LM3477EVAL National Semiconductor, LM3477EVAL Datasheet - Page 17

• Manufacturer Part Number: LM3477EVAL
• Description: BOARD EVALUATION LM3477
• Manufacturer: National Semiconductor

Specifications of LM3477EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• Current - Output: 1.6A
• Voltage - Input: 4.5 ~ 15V
• Regulator Topology: Buck
• Frequency - Switching: 500kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM3477
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

Output Capacitor Selection
The ESR and the capacitance of the output capacitor must
be carefully chosen so that the output voltage overshoot is
within the design’s specification V
bined ESR of the output capacitors is not low enough, the
initial output voltage excursion will violate the specification,
see ∆V
output capacitance, the output voltage will travel outside the
specification window due to the extra charge being dumped
into the capacitor, see ∆V
voltage protection (OVP) which could trigger if the transient
overshoot is high enough. If this happens, the controller will
operate in hysteretic mode (see OVER VOLTAGE PROTEC-
TION section) for a few cycles before the output voltage
settles to its steady state. If this behavior is not desired,
substitute V
found in the ELECTRICAL CHARACTERISTICS table) to
find the minimum capacitance and maximum ESR of the
output capacitor.
CALCULATIONS FOR THE OUTPUT CAPACITOR
During a loading transient, the delta output voltage ∆V
two changing components. One is the voltage difference
across the ESR (∆V
caused by the gained charge (∆V
The design objective is to keep ∆V
mum overshoot (V
the output load requirements.
Both voltages ∆V
equation is:
where,
R
∆I
I
D
Evaluating this equation at t = 0 gives ∆V
V
The expression for ∆V
OUT(MAX)
(Continued)
OS(MAX)
ESR
MIN
OUT
FIGURE 10. Output Voltage Overshoot Violation
= Minimum duty cycle of device (0.165 typical)
= the output capacitor ESR
= the difference between the load current change
C1
. If the ESR is low enough, but there is not enough
for ∆V
− I
OVP
OUT(MIN)
(referred to the output) for V
r(MAX)
r
and ∆V
OS(MAX)
r
∆V
), the other is the voltage difference
and solving for R
q
is:
c
C2
q
= ∆V
). V
will change with time. For ∆V
. The LM3477/A has output over
OS(MAX)
r
+ ∆V
q
OS(MAX)
). This gives:
c
lower than some maxi-
q
is chosen based on
ESR
r(max)
. If the total com-
OS(MAX)
gives:
. Substituting
200033B5
(V
OVP
c
r
has
the
is
17
From Figure 11 it can be told that ∆V
value at some point in time and then decrease. The larger
the output capacitance is, the earlier the peak will occur. To
find the peak position, let the derivative of ∆V
and the result is:
The intention is to find the capacitance value that will yield, at
t
equating ∆V
C
The chosen output capacitance should not be less than
47µF, even if the solution for C
Notice it is already assumed that the total ESR is no greater
than R
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
provides the gate drive needed for an external N-Channel
MOSFET. The gate drive voltage depends on the input volt-
age (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
2. On-resistance, R
3. Total gate charge, Q
4. Reverse transfer capacitance, C
5. Maximum drain to source voltage, V
The off-state voltage of the MOSFET is approximately equal
to the input voltage. V
greater than the input voltage. The power losses in the
peak
OUT(MIN)
, a ∆V
FIGURE 11. Output Voltage Overshoot Peak
ESR(MAX)
:
C
that equals V
C
to V
, otherwise the term under the square root
OS(max)
DS
(ON)
DS(MAX)
g
OS(max)
gives the following solution for
. Substituting tpeak for t and
OUT(MIN)
of the MOSFET must be
TH
(MIN)
RSS
C
DS(MAX)
will reach its peak
is less than 47µF.
C
B
www.national.com
go to zero,
pin voltage
200033B6