LM3478MM National Semiconductor, LM3478MM Datasheet - Page 15

• 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

Typical Applications
If V
given input voltage and output voltage, the current in the
inductor changes at a constant rate.
The important quantities in determining a proper inductance
value are I
inductor current ripple). If ∆i
current will drop to zero for a portion of the cycle and the
converter will operate in discontinuous conduction mode. If
∆i
zero and the converter will operate in continuous conduction
mode. All the analysis in this datasheet assumes operation
in continuous conduction mode. To operate in continuous
conduction mode, the following conditions must be met:
Choose the minimum I
common choice is to set ∆i
appropriate core size for the inductor involves calculating the
average and peak currents expected through the inductor. In
a boost converter,
and I
where
A core size with ratings higher than these values should be
chosen. If the core is not properly rated, saturation will
dramatically reduce overall efficiency.
The LM3478 can be set to switch at very high frequencies.
When the switching frequency is high, the converter can be
operated with very small inductor values. With a small induc-
tor value, the peak inductor current can be higher than the
output currents, especially under light load conditions.
The LM3478 senses the peak current through the switch.
The peak current through the switch is the same as the peak
current calculated above.
L
L
is smaller than I
(t) is constant, di
L_peak
L
= I
(the average inductor current) and ∆i
L
(max) + ∆i
L
L
, the inductor current will stay above
(t)/dt must be constant. Hence, for a
OUT
I
L
L
(max),
to determine the minimum L. A
L
>
L
is larger than I
∆i
to 30% of I
L
(Continued)
L
. Choosing an
L
, the inductor
L
(the
15
PROGRAMMING THE OUTPUT VOLTAGE AND OUTPUT
CURRENT
The output voltage can be programmed using a resistor
divider between the output and the feedback pins, as shown
in Figure 10. The resistors are selected such that the voltage
at the feedback pin is 1.26V. R
using the equation,
A 100pF capacitor may be connected between the feedback
and ground pins to reduce noise.
The maximum amount of current that can be delivered at the
output can be controlled by the sense resistor, R
limit occurs when the voltage that is generated across the
sense resistor equals the current sense threshold voltage,
V
trical characteristics. This can be expressed as:
V
as shown in Figure 2. This control signal, however, is not a
constant value and changes over the course of a period as a
result of the internal compensation ramp (see Figure 3).
Therefore the current limit will also change as a result of the
internal compensation ramp. The actual command signal,
V
voltage and the internal compensation ramp:
V
limits are specified in the electrical characteristics.
The peak current through the switch is equal to the peak
inductor current.
Therefore for a boost converter
Combining the three equation yields an expression for R
SENSE
SENSE
CS
SL
, can be better expressed as a function of the sense
is defined as the internal compensation ramp voltage,
. Limits for V
represents the maximum value of the control signal
V
I
sw(peak)
CS
SENSE
I
= V
sw(peak)
SENSE
*
R
have been specified in the elec-
SEN
= I
F1
− (D
L
= V
+ ∆i
and R
SENSE
*
L
V
SL
F2
)
can be selected
SEN
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. Current
SEN