LM2743MTC National Semiconductor, LM2743MTC Datasheet - Page 14

• 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

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Application Information
Input Capacitor
The input capacitors in a Buck switching converter are sub-
jected to high stress due to the input current square wave-
form. Hence input capacitors are selected for their ripple
current capability and their ability to withstand the heat gen-
erated as that ripple current runs through their ESR. Input
rms ripple current is approximately:
where D is the duty cycle.
The power dissipated by each input capacitor is:
where, n is the number of capacitors, and ESR is the equiva-
lent series resistance of C
The equation indicates that power loss in each capacitor
decreases rapidly as the number of input capacitors in-
creases. The worst-case ripple for a Buck converter occurs
during full load and when the duty cycle (D) is 0.5. For design
3.3V (V
maximum load the ripple current is around 2A. The Sanyo
20SP120M aluminum electrolytic capacitor works fine here.
It has a ripple current rating of 3A and maximum ESR of
24mΩ at 100kHz. The power dissipated by the Sanyo’s
capacitor is then 0.088W. Other options for input and output
capacitors include MLCC, Tantalum, OSCON, SP, and POS-
CAPS.
Support Components: Capacitors (C
C
Schottky Diode (D
C
pass device designed to filter harmonics of the switching
frequency and input noise. 0.1µF - 1µF ceramic capacitor
with a sufficient voltage rating will work well in almost any
case.
R
tors are standard filter components designed to ensure
smooth DC voltage for the chip supply and for the bootstrap
structure, if it is used. Recommended values: R
C
R
drain power good signal (PWGD). The recommended value:
100Ω: connected to V
resistor can be omitted.
D
allows the minimum drop for both, high and low side drivers.
The MBR0520 works well here.
R
calls for a peak current magnitude (I
a safe setting would be 6A. (This is below the saturation
current of the output inductor, which is 7.8 A.) Following the
equation from the Current Limit section, use a 1.5kΩ resistor.
R
(F
equation in Normal Operation section. To obtain the switch-
ing frequency of 300kHz, 110kΩ, 1% resistor is needed.
SS
IN2
CC
CC
PULL-UP
1
CS
FADJ
OSC
- Schottky diode should be used for the bootstrap. It
), Resistors (R
, C
= 0.1µF, and C
- resistor used to set the current limit. Since the design
- the MOSFET’s input capacitor is high frequency by-
) of the chip. The resistor value is calculated from
- this resistor is used to set the switching frequency
CC
CC
, and C
) to 1.2V (V
– this is a standard pull-up resistor for the open-
BOOT
BOOT
CC
1
)
OUT
CC
, R
- bypass resistor and bypass capaci-
. If this feature is not necessary, the
= 0.1µF.
) the duty cycle is 0.364. With a 4A
CS
IN1
, R
.
FADJ
, R
OUT
PULL-UP
IN2
+0.5*∆I
, C
(Continued)
CC
), and
OUT
, C
CC
BOOT
) of 4.8A,
= 10Ω,
,
14
C
ments and is calculated based on the equation from the Start
Up section. For a 7ms delay, a 12nF capacitor will be suit-
able.
Output Inductor
The output inductor forms the first half of the power stage in
a Buck converter. It is responsible for smoothing the square
wave created by the switching action and for controlling the
output current ripple (∆I
selecting between tradeoffs in efficiency and response time.
The smaller the output inductor, the more quickly the con-
verter can respond to transients in the load current. How-
ever, as shown in the efficiency calculations, a smaller in-
ductor requires a higher switching frequency to maintain the
same level of output current ripple. An increase in frequency
can mean increasing loss in the FETs due to the charging
and discharging of the gates. Generally the switching fre-
quency is chosen so that conduction loss outweighs switch-
ing loss. The equation for output inductor selection is:
Plugging in the values for output current ripple, input voltage,
output voltage, switching frequency, and assuming a 40%
peak-to-peak output current ripple yields an inductance of
1.6µH. The output inductor must be rated to handle the peak
current (also equal to the peak switch current), which is (I
+ 0.5*∆I
222P is 2.2µH, is rated to 7.4A rms, and has a direct current
resistance (DCR
Output Capacitor
The output capacitor forms the second half of the power
stage of a Buck switching converter. It is used to control the
output voltage ripple (∆V
during fast load transients.
In this example the output current is 4A and the expected
type of capacitor is an aluminum electrolytic, as with the
input capacitors. Other possibilities include ceramic, tanta-
lum, 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 ca-
pacitors 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 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, ∆V
ripple, ∆I
SS
- the soft start capacitor depends on the user require-
OUT
OUT
) 4.8A for a 4A design. The Coilcraft DO3316P-
, is:
IOUT
) of 11mΩ.
OUT
OUT
L = 16µH
OUT
). The inductance is chosen by
and the designed output current
) and to supply load current
OUT