lm2746mxax National Semiconductor Corporation, lm2746mxax Datasheet - Page 19

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

Application Information
In practice, a good trade off between phase margin and
bandwidth can be obtained by selecting the closest
capacitor values above what are suggested for C
the closest
C
tions for R
less than 100Ω, it should be replaced by a short circuit.
Following this guideline, the compensation components will
be:
C
C
R
The transfer function of the compensation block can be
derived by considering the compensation components as
impedance blocks Z
As with the generic equation, G
to take into account the limited bandwidth of the error ampli-
fier. The result is:
The total control loop transfer function H is equal to the
power stage transfer function multiplied by the error amplifier
transfer function.
The bandwidth and phase margin can be read graphically
from Bode plots of H
C3
C1
C3
C2
, and the closest
= 27pF
= 2.7nF
= 2.55kΩ
C1
±
, R
±
10% capacitor value below the suggestion for
±
10%, C
±
10%, R
C2
1%
. Note that if the suggested value for R
F
±
EA
C2
and Z
C1
H = G
1% resistor values below the sugges-
are shown in Figure 15.
= 820pF
= 39.2kΩ
I
PS
around an inverting op-amp:
x H
EA-ACTUAL
±
EA
±
10%
1%
(Continued)
must be modified
C1
and C
±
10%
C2
C2
is
,
19
The bandwidth of this example circuit is 59kHz, with a phase
margin of 60˚.
EFFICIENCY CALCULATIONS
The following is a sample calculation.
A reasonable estimation of the efficiency of a switching buck
controller can be obtained by adding together the Output
Power (P
The Output Power (P
design is (1.2V x 4A) = 4.8W. The Total Power (P
an efficiency calculation to complement the design, is shown
below.
The majority of the power losses are due to low and high
side of MOSFET’s losses. The losses in any MOSFET are
group of switching (P
FET Switching Loss (P
P
FIGURE 15. Overall Loop Gain and Phase
FET
OUT
= P
) loss and the Total Power (P
SW
P
SW
+ P
SW
P
= P
OUT
FET
CND
) and conduction losses(P
SW
SW(ON)
) for theTypical Application Circuit
= 331.8mW
)
= 61.38mW + 270.42mW
+ P
SW(OFF)
TOTAL
20147785
20147786
) loss:
TOTAL
www.national.com
CND
), with
).