LM3370SD-3021/NOPB National Semiconductor, LM3370SD-3021/NOPB Datasheet - Page 22

• Manufacturer Part Number: LM3370SD-3021/NOPB
• Description: IC CONV DC/DC DUAL STPDN 16-LLP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Step-Down (Buck)r

Specifications of LM3370SD-3021/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 1.2V, 3.3V
• Current - Output: 600mA
• Frequency - Switching: 2MHz
• Voltage - Input: 2.7 ~ 5.5 V
• Operating Temperature: -30°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-LLP
• Power - Output: 1.54W
• For Use With: LM3370SD-3021EV - BOARD EVALUATION LM3370SD-3021
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM3370SD-3021TR

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bias characteristics vary from manufacturer to manufacturer
and dc bias curves should be requested from them as part of
the capacitor selection process.
The output filter capacitor smoothes out current flow from the
inductor to the load, helps maintain a steady output voltage
during transient load changes and reduces output voltage
ripple. These capacitors must be selected with sufficient ca-
pacitance and sufficiently low ESR to perform these functions.
The output ripple voltage can be calculated as:
Voltage peak-to-peak ripple due to capacitance =
Voltage peak-to-peak ripple due to ESR = V
Voltage peak-to-peak ripple, root mean squared =
Note that the output ripple is dependent on the current ripple
and the equivalent series resistance of the output capacitor
(R
ature dependent); make sure that the frequency of the R
given is the same order of magnitude as the switching fre-
quency.
POR (POWER ON RESET)
The LM3370 has an independent POR functions (nPOR) for
each buck converter. The nPOR1 and nPOR2 are open drain
circuits which pull low when the outputs are below 94% (rising
4.7 µF for C
10µF C
TABLE 3. Suggested Capacitors and Their Suppliers
ESR
GRM21BR60J475
GRM21BR60J106
C1608X5R0J475
C2012X5R0J475
C2012X5R0J106
GRM219R60J-
GRM219R60J-
JMK212BJ475
JMK212BJ106
). The R
475KE19D
106KE19D
OUT
Model
IN
ESR
is frequency dependent (as well as temper-
Ceramic, X5R,
6.3V Rating
Ceramic, X5R,
6.3V Rating
Ceramic, X5R,
6.3V Rating
Ceramic, X5R,
6.3V Rating
Ceramic, X5R,
6.3V Rating
Ceramic, X5R,
6.3V Rating
Ceramic, X5R,
6.3V Rating
Ceramic, X5R,
6.3V Rating
Ceramic, X5R,
6.3V Rating
Description
< 1mm
<1mm
Height
Height
(Thin)
(Thin)
Case
0603
0805
0805
0805
0805
0805
0805
0805
0805
Size
PP-ESR
= I
Vendor
muRata
muRata
Yuden
Yuden
PP
Taiyo
Taiyo
TDK
TDK
*R
ESR
ESR
22
V
delay between the output (at 94% of V
the nPOR is enabled is about 50 ms. A pull up resistor of 100
kΩ at nPOR pin is required. Please refer to the electrical
specification table for other timing options. The diagram be-
low illustrates the timing response of the POR function.
SPREAD SPECTRUM (SS)
The LM3370 features Spread Spectrum capability, via I
reduce the noise amplitude of the switching frequency during
data transmission. The feature can be enabled by activating
the appropriate control register bit (see register information
section for detail). The main clock of the LM3370 features
spread spectrum at F
deviation) with the modulation frequency of either 1 kHz(de-
fault) or 2 kHz via I
harmonics present in the waveforms at the switch pins of the
buck regulators. It is controlled by I
BOARD LAYOUT CONSIDERATIONS
PC board layout is an important part of DC-DC converter de-
sign. Poor board layout can disrupt the performance of a DC-
DC converter and surrounding circuitry by contributing to EMI,
ground bounce, and resistive voltage loss in the traces. These
can send erroneous signals to the DC-DC converter IC, re-
sulting in poor regulation or instability.
Good layout for the LM3370 can be implemented by following
a few simple design rules:
1.
2.
3.
I
SS_fmod = 1 (default)
SS_fmod = 0
2
OUT
C bit
Place the LM3370, inductor and filter capacitors close
together and make the traces short. The traces between
these components carry relatively high switching
currents and act as antennas. Following this rule reduces
radiated noise. Place the capacitors and inductor within
0.2 in. (5mm) of the LM3370.
Arrange the components so that the switching current
loops curl in the same direction. During the first half of
each cycle, current flows from the input filter capacitor,
through the LM3370 and inductor to the output filter
capacitor and back through ground, forming a current
loop. In the second half of each cycle, current is pulled
up from ground, through the LM3370 by the inductor, to
the output filter capacitor and then back through ground,
forming a second current loop. Routing these loops so
the current curls in the same direction prevents magnetic
field reversal between the two half-cycles and reduces
radiated noise.
Connect the ground pins of the LM3370, and filter
capacitors together using generous component-side
) or 85% (falling V
2
C. This help reduce noise caused by the
OSC
OUT
= 2 MHz ± 22 kHz ( peak frequency
) of the desire output. The inherent
Modulation Frequency
1 kHz
2 kHz
2
C in the following manner:
OUT
) to the time at which
20167319
2
C, to