LM3670MFX-2.5 National Semiconductor, LM3670MFX-2.5 Datasheet - Page 13

• Manufacturer Part Number: LM3670MFX-2.5
• Description: Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits
• Manufacturer: National Semiconductor

Application Information
OUTPUT VOLTAGE SELECTION FOR ADJUSTABLE
LM3670
The output voltage of the adjustable parts can be pro-
grammed through the resistor network connected from V
to V
to .5V. The resistor from V
100KΩ to keep the current sunk through this network well
below the 15µA quiescent current level (PFM mode with no
switching) but large enough that it is not susceptible to noise.
If R
through the resistor feedback network will be 2.5µA ( I
=.5V/R
For any output voltage greater than or equal to .8V a fre-
quency zero must be added at 10KHz for stability. The
formula is:
For output voltages between .7 and .8V a pole must also be
placed at 10KHz as well. The lowest output voltage possible
is .7V. At the low voltages the duty cycle is very small. In
addition, as the input voltage increases the duty cycle de-
creases even further. Since the duty cycle is so low any
change due to noise is an appreciable percentage. In other
words, it is susceptible to noise. The C
filters at this point rather than frequency poles and zeroes. If
tghe pole and zero are at the sasme frequency the formula
is:
A pole can be usesd at higher output voltages too. For
example, in the table "Adjustable LM3670 Configurations for
Various VOUT"Table 3 there is an entry for 1.24V with both a
pole and zero at approximately 10KHz for noise rejection.
INDUCTOR SELECTION
There are two main considerations when choosing an induc-
tor; the inductor current should not saturate, and the inductor
current ripple is small enough to achieve the desired output
voltage ripple.
There are two methods to choose the inductor current rating.
Method 1:
The total current is the sum of the load and the inductor
ripple current. This can be written as
• V
• V
• R
• R
2
FB
OUT
FB
1
2
is 200KΩ, and given the V
Resistor from V
Resistor from V
the to GND. V
2
). The output voltage formula is:
Feedback Voltage (.5V typ)
Output Voltage (V)
OUT
OUT
OUT
will be adjusted to make V
FB
to V
to GND (Ω)
to GND (R2) should be at least
FB
FB
(Ω)
is .5V, then the current
1
and C
2
act as noise
FB
equal
OUT
FB
13
Method 2:
A more conservative approach is to choose an inductor that
can handle the current limit of 700 mA.
Given a peak-to-peak current ripple (I
to be at least
A 10 µH inductor with a saturation current rating of at least
800 mA is recommended for most applications. The induc-
tor’s resistance should be less than around 0.3Ω for good
efficiency. Table 1 lists suggested inductors and suppliers.
For low-cost applications, an unshielded bobbin inductor is
suggested. For noise critical applications, a toroidal or
shielded-bobbin inductor should be used. A good practice is
to lay out the board with overlapping footprints of both types
for design flexibility. This allows substitution of a low-noise
toroidal inductor, in the event that noise from low-cost bobbin
models is unacceptable.
INPUT CAPACITOR SELECTION
A ceramic input capacitor of 4.7 µF is sufficient for most
applications. A larger value may be used for improved input
voltage filtering. The input filter capacitor supplies current to
the PFET switch of the LM3670 in the first half of each cycle
and reduces voltage ripple imposed on the input power
source. A ceramic capcitor’s low ESR provides the best
noise filtering of the input voltage spikes due to this rapidly
changing current. Select an input filter capacitor with a surge
current rating sufficient for the power-up surge from the input
power source. The power-up surge current is approximately
the capacitor’s value (µF) times the voltage rise rate (V/µs).
The input current ripple can be calculated as:
• I
• V
• L inductor
• f switching frequency
• I
LOAD
RIPPLE
IN
input voltage
load current
peak-to-peak
PP
) the inductor needs
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