LM3478MAX/NOPB National Semiconductor, LM3478MAX/NOPB Datasheet - Page 17

• Manufacturer Part Number: LM3478MAX/NOPB
• Description: IC CTRLR SW REG N-CH 8SOIC
• Manufacturer: National Semiconductor
• Series: -r

Specifications of LM3478MAX/NOPB

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 1MHz
• Duty Cycle: 100%
• Voltage - Supply: 2.97 V ~ 40 V
• Buck: No
• Boost: Yes
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

This changes the equation for current limit (or R
The R
in order to achieve both the desired current limit and stable
operation. In some designs R
on the ISEN pin.
If the inductor is selected such that ripple current is the rec-
ommended 30% value, and the current limit threshold is 120%
of the maximum peak, a simpler method can be used to de-
termine R
bility without RSL, provided that the above 2 conditions are
met:
POWER DIODE SELECTION
Observation of the boost converter circuit shows that the av-
erage current through the diode is the average load current,
and the peak current through the diode is the peak current
through the inductor. The diode should be rated to handle
more than its peak current. The peak diode current can be
calculated using the formula:
Thermally the diode must be able to handle the maximum av-
erage current delivered to the output. The peak reverse volt-
age for boost converters is equal to the regulated output
voltage. The diode must be capable of handling this voltage.
To improve efficiency, a low forward drop schottky diode is
recommended.
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.
2.
3.
4.
5.
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:
The temperature effect on the R
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.
Minimum threshold voltage, V
On-resistance, R
Total gate charge, Q
Reverse transfer capacitance, C
Maximum drain to source voltage, V
SEN
and R
SEN
. The equation below will provide optimum sta-
SL
Pcond = I
I
D(Peak)
values may have to be calculated iteratively
DS(ON)
= I
g
2
OUT
x R
SL
/ (1−D) + ΔI
DS(ON)
DS(ON)
can also help to filter noise
TH
(MIN)
RSS
x D x f
usually is quite signif-
DS(MAX)
L
S
SEN
) to:
17
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.
FIGURE 11. Reducing IC Input Noise
10135593
www.national.com