LM2743MTC National Semiconductor, LM2743MTC Datasheet - Page 12

• Manufacturer Part Number: LM2743MTC
• Description: SYNC SW CONTROL, 3V:6V, POWERWISE
• Manufacturer: National Semiconductor

Specifications of LM2743MTC

• Primary Input Voltage: 16V
• No. Of Outputs: 1
• Output Voltage: 13.5V
• Output Current: 20A
• Voltage Regulator Case Style: TSSOP
• No. Of Pins: 14
• Operating Temperature Range: -40°C To +125°C
• Svhc: No SVHC
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

www.national.com
Application Information
If the tracking through SS/TRACK pin is not used, a capaci-
tor to ground is needed to limit the inrush current.
MOSFET GATE DRIVERS
The LM2743 has two gate drivers designed for driving
N-channel MOSFETs in a synchronous mode. Power for the
drivers is supplied through the BOOT pin. For the high side
gate (HG), to fully turn the top FET on, the BOOT voltage
must be at least one V
V
source or from a local charge pump structure, the bootstrap.
A charge pump can be built using a diodes and small ca-
pacitors, as shown in the below figures. 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 LM2743 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 cou-
pling.
The LM2743 can operate its internal circuitry at the V
range from 3.0V to 6.0V. However, the external FETs may
operate more efficiently with higher gate drive voltage. Fig-
ure 9 shows a typical bootstrap method where the voltage
applied to the FETs is V
IN
). This voltage can be supplied from a separate voltage
FIGURE 8. Timing Behavior of Case 3
GS(th)
CC
- V
greater than V
D
.
(Continued)
IN
(V
BOOT
20095211
≥ 2V +
CC
12
This means that if V
approximately 2.5V. This voltage could be too low to fully
turn the FET on. As a result I
expected.
In the next bootstrap configuration (Figure 10) the voltage
applied to the high side FET is V
FET is V
If V
pin and the low side FET can be driven fully on. The high
side FET however may have difficulty turning on since the
applied gate drive is only 2V in this case.
The Figure 11 shows next example of bootstrap configura-
tion. In this case the low gate drive voltage on the top FET is
resolved. Now the gate drive on both, the low and high, side
FETs is V
CC
and V
CC
FIGURE 10. Bootstrap Configuration 2
CC
FIGURE 9. Bootstrap Configuration 1
- 2V
IN
- 3V
are both 3.0V, then 5V is developed at BOOT
D
D
+ V
+ V
CC
IN
IN
is 3.0V, the gate drive for the FETs is
.
.
2
R losses could be higher than
CC
- 2V
D
and to the low side
20095213
20095212