參數(shù)資料
型號(hào)： LM3477AMM
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 穩(wěn)壓器
英文描述： High Efficiency High-Side N-Channel Controller for Switching Regulator
中文描述： 1 A SWITCHING CONTROLLER, 575 kHz SWITCHING FREQ-MAX, PDSO8
封裝： MSOP-8
文件頁(yè)數(shù)： 18/23頁(yè)
文件大?。?/td> 815K
代理商： LM3477AMM
Power MOSFET Selection
(Continued)
3.
4.
5.
The off-state voltage of the MOSFET is approximately equal
to the input voltage. V
of the MOSFET must be
greater than the input voltage. The power losses in the
MOSFET can be categorized into conduction losses and ac
switching or transition losses. R
is needed to estimate
the conduction losses. The conduction loss, P
, is the
I
2
R loss across the MOSFET. The maximum conduction loss
is given by:
Total gate charge, Q
g
Reverse transfer capacitance, C
RSS
Maximum drain to source voltage, V
DS(MAX)
where D
MAX
is the maximum operating duty cycle:
The turn-on and turn-off transition times of a MOSFET from
the MOSFET specifications require tens of nano-seconds.
C
RSS
and Q
g
are needed from the MOSFET specifications to
estimate the large instantaneous power loss that occurs
during these transitions.
The average amount of gate current required to turn the
MOSFET on can be calculated using the formula:
I
G
= Q
g
.F
S
The required gate drive power to turn the MOSFET on is
equal to the switching frequency times the energy required
to deliver the charge to bring the gate charge voltage to V
(see
ELECTRICAL CHARACTERISTICS
and
TYPICAL
PERFORMANCE CHARACTERISTICS
for the drive voltage
specification).
P
Drive
= F
S
.Q
g
.V
DR
It is sometimes helpful or necessary to slow down the turn on
transition of the FET so that less switching noise appears at
the I
SEN
pin. This can be done by inserting a drive resistor
R
Figure 6
).
This can help reduce sensing noise that may be preventing
designs from operating at or near the LM3477/A’s minimum
duty cycle limit. Gate drive resistors from 2.2
to 51
are
recommended.
Power Diode Selection
The output current commutates through the diode when the
external MOSFET turns off. The three most important pa-
rameters for the diode are the peak current, peak inverse
voltage, and average power dissipation. Exceeding these
ratings can cause damage to the diode. The average current
through the diode is given by:
I
D(AVG)
= I
OUT
x (1-D)
where D is the duty cycle and I
is the output current. The
diode must be rated to handle this current.
The off-state voltage across the diode in a buck converter is
approximately equal to the input voltage. The peak inverse
voltage rating of the diode must be greater than the off-state
voltage of the diode. To improve efficiency, a low forward
drop schottky diode is recommended.
Input Capacitor Selection
In a buck converter, the high side switch draws large ripple
currents from the input capacitor. The input capacitor must
be rated to handle this RMS current.
The power dissipated in the input capacitor is given by:
P
D(CIN)
=I
RMS_CIN2
R
ESR_CIN
,
where R
ESR_CIN
is the ESR of the input capacitor. The input
capacitor must be selected to handle the rms current and
must be able to dissipate the power. P
D(CIN)
must be lower
than the rated power dissipation of the selected input capaci-
tor. In many cases, several capacitors have to be paralleled
to handle the rms current. In that case, the power dissipated
in each capacitor is given by:
P
= (I
2RMS_CIN
R
)/n
2
, where n is the total num-
ber of capacitors paralled at the input.
A 0.1μF or 1μF ceramic bypass capacitor is also recom-
mended on the V
pin (pin 8) of the IC. This capacitor must
be connected very close to pin 8.
Compensation
The LM3477/A is a current mode controller, therefore the
control block diagram representation involves 2 feedback
loops (see
Figure 12
). The inner feedback loop derives its
feedback from the sensed inductor current, while the outer
loop monitors the output voltage. This section will not give a
rigorous analysis of current mode control, but rather a sim-
plified but accurate method to determine the compensation
network. The first part reveals the results of the model, giving
expressions for solving for component values in the compen-
sation network.
The compensation network is designed around the power
components, or the power stage. An isolated schematic of
the error amplifier and the various compensation compo-
nents is shown in
Figure 13
. The error amplifier in conjunc-
tion with the compensation network makes up the compen-
sator block in
Figure 12
. The purpose of the compensator
block is to stabilize the control loop and achieve high perfor-
mance in terms of the transient response, audio susceptibil-
ity and output impedance.
20003391
FIGURE 12. Control Block Diagram of a Current Mode
Controlled Buck Converter
L
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