參數(shù)資料
型號(hào)： LM2744
廠(chǎng)商： National Semiconductor Corporation
英文描述： Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller with External Reference
中文描述： 低壓N溝道MOSFET同步降壓穩(wěn)壓控制器與外部參考
文件頁(yè)數(shù)： 18/22頁(yè)
文件大?。?/td> 663K
代理商： LM2744
Application Information
(Continued)
In practice, a good trade off between phase margin and
bandwidth can be obtained by selecting the closest
±
10%
capacitor values above what are suggested for C
and C
,
the closest
±
10% capacitor value below the suggestion for
C
, and the closest
±
1% resistor values below the sugges-
tions for R
, R
C2
. Note that if the suggested value for R
is
less than 100
, it should be replaced by a short circuit.
Following this guideline, the compensation components will
be:
C
C1
= 27pF
±
10%, C
C2
= 820pF
±
10%
C
C3
= 2.7nF
±
10%, R
C1
= 39.2k
±
1% R
C2
= 2.55k
±
1%
The transfer function of the compensation block can be
derived by considering the compensation components as
impedance blocks Z
F
and Z
I
around an inverting op-amp:
As with the generic equation, G
must be modified
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.
H = G
PS
x H
EA
The bandwidth and phase margin can be read graphically
from Bode plots of H
EA
are shown in
Figure 14
.
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
OUT
) loss and the Total Power (P
TOTAL
) loss:
The Output Power (P
OUT
) for theTypical Application Circuit
design is (1.2V * 4A) = 4.8W. The Total Power (P
TOTAL
), with
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
SW
) and conduction losses(P
CND
).
P
FET
= P
SW
+ P
CND
= 61.38mW + 270.42mW
P
FET
= 331.8mW
FET Switching Loss (P
SW
)
P
SW
= P
SW(ON)
+ P
SW(OFF)
20106085
20106086
FIGURE 14. Overall Loop Gain and Phase
L
www.national.com
18
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