Datasheet

LM3488
LM3488-Q1
www.ti.com SNVS089M –JULY 2000–REVISED MARCH 2013
POWER MOSFET SELECTION
The drive pin of LM3488 must be connected to the gate of an external MOSFET. In a boost topology, the drain of
the external N-Channel MOSFET is connected to the inductor and the source is connected to the ground. 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 directly controls the efficiency. The critical parameters for selection of 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. V
DS(MAX)
of the MOSFET must
be greater than the output voltage. The power losses in the MOSFET can be categorized into conduction losses
and ac switching or transition losses. R
DS(ON)
is needed to estimate the conduction losses. The conduction loss,
P
COND
, is the I
2
R loss across the MOSFET. The maximum conduction loss is given by:
where
• D
MAX
is the maximum duty cycle.
(26)
(27)
The turn-on and turn-off transitions of a MOSFET require times of tens of nano-seconds. C
RSS
and Q
g
are
needed to estimate the large instantaneous power loss that occurs during these transitions.
The amount of gate current required to turn the MOSFET on can be calculated using the formula:
I
G
= Q
g
.F
S
(28)
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
DR
(see Electrical Characteristics and Typical
Performance Characteristics for the drive voltage specification).
P
Drive
= F
S
.Q
g
.V
DR
(29)
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 33. 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:
(30)
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 V
IN
below 8 volts,
it is recommended to use a 20Ω resistor at the input to provide a RC filter. The resistor is placed in series with
the V
IN
pin with only a bypass capacitor attached to the V
IN
pin directly (see Figure 35). 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 with the input power supply.
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