LM2743MTCX/NOPB National Semiconductor, LM2743MTCX/NOPB Datasheet - Page 16

• 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

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voltage ripple (ΔV
load transients.
In this example the output current is 4A and the expected type
of capacitor is an aluminum electrolytic, as with the input ca-
pacitors. Other possibilities include ceramic, tantalum, and
solid electrolyte capacitors, however the ceramic type often
do not have the large capacitance needed to supply current
for load transients, and tantalums tend to be more expensive
than aluminum electrolytic. Aluminum capacitors tend to have
very high capacitance and fairly low ESR, meaning that the
ESR zero, which affects system stability, will be much lower
than the switching frequency. The large capacitance means
that at the switching frequency, the ESR is dominant, hence
the type and number of output capacitors is selected on the
basis of ESR. One simple formula to find the maximum ESR
based on the desired output voltage ripple, ΔV
designed output current ripple, ΔI
In this example, in order to maintain a 2% peak-to-peak output
voltage ripple and a 40% peak-to-peak inductor current ripple,
the required maximum ESR is 20 mΩ. The Sanyo 4SP560M
electrolytic capacitor will give an equivalent ESR of 14 mΩ.
The capacitance of 560 µF is enough to supply energy even
to meet severe load transient demands.
MOSFETs
Selection of the power MOSFETs is governed by a tradeoff
between cost, size, and efficiency. One method is to deter-
mine the maximum cost that can be endured, and then select
the most efficient device that fits that price. Breaking down the
losses in the high-side and low-side MOSFETs and then cre-
ating spreadsheets is one way to determine relative efficien-
cies between different MOSFETs. Good correlation between
the prediction and the bench result is not guaranteed, how-
ever. Single-channel buck regulators that use a controller IC
and discrete MOSFETs tend to be most efficient for output
currents of 2A to 10A.
Losses in the high-side MOSFET can be broken down into
conduction loss, gate charging loss, and switching loss. Con-
duction loss, or I
In the above equations the factor 1.3 accounts for the in-
crease in MOSFET R
1.3 can be ignored and the R
using the R
datasheets.
Gate charging loss results from the current driving the gate
capacitance of the power MOSFETs, and is approximated as:
where ‘n’ is the number of MOSFETs (if multiple devices have
been placed in parallel), V
FET Gate Drivers section) and Q
MOSFET. If different types of MOSFETs are used, the ‘n’ term
can be ignored and their gate charges simply summed to form
a cumulative Q
P
DSON
C
= (1 - D) x ((I
P
G
2
C
P
. Gate charge loss differs from conduction
R loss, is approximately:
OUT
Vs. Temperature curves in the MOSFET
GC
= D ((I
(High-Side MOSFET)
(Low-Side MOSFET)
) and to supply load current during fast
= n x (V
DSON
O
DD
)
2
due to heating. Alternatively, the
O
x R
is the driving voltage (see MOS-
DSON
)
DD
2
x R
DSON-HI
) x Q
GS
OUT
of the MOSFET estimated
DSON-LO
is the gate charge of the
G
, is:
x f
x 1.3)
SW
x 1.3)
OUT
and the
16
and switching losses in that the actual dissipation occurs in
the LM2743, and not in the MOSFET itself.
Switching loss occurs during the brief transition period as the
high-side MOSFET turns on and off, during which both current
and voltage are present in the channel of the MOSFET. It can
be approximated as:
where t
Switching loss occurs in the high-side MOSFET only.
For this example, the maximum drain-to-source voltage ap-
plied to either MOSFET is 3.6V. The maximum drive voltage
at the gate of the high-side MOSFET is 3.1V, and the maxi-
mum drive voltage for the low-side MOSFET is 3.3V. Due to
the low drive voltages in this example, a MOSFET that turns
on fully with 3.1V of gate drive is needed. For designs of 5A
and under, dual MOSFETs in SO-8 package provide a good
trade-off between size, cost, and efficiency.
Support Components
C
be placed as close as possible to the drain of the high-side
MOSFET and source of the low-side MOSFET (dual MOS-
FETs make this easy). This capacitor should be X5R type
dielectric or better.
R
ensure smooth DC voltage for the chip supply. R
1Ω to 10Ω. C
C
R
drain power good signal (PWGD). The recommended value
is 10 kΩ connected to V
resistor can be omitted.
D
It allows for a minimum drop for both high and low-side
drivers. The MBR0520 or BAT54 work well in most designs.
R
calls for a peak current magnitude (I
4.8A, a safe setting would be 6A. (This is below the saturation
current of the output inductor, which is 7A.) Following the
equation from the Current Limit section, a 1.3 kΩ resistor
should be used.
R
the chip. The resistor value is calculated from equation in
Normal Operation section. For 300 kHz operation, a 97.6
kΩ resistor should be used.
C
ments and is calculated based on the equation given in the
section titled START UP/SOFT-START. Therefore, for a 700
μs delay, a 12 nF capacitor is suitable.
Control Loop Compensation
The LM2743 uses voltage-mode (‘VM’) PWM control to cor-
rect changes in output voltage due to line and load transients.
One of the attractive advantages of voltage mode control is
its relative immunity to noise and layout. However VM re-
quires careful small signal compensation of the control loop
for achieving high bandwidth and good phase margin.
The control loop is comprised of two parts. The first is the
power stage, which consists of the duty cycle modulator, out-
put inductor, output capacitor, and load. The second part is
the error amplifier, which for the LM2743 is a 9 MHz op-amp
used in the classic inverting configuration.
the regulator and control loop components.
IN
CC
BOOT
PULL-UP
1
CS
FADJ
SS
- A small Schottky diode should be used for the bootstrap.
2 - A small value (0.1 µF to 1 µF) ceramic capacitor should
, C
- Resistor used to set the current limit. Since the design
- The soft-start capacitor depends on the user require-
- Bootstrap capacitor, typically 100 nF.
- This resistor is used to set the switching frequency of
CC
R
- These are standard filter components designed to
and t
– This is a standard pull-up resistor for the open-
P
CC
SW
F
should 1 µF, X5R type or better.
are the rise and fall times of the MOSFET.
= 0.5 x V
CC
. If this feature is not necessary, the
IN
x I
O
x (t
r
+ t
OUT
f
) x f
+ (0.5 x ΔI
Figure 13
SW
CC
should be
OUT
shows
)) of