LM3478 MDC National Semiconductor, LM3478 MDC Datasheet - Page 15

• Manufacturer Part Number: LM3478 MDC
• Description: LM3478/LM3478Q High Efficiency Low-Side N-Channel Controller for Switching Regulator; ; Container: Tray
• Manufacturer: National Semiconductor

PROGRAMMING THE OUTPUT VOLTAGE AND OUTPUT
CURRENT
The output voltage can be programmed using a resistor di-
vider between the output and the FB pin. The resistors are
selected such that the voltage at the FB pin is 1.26V. Pick
R
pin) and R
ground) can be selected using the following equation,
A 100pF capacitor may be connected between the feedback
and ground pins to reduce noise.
CURRENT LIMIT WITH ONLY THE INTERNAL SLOPE
COMPENSATION
The maximum amount of current that can be delivered at the
output is controlled by the sense resistor, R
occurs when the voltage that is generated across the sense
resistor equals the current sense threshold voltage, V
Limits for V
acteristics section. This can be expressed as:
V
V
not a constant value and changes over the course of a period
as a result of the internal compensation ramp. Therefore the
current limit will also change as a result of the internal com-
pensation ramp. The actual V
a function of the sense voltage (V
pensation ramp:
Figure 10 shows how V
shows the ramp S
0% dutycycle) and by the internally generated slope V
which changes V
V
V
The graph also shows the increased current limit of 343 mV
(typical) during the 325 ns (typical) blank out time. For differ-
ent frequencies this fixed blank out time obviously occupies
more duty cycle, percentage wise.
The peak current through the switch is equal to the peak in-
ductor current.
SENSE
CS
SENSE
SENSE
F1
FIGURE 10. Current Sense Voltage vs Duty Cycle
(the resistor between the output voltage and the feedback
as shown in Figure 10. This control signal, however, is
. At 100% duty cycle, the current sense voltage will be
minus V
represents the maximum value of the control signal
F2
SENSE
R
(the resistor between the feedback pin and
F2
SL
= (1.26V x R
V
CS
V
have been specified in the electrical char-
e
.
SENSE
CS
which is defined by the voltage V
with duty cycle. The dotted line shows
= V
CS
= I
SENSE
changes with duty cycle. The curve
SW(PEAK)
F1
CS
) / (V
− (D x V
can be better expressed as
SENSE
x R
out
) and the internal com-
- 1.26V)
SEN
SL
)
SEN
. Current limit
10135552
SENSE
SENSE
(at
SL
.
15
The expression for R
The numerator of the above equation is the V
nominator is the peak current.
CURRENT LIMIT WITH THE INTERNAL SLOPE
COMPENSATION PLUS ADDITIONAL EXTERNAL SLOPE
COMPENSATION
If an external slope compensation resistor R
internal command signal will be modified and this will have an
effect on the current limit. The command signal then includes
the external slope ΔV
Where ΔV
as discussed in the section slope compensation ramp. This
changes the equation for R
Note that R
ting the current limit.
In some designs R
I
POWER DIODE SELECTION
Observation of the boost converter circuit shows that the av-
erage current through the diode is the average load current,
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:
Thermally the diode must be able to handle the maximum av-
erage current delivered to the output. The peak reverse volt-
age for boost converters is equal to the regulated 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 the LM3478 must be connected to the gate
of an external MOSFET. The drive pin (DR) 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 has a great influence on the
system efficiency. The critical parameters for selecting a
MOSFET are:
1.
2.
3.
4.
5.
SEN
pin quiet.
Minimum threshold voltage, V
On-resistance, R
Total gate charge, Q
Reverse transfer capacitance, C
Maximum drain to source voltage, V
SL
SL
is the additional slope compensation generated
V
which defines ΔV
CS
I
= V
D(Peak)
SL
SEN
SENSE
can also help to filter noise to keep the
DS(ON)
SL
:
= I
is:
g
– (D x (V
OUT
SEN
/ (1−D) + ΔI
to:
SL
is an additional way of set-
TH
SL
(MIN)
RSS
+ ΔV
DS(MAX)
L
SL
SL
))
CS
is used, the
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and the de-