LM3478_09 Datasheet, PDF (16/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulator

The off-state voltage of the MOSFET is approximately equal
to the output voltage. Vds(max) must be greater than the out-
put voltage. The power losses in the MOSFET can be cate-
gorized into conduction losses and switching losses. Rds(on)
is needed to estimate the conduction losses, Pcond:
Pcond = I² x R_DS(ON) x D x f_S
The temperature effect on the R_DS(ON) usually is quite signif-
icant. Assume 30% increase at hot.
For the current I in the formula above the average inductor
current may be used.
Especially at high switching frequencies the switching losses
may be the largest portion of the total losses.
The switching losses are very difficult to calculate due to
changing parasitics of a given MOSFET in operation. Often
the individual MOSFETS datasheet does not give enough in-
formation to yield a useful result. The following formulas give
a rough idea how the switching losses are calculated:
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 9. 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
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 Vin below 8 volts, it is recommended to use a 20
Ohm resistor at the input to provide an RC filter. The resistor
is placed in series with the VIN pin with only a bypass capac-
itor attached to the VIN pin directly (see figure 11). A 0.1µF
or 1µF ceramic capacitor is necessary in this configuration.
The bulk input capacitor and inductor will connect on the other
side of the resistor at 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
The ESR and ESL of the 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 tanta-
lums, surface mount polymer electrolytic, polymer tantalum,
or multi-layer ceramic capacitors are recommended at the
output.
For applications that require very low output voltage ripple, a
second stage LC filter often is a good solution. Most of the
time it is lower cost to use a small second Inductor in the
power path and an additional final output capacitor than to
reduce the output voltage ripple by purely increasing the out-
put capacitor without an additional LC filter.
LAYOUT GUIDELINES
Good board layout is critical for switching controllers. First the
ground plane area must be sufficient for thermal dissipation
purposes and second, appropriate guidelines must be fol-
lowed to reduce the effects of switching noise. Switching
converters are very fast switching devices. In such devices,
the rapid increase of input current combined with the parasitic
trace inductance generates unwanted Ldi/dt noise spikes.
The magnitude of this noise tends to increase as the output
current increases. This parasitic spike noise may turn into
electromagnetic interference (EMI), and can also cause prob-
lems in device performance. Therefore, care must be taken
in layout to minimize the effect of this switching noise. The
current sensing circuit in current mode devices can be easily
affected by switching noise. This noise can cause duty cycle
jittering which leads to increased spectral noise. Although the
LM3478 has 325ns blanking time at the beginning of every
cycle to ignore this noise, some noise may remain after the
blanking time.
The most important layout rule is to keep the AC current loops
as small as possible. Figure 12 shows the current flow of a
boost converter. The top schematic shows a dotted line which
represents the current flow during on-state and the middle
schematic shows the current flow during off-state. The bottom
schematic shows the currents we refer to as AC currents.
They are the most critical ones since current is changing in
very short time periods. The dotted lined traces of the bottom
schematic are the once to make as short as possible.
10135593
FIGURE 11. Reducing IC Input Noise
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