LM2670SDX-3.3/NOPB National Semiconductor, LM2670SDX-3.3/NOPB Datasheet - Page 12

• Manufacturer Part Number: LM2670SDX-3.3/NOPB
• Description: IC REG STEP DOWN 3A 3.3V 14-LLP
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r
• Type: Step-Down (Buck)r

Specifications of LM2670SDX-3.3/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 3.3V
• Current - Output: 3A
• Frequency - Switching: 260kHz
• Voltage - Input: 8 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 14-LLP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: LM2670SDX-3.3

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The inductors offered have been specifically manufactured to
provide proper operation under all operating conditions of in-
put and output voltage and load current. Several part types
are offered for a given amount of inductance. Both surface
mount and through-hole devices are available. The inductors
from each of the three manufacturers have unique character-
istics.
Renco: ferrite stick core inductors; benefits are typically low-
est cost and can withstand ripple and transient peak currents
above the rated value. These inductors have an external
magnetic field, which may generate EMI.
Pulse Engineering: powdered iron toroid core inductors;
these also can withstand higher than rated currents and, be-
ing toroid inductors, will have low EMI.
Coilcraft: ferrite drum core inductors; these are the smallest
physical size inductors and are available only as surface
mount components. These inductors also generate EMI but
less than stick inductors.
OUTPUT CAPACITOR
The output capacitor acts to smooth the dc output voltage and
also provides energy storage. Selection of an output capaci-
tor, with an associated equivalent series resistance (ESR),
impacts both the amount of output ripple voltage and stability
of the control loop.
The output ripple voltage of the power supply is the product
of the capacitor ESR and the inductor ripple current. The ca-
pacitor types recommended in the tables were selected for
having low ESR ratings.
In addition, both surface mount tantalum capacitors and
through-hole aluminum electrolytic capacitors are offered as
solutions.
Impacting frequency stability of the overall control loop, the
output capacitance, in conjunction with the inductor, creates
a double pole inside the feedback loop. In addition the ca-
pacitance and the ESR value create a zero. These frequency
response effects together with the internal frequency com-
pensation circuitry of the LM2670 modify the gain and phase
shift of the closed loop system.
As a general rule for stable switching regulator circuits it is
desired to have the unity gain bandwidth of the circuit to be
limited to no more than one-sixth of the controller switching
frequency. With the fixed 260KHz switching frequency of the
LM2670, the output capacitor is selected to provide a unity
gain bandwidth of 40KHz maximum. Each recommended ca-
pacitor value has been chosen to achieve this result.
In some cases multiple capacitors are required either to re-
duce the ESR of the output capacitor, to minimize output
ripple (a ripple voltage of 1% of Vout or less is the assumed
performance condition), or to increase the output capacitance
to reduce the closed loop unity gain bandwidth (to less than
40KHz). When parallel combinations of capacitors are re-
quired it has been assumed that each capacitor is the exact
same part type.
The RMS current and working voltage (WV) ratings of the
output capacitor are also important considerations. In a typi-
cal step-down switching regulator, the inductor ripple current
(set to be no more than 30% of the maximum load current by
the inductor selection) is the current that flows through the
output capacitor. The capacitor RMS current rating must be
greater than this ripple current. The voltage rating of the out-
put capacitor should be greater than 1.3 times the maximum
output voltage of the power supply. If operation of the system
at elevated temperatures is required, the capacitor voltage
rating may be de-rated to less than the nominal room tem-
perature rating. Careful inspection of the manufacturer's
12
specification for de-rating of working voltage with temperature
is important.
INPUT CAPACITOR
Fast changing currents in high current switching regulators
place a significant dynamic load on the unregulated power
source. An input capacitor helps to provide additional current
to the power supply as well as smooth out input voltage vari-
ations.
Like the output capacitor, the key specifications for the input
capacitor are RMS current rating and working voltage. The
RMS current flowing through the input capacitor is equal to
one-half of the maximum dc load current so the capacitor
should be rated to handle this. Paralleling multiple capacitors
proportionally increases the current rating of the total capac-
itance. The voltage rating should also be selected to be 1.3
times the maximum input voltage. Depending on the unregu-
lated input power source, under light load conditions the
maximum input voltage could be significantly higher than nor-
mal operation and should be considered when selecting an
input capacitor.
The input capacitor should be placed very close to the input
pin of the LM2670. Due to relative high current operation with
fast transient changes, the series inductance of input con-
necting wires or PCB traces can create ringing signals at the
input terminal which could possibly propagate to the output or
other parts of the circuitry. It may be necessary in some de-
signs to add a small valued (0.1μF to 0.47μF) ceramic type
capacitor in parallel with the input capacitor to prevent or min-
imize any ringing.
CATCH DIODE
When the power switch in the LM2670 turns OFF, the current
through the inductor continues to flow. The path for this cur-
rent is through the diode connected between the switch output
and ground. This forward biased diode clamps the switch out-
put to a voltage less than ground. This negative voltage must
be greater than −1V so a low voltage drop (particularly at high
current levels) Schottky diode is recommended. Total effi-
ciency of the entire power supply is significantly impacted by
the power lost in the output catch diode. The average current
through the catch diode is dependent on the switch duty cycle
(D) and is equal to the load current times (1-D). Use of a diode
rated for much higher current than is required by the actual
application helps to minimize the voltage drop and power loss
in the diode.
During the switch ON time the diode will be reversed biased
by the input voltage. The reverse voltage rating of the diode
should be at least 1.3 times greater than the maximum input
voltage.
BOOST CAPACITOR
The boost capacitor creates a voltage used to overdrive the
gate of the internal power MOSFET. This improves efficiency
by minimizing the on resistance of the switch and associated
power loss. For all applications it is recommended to use a
0.01μF/50V ceramic capacitor.
SYNC COMPONENTS
When synchronizing the LM2670 with an external clock it is
recommended to connect the clock to pin 5 through a series
100pf capacitor and connect a 1KΩ resistor to ground from
pin 5. This RC network creates a short 100nS pulse on each
positive edge of the clock to reset the internal ramp oscillator.
The reset time of the oscillator is approximately 300nS.
ADDITIONAL APPLICATION INFORMATION
When the output voltage is greater than approximately 6V,
and the duty cycle at minimum input voltage is greater than