Datasheet

V
IN(MIN)
V
OUT
D
max
|
P
COND(MAX)
= I
2
D
MAX
1+
'i
(PK-PK)
> @
1
12
I
2
R
DS(ON)
V
OS(MAX)
-
V
OS(MAX)
2
- ('I
OUT(MAX)
x R
ESR
)
2
C
OUT(MIN)
=
(V
OUT
) R
ESR
2
(F)
L
'V
q
'V
r
'V
c
Time
Voltage
0
t
peak
LM3477
www.ti.com SNVS141K –OCTOBER 2000–REVISED MARCH 2013
Figure 30. Output Voltage Overshoot Peak
The intention is to find the capacitance value that will yield, at t
peak
, a ΔV
C
that equals V
OS(max)
. Substituting tpeak
for t and equating ΔV
C
to V
OS(max)
gives the following solution for C
OUT(MIN)
:
(29)
The chosen output capacitance should not be less than 47µF, even if the solution for C
OUT(MIN)
is less than 47µF.
Notice it is already assumed that the total ESR is no greater than R
ESR(MAX)
, otherwise the term under the square
root will be a negative number.
Power Mosfet Selection
The drive pin of LM3477/A must be connected to the gate of an external MOSFET. In a buck topology, the drain
of the external N-Channel MOSFET is connected to the input and the source is connected to the inductor. The
C
B
pin voltage provides the gate drive needed for an external N-Channel MOSFET. The gate drive voltage
depends on the input voltage (see Typical Performance Characteristics). In most applications, a logic level
MOSFET can be used. For very low input voltages, a sub-logic level MOSFET should be used.
The selected MOSFET directly controls the efficiency. The critical parameters for selection of a MOSFET are:
1. Minimum threshold voltage, V
TH
(MIN)
2. On-resistance, R
DS
(ON)
3. Total gate charge, Q
g
4. Reverse transfer capacitance, C
RSS
5. Maximum drain to source voltage, V
DS(MAX)
The off-state voltage of the MOSFET is approximately equal to the input voltage. V
DS(MAX)
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
DS(ON)
is needed to estimate the conduction losses. The conduction loss,
P
COND
, is the I
2
R loss across the MOSFET. The maximum conduction loss is given by:
(30)
where D
MAX
is the maximum operating duty cycle:
(31)
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
(32)
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
DR
(see Electrical Characteristics and Typical
Performance Characteristics for the drive voltage specification).
P
Drive
= F
S
.Q
g
.V
DR
(33)
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