LM3478MM National Semiconductor, LM3478MM Datasheet - Page 16

• Manufacturer Part Number: LM3478MM
• Description: Pulse Width Modulation (PWM) Controller IC
• Manufacturer: National Semiconductor

Specifications of LM3478MM

• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Mounting Type: Surface Mount
• Features: High Efficiency
• Package / Case: 8-MSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Typical Applications
CURRENT LIMIT WITH ADDITIONAL SLOPE
COMPENSATION
If an external slope compensation resistor is used (see
Figure 4) the internal control signal will be modified and this
will have an effect on the current limit. The control signal is
given by:
Where V
trical characteristics section. If R
to the existing slope compensation. The command voltage
will then be given by:
Where ∆V
and can be calculated by use of Figure 5 or is equal to 40 x
10
Therefore R
for setting the current limit.
POWER DIODE SELECTION
Observation of the boost converter circuit shows that the
average current through the diode is the average load cur-
rent, and the peak current through the diode is the peak
current through the inductor. The diode should be rated to
handle more than its peak current. The peak diode current
can be calculated using the formula:
−6
*
R
SL
SENSE
SL
. This changes the equation for R
V
SL
CS
is the additional slope compensation generated
can be used to provide an additional method
and V
= V
I
V
D(Peak)
CS
SENSE
= V
SL
= I
are defined parameters in the elec-
SENSE
− (D
OUT
/ (1−D) + ∆I
*
− (D
SL
( V
is used, then this will add
SL
(Continued)
*
V
+ ∆V
SL
FIGURE 10. Adjusting the Output Voltage
)
L
SL
SEN
) )
to:
16
In the above equation, I
been defined in Figure 9.
The peak reverse voltage for boost converter is equal to the
regulator output voltage. The diode must be capable of
handling this voltage. To improve efficiency, a low forward
drop schottky diode is recommended.
POWER MOSFET SELECTION
The drive pin of LM3478 must be connected to the gate of an
external MOSFET. In a boost topology, the drain of the
external N-Channel MOSFET is connected to the inductor
and the source is connected to the ground. The drive pin
(DR) voltage depends on the input voltage (see typical per-
formance 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 output voltage. V
greater than the output voltage. The power losses in the
MOSFET can be categorized into conduction losses and ac
switching or transition losses. R
the conduction losses. The conduction loss, P
I
is given by:
2
R loss across the MOSFET. The maximum conduction loss
DS
OUT
(ON)
g
10135520
DS(MAX)
is the output current and ∆I
DS(ON)
of the MOSFET must be
TH
(MIN)
RSS
is needed to estimate
DS(MAX)
COND
, is the
L
has