LM3478MA_15 Datasheet, PDF (18/26 Pages) Texas Instruments – High-Efficiency Low-Side N-Channel Controller for Switching Regulator

LM3478MA
SNVS705B – FEBRUARY 2011 – REVISED FEBRUARY 2013
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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. Minimum threshold voltage, V_TH(MIN)
2. On-resistance, R_DS(ON)
3. Total gate charge, Q_g
4. Reverse transfer capacitance, C_RSS
5. Maximum drain to source voltage, V_DS(MAX)
The off-state voltage of the MOSFET is approximately equal to the output voltage. Vds(max) must be greater
than the output voltage. The power losses in the MOSFET can be categorized 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
(28)
The temperature effect on the R_DS(ON) usually is quite significant. 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 MOSFET's datasheet does not give enough information to yield a useful result. The following
formulas give a rough idea how the switching losses are calculated:
P_SW
I_Lmax
=
x V_out
2
x f_SW
x
(t_LH
+ t_HL)
(29)
Qgs
t_LH = Qgd + ₂
x ^RdrOn
Vdr - Vgs_th
(30)
INPUT CAPACITOR SELECTION
Due to the presence of an inductor at the input of a boost converter, the input current waveform is continuous
and triangular as shown in Figure 31. 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:
(31)
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 interactions. Therefore a good quality capacitor should
be chosen in the range of 10µF to 20µF. If a value lower than 10µF is used, then problems with impedance
interactions or switching noise can affect the LM3478. To improve performance, especially 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 capacitor attached to the VIN pin directly (see Figure 33). 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.
VIN
LM3478
RIN
CBYPASS
VIN
CIN
Figure 33. Reducing IC Input Noise
18
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