LM2743MTCX/NOPB National Semiconductor, LM2743MTCX/NOPB Datasheet - Page 12

• Manufacturer Part Number: LM2743MTCX/NOPB
• Description: IC REG CTLR BUCK N-CH 14-TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Step-Down (Buck)r

Specifications of LM2743MTCX/NOPB

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 13.5 V
• Current - Output: 20A
• Frequency - Switching: 50kHz ~ 1MHz
• Voltage - Input: 1 ~ 16 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 14-TSSOP
• For Use With: LM2743EVAL - BOARD EVALUATION LM2743LM2743-19AEVAL - BOARD EVALUATION LM2743-19A
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: LM2743MTCX

www.national.com
SD PIN IMPEDANCE
When connecting a resistor divider to the SD pin of the
LM2743 some care has to be taken. Once the SD voltage
goes above V
shown in
hysteresis (
will affect the SD pin logic thresholds as well. The external
impedance used for the sequencing divider network should
preferably be a small fraction of the impedance of the SD pin
for good performance (around 1kΩ).
MOSFET GATE DRIVERS
The LM2743 has two gate drivers designed for driving N-
channel MOSFETs in a synchronous mode. Note that unlike
most other synchronous controllers, the bootstrap capacitor
of the LM2743 provides power not only to the driver of the
upper MOSFET, but the lower MOSFET driver too (both
drivers are ground referenced, i.e. no floating driver). To fully
turn the top MOSFET on, the BOOT voltage must be at least
one gate threshold greater than V
Figure
≊
FIGURE 5. Delay for Sequencing
170 mV); however, high external impedances
SD-IH
6. This current is used to create the internal
FIGURE 6. SD Pin Logic
, a 17 µA pull-up current is activated as
IN
when the high-side drive
20095206
20095211
12
goes high. This bootstrap voltage is usually supplied from a
local charge pump structure. But looking at the Typical Appli-
cation schematic, this also means that the difference voltage
V
charges up to, must be always greater than the maximum tol-
erance limit of the threshold voltage of the upper MOSFET.
Here V
diode D1. This therefore may place restrictions on the mini-
mum input voltage and/or type of MOSFET used.
The most basic charge bootstrap pump circuit can be built
using one Schottky diode and a small capacitor, as shown in
Figure
voltage between the top MOSFET gate and source to control
the device even when the top MOSFET is on and its source
has risen up to the input voltage level. The charge pump cir-
cuitry is fed from V
3.0V to 6.0V. Using this basic method the voltage applied to
the gates of both high-side and low-side MOSFETs is V
V
the gate drives will get at least 4.0V of drive voltage during
the worst case of V
level MOSFETs generally specify their on-resistance at V
= 4.5V. When V
could go as low as 2.5V. Logic level MOSFETs are not guar-
anteed to turn on, or may have much higher on-resistance at
2.5V. Sub-logic level MOSFETs, usually specified at V
2.5V, will work, but are more expensive, and tend to have
higher on-resistance. The circuit in
input voltages ranging from 1V up to 16V and V
because the drive voltage depends only on V
Note that the LM2743 can be paired with a low cost linear
regulator like the LM78L05 to run from a single input rail be-
tween 6.0 and 14V. The 5V output of the linear regulator
powers both the V
cient drive for logic level MOSFETs. An example of this circuit
is shown in
CC
D
. This method works well when V
- V
FIGURE 7. Basic Charge Pump (Bootstrap)
D1
7. The capacitor C
D1
is the forward voltage drop across the bootstrap
, which is the voltage the bootstrap capacitor
Figure
CC
CC
= 3.3V±10%, the gate drive at worst case
8.
CC
CC-MIN
and the bootstrap circuit, providing effi-
, which can operate over a range from
= 4.5V and V
BOOT
serves to maintain enough
CC
Figure 7
is 5V±10%, because
D-MAX
CC
CC
works well for
= 0.5V. Logic
.
= 5V ±10%,
20095212
GS
CC
GS
=
-