Datasheet

Fc(s)
Power Stage
Compensator
Feedback
-
+
V
REF
V
C
V
O
+
-
Loop (T)
H
F
p
(s) x F
h
(s)
I
RMS_CIN
= I
OUT
V
OUT
(V
IN
-V
OUT
)
V
IN
LM3477
SNVS141K –OCTOBER 2000–REVISED MARCH 2013 www.ti.com
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
DR
in series with the boot-strap capacitor
(see Figure 25). 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 2.2Ω to 51Ω are recommended.
Power Diode Selection
The output current commutates through the diode when the external MOSFET turns off. The three most
important parameters 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) (34)
where D is the duty cycle and I
OUT
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.
(35)
The power dissipated in the input capacitor is given by:
P
D(CIN)
=I
RMS_CIN
2
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 capacitor. 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
D(CIN)
= (I
2
RMS_CIN
R
ESR_CIN
)/n
2
, where n is the total number of capacitors paralled at the input.
A 0.1µF or 1µF ceramic bypass capacitor is also recommended on the V
IN
pin (pin 8) of the IC. This capacitor
must be connected very close to pin 8.
Compensation
Figure 31. Control Block Diagram of a Current Mode Controlled Buck Converter
The LM3477/A is a current mode controller, therefore the control block diagram representation involves 2
feedback loops (see Figure 31). 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 simplified 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 compensation network.
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