LM3488QMM National Semiconductor, LM3488QMM Datasheet - Page 18

• 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
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:
where D
The turn-on and turn-off transitions of a MOSFET require
times of tens of nano-seconds. C
estimate the large instantaneous power loss that occurs dur-
ing these transitions.
The amount of gate current required to turn the MOSFET on
can be calculated using the formula:
The required gate drive power to turn the MOSFET on is equal
to the switching frequency times the energy required to deliver
the charge to bring the gate charge voltage to V
trical characteristics and typical performance characteristics
for the drive voltage specification).
INPUT CAPACITOR SELECTION
Due to the presence of an inductor at the input of a boost
converter, the input current waveform is continuous and tri-
angular, as shown in Figure 11. The inductor ensures that the
input capacitor sees fairly low ripple currents. However, as the
input capacitor gets smaller, the input ripple goes up. The rms
current in the input capacitor is given by:
The input capacitor should be capable of handling the rms
current. Although the input capacitor is not as critical in a
boost application, low values can cause impedance interac-
tions. Therefore a good quality capacitor should be chosen in
Designing SEPIC Using LM3488
Since the LM3488 controls a low-side N-Channel MOSFET,
it can also be used in SEPIC (Single Ended Primary Induc-
tance Converter) applications. An example of SEPIC using
LM3488 is shown in Figure 14. As shown in Figure 14, the
output voltage can be higher or lower than the input voltage.
The SEPIC uses two inductors to step-up or step-down the
input voltage. The inductors L1 and L2 can be two discrete
inductors or two windings of a coupled transformer since
equal voltages are applied across the inductor throughout the
switching cycle. Using two discrete inductors allows use of
catalog magnetics, as opposed to a custom transformer. The
input ripple can be reduced along with size by using the cou-
pled windings of transformer for L1 and L2.
Due to the presence of the inductor L1 at the input, the SEPIC
inherits all the benefits of a boost converter. One main ad-
2
R loss across the MOSFET. The maximum conduction loss
MAX
is the maximum duty cycle.
P
Drive
I
G
= F
= Q
S
g
.Q
DS(ON)
.F
RSS
g
S
.V
DR
and Q
is needed to estimate
g
are needed to
DR
COND
(see elec-
, is the
18
the range of 100µF to 200µF. If a value lower than 100µF is
used, then problems with impedance interactions or switching
noise can affect the LM3478. To improve performance, es-
pecially with V
20Ω resistor at the input to provide a RC filter. The resistor is
placed in series with the V
attached to the V
ceramic capacitor is necessary in this configuration. The bulk
input capacitor and inductor will connect on the other side of
the resistor with the input power supply.
OUTPUT CAPACITOR SELECTION
The output capacitor in a boost converter provides all the out-
put current when the inductor is charging. As a result it sees
very large ripple currents. The output capacitor should be ca-
pable of handling the maximum rms current. The rms current
in the output capacitor is:
Where
and D, the duty cycle is equal to (V
The ESR and ESL of the output capacitor directly control the
output ripple. Use capacitors with low ESR and ESL at the
output for high efficiency and low ripple voltage. Surface
Mount tantalums, surface mount polymer electrolytic and
polymer tantalum, Sanyo- OSCON, or multi-layer ceramic ca-
pacitors are recommended at the output.
vantage of SEPIC over boost converter is the inherent input
to output isolation. The capacitor CS isolates the input from
the output and provides protection against shorted or mal-
functioning load. Hence, the A SEPIC is useful for replacing
boost circuits when true shutdown is required. This means
that the output voltage falls to 0V when the switch is turned
off. In a boost converter, the output can only fall to the input
voltage minus a diode drop.
The duty cycle of a SEPIC is given by:
In the above equation, V
FET, Q, and V
FIGURE 13. Reducing IC Input Noise
DIODE
IN
IN
below 8 volts, it is recommended to use a
pin directly (see Figure 13). A 0.1µF or 1µF
is the forward voltage drop of the diode.
Q
is the on-state voltage of the MOS-
IN
pin with only a bypass capacitor
OUT
− V
IN
10138893
)/V
OUT
.