRENT LIMIT

Current limit is realized by sensing the voltage across the

low-side MOSFET while it is on. The R

DSON of the MOSFET

is a known value; hence the current through the MOSFET

can be determined as:

DS =IOUT *RDSON

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, R

CS,

connected between the switching node and the I

SEN pin. A

constant current of 40 µA is forced through R

CS, causing a

fixed voltage drop. This fixed voltage is compared against

DS and if the latter is higher, the current limit of the chip has

been reached. To obtain a more accurate value for R

CS you

must consider the operating values of R

DSON and ISEN-TH at

their operating temperatures in your application and the

effect of slight parameter differences from part to part. R

can be found by using the following equation using the

DSON 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 I

SEN-TH which is 25

µA:

CS =RDSON-HOT xILIM /40µA

For example, a conservative 15A current limit in a 10A

design with a minimum R

DSON of 10m

Ω would require a 6kΩ

resistor. The minimum value for R

CS in any application is 1

Ω. Because current sensing is done across the low-side

MOSFET, no minimum high-side on-time is necessary. The

LM2746 enters current limit mode if the inductor current

exceeds the current limit threshold at the point where the

high-side MOSFET turns off and the low-side MOSFET turns

on. (The point of peak inductor current, see Figure 10). Note

that in normal operation mode the high-side MOSFET al-

ways turns on at the beginning of a clock cycle. In current

limit mode, by contrast, the high-side MOSFET on-pulse is

skipped. This causes inductor current to fall. Unlike a normal

operation switching cycle, however, in a current limit mode

switching cycle the high-side MOSFET will turn on as soon

20147713

FIGURE 8. LM78L05 Feeding Basic Charge Pump

20147719

FIGURE 9. Charge Pump with Added Gate Drive

LM2746 датащи(PDF) 13 Page - National Semiconductor (TI)

LM2746 датащи(HTML) 13 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