lm2746mxax National Semiconductor Corporation, lm2746mxax Datasheet - Page 13

• Manufacturer Part Number: lm2746mxax
• Description: Low Voltage N-channel Mosfet Synchronous Buck Regulator Controller
• Manufacturer: National Semiconductor Corporation

Application Information
Figure 9 shows a second possibility for bootstrapping the
MOSFET drives using a doubler. This circuit provides an
equal voltage drive of V
and low-side MOSFET drives. This method should only be
used in circuits that use 3.3V for both V
V
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
All the gate drive circuits shown in the above figures typically
use 100nF ceramic capacitors in the bootstrap locations.
IN
FIGURE 8. LM78L05 Feeding Basic Charge Pump
= V
FIGURE 9. Charge Pump with Added Gate Drive
CC
= 3.0V (10% lower tolerance on 3.3V) and V
CC
CC
or V
- 3V
IN
voltages over 5V.
D
+ V
IN
CC
to both the high-side
and V
(Continued)
IN
20147719
. Even with
20147713
D
=
13
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 kΩ 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
value (72 %x V
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
of about 300mV. If V
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
is a known value; hence the current through the MOSFET
can be determined as:
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
connected between the switching node and the I
constant current of 40 µA is forced through R
fixed voltage drop. This fixed voltage is compared against
V
been reached. To obtain a more accurate value for R
must consider the operating values of R
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
R
temperature and the absolute minimum value expected over
the full temperature range for the for the I
µA:
For example, a conservative 15A current limit in a 10A
design with a minimum R
resistor. The minimum value for R
kΩ. 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
DS
DSON
and if the latter is higher, the current limit of the chip has
value of the low side MOSFET at it’s expected hot
R
CS
FB
= R
= 0.42V) the Power Good flag goes low.
V
CC
DS
DSON-HOT
FB
drops below 2.42V, the chip enters
= I
DSON
= 0.708V) or falls 28% below that
OUT
of 10mΩ would require a 6kΩ
* R
x I
CC
CS
DSON
LIM
has a built in hysteresis
in any application is 1
DSON
/ 40 µA
DSON
SEN-TH
of the MOSFET
and I
CS
www.national.com
, causing a
which is 25
SEN
SEN-TH
CS
pin. A
you
CS
CC
CS
at
,