LM3488QMM National Semiconductor, LM3488QMM Datasheet - Page 16

• Manufacturer Part Number: LM3488QMM
• Description: LM3488/LM3488Q High Efficiency Low-Side N-Channel Controller for Switching Regulators; Package: MINI SOIC; No of Pins: 8; Container: Reel
• Manufacturer: National Semiconductor

www.national.com
will operate in discontinuous conduction mode. If Δi
er than I
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 Δi
propriate 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 dra-
matically reduce overall efficiency.
The LM3488 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 extremely higher
than the output currents, especially under light load condi-
tions.
The LM3488 senses the peak current through the switch. The
peak current through the switch is the same as the peak cur-
rent calculated above.
PROGRAMMING THE OUTPUT VOLTAGE AND OUTPUT
CURRENT
The output voltage can be programmed using a resistor di-
vider between the output and the feedback pins, as shown in
L_peak
L
, the inductor current will stay above zero and the
= I
L
(max) + Δi
OUT
L
(max),
I
L
L
to determine the minimum L. A
> Δi
to 30% of I
L
L
. Choosing an ap-
L
is small-
16
Figure 12. The resistors are selected such that the voltage at
the feedback pin is 1.26V. R
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
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
age and the internal compensation ramp:
V
limits are specified in the electrical characteristics.
The peak current through the switch is equal to the peak in-
ductor 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 volt-
is defined as the internal compensation ramp voltage,
. Limits for V
represents the maximum value of the control signal
V
I
sw(peak)
SENSE
CS
I
= V
sw(peak)
have been specified in the electrical
SENSE
* R
F1
SEN
= I
and R
− (D * V
L
= V
+ Δi
F2
SENSE
L
can be selected using
SL
)
SEN
. Current
SEN