LM3687TL-1813 NSC [National Semiconductor], LM3687TL-1813 Datasheet - Page 16

• Manufacturer Part Number: LM3687TL-1813
• Description: Step-Down DC-DC Converter with Integrated Low Dropout Regulator and Startup Mode
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Hints
INDUCTOR SELECTION
There are two main considerations when choosing an induc-
tor; the inductor should not saturate, and the inductor current
ripple should be small enough to achieve the desired output
voltage ripple. Different saturation current rating specifica-
tions are followed by different manufacturers so attention
must be given to details. Saturation current ratings are typi-
cally specified at 25°C. However, ratings at the maximum
ambient temperature of application should be requested from
the manufacturer. The minimum value of inductance to
guarantee good performance is 1.76µH at I
current over the ambient temperature range. Shielded in-
ductors radiate less noise and should be preferred. There are
two methods to choose the inductor saturation current rating.
Method 1
The saturation current should be greater than the sum of the
maximum load current and the worst case average to peak
inductor current. This can be written as:
I
where
EXTERNAL CAPACITORS
As is common with most regulators, the LM3687 requires ex-
ternal capacitors to ensure stable operation. The LM3687 is
specifically designed for portable applications requiring mini-
mum board space and the smallest size components. These
capacitors must be correctly selected for good performance.
INPUT CAPACITOR SELECTION
V
A ceramic input capacitor of 4.7 µF, 6.3V is sufficient for most
applications. Place the input capacitor as close as possible to
the V
improved input voltage filtering. Use X7R or X5R types; do
not use Y5V. DC bias characteristics of ceramic capacitors
must be considered when selecting case sizes like 0805 and
0603. The minimum input capacitance to guarantee good
performance is 2.2µF at 3V dc bias; 1.5µF at 5V dc bias in-
cluding tolerances and over ambient temperature range. The
input filter capacitor supplies current to the PFET switch of the
LM3687 DC-DC converter in the first half of each cycle and
reduces voltage ripple imposed on the input power source. A
ceramic capacitor’s low ESR provides the best noise filtering
of the input voltage spikes due to this rapidly changing cur-
rent. Select a capacitor with sufficient ripple current rating.
The input current ripple can be calculated as:
SAT
BATT
> I
LPS3015-222ML
BATT
DO3314-222MX
NR3015T2R2M
OUT_DCDC_MAX
pin of the device. A larger value may be used for
Model
+ I
RIPPLE
TABLE 1. Suggested Inductors and their Suppliers
Taiyo Yuden
Coilcraft
Coilcraft
Vendor
LIM
(typ) dc
16
Dimensions LxWxH (mm)
•
•
•
•
•
Method 2
A more conservative and recommended approach is to
choose an inductor that has a saturation current rating greater
than the maximum current limit of 1380mA.
A 2.2 µH inductor with a saturation current rating of at least
1380mA is recommended for most applications. The
inductor’s resistance should be less than 0.3Ω for good effi-
ciency. Table 1 lists suggested inductors and suppliers. For
low-cost applications, an unshielded bobbin inductor could be
considered. For noise critical applications, a toroidal or shield-
ed- bobbin inductor should be used. A good practice is to lay
out the board with overlapping footprints of both types for de-
sign flexibility. This allows substitution of a low-noise shielded
inductor, in the event that noise from low-cost bobbin models
is unacceptable.
V
If the linear regulator is used as post regulation no additional
capacitor is needed at V
the DC-DC converter is close by and therefore sufficient.
In case of independent use, a 1.0µF ceramic capacitor is rec-
ommended at V
the V
tance of not more than 1 cm from the V
returned to a clean analogue ground. Any good quality ce-
ramic, tantalum, or film capacitor may be used at this input.
Important
Tantalum capacitors can suffer catastrophic failures due to
surge current when connected to a low-impedance source of
power (like a battery or a very large capacitor). If a tantalum
capacitor is used at this input, it must be guaranteed by the
IN_LIN
I
I
V
L: minimum inductor value including worst case tolerances
(30% drop can be considered for method 1)
f: minimum switching frequency (1.3MHz)
RIPPLE
OUT_DCDCMAX
BATT
IN_LIN
3.0 x 3.0 x 1.5
3.0 x 3.0 x 1.5
3.3 x 3.3 x 1.4
: maximum input voltage in application
: average to peak inductor current
supply path. This capacitor must be located a dis-
IN_LIN
: maximum load current (750mA)
if no other filter capacitor is present in
IN_LIN
as the output filter capacitor of
DCR (max)
IN_LIN
110mΩ
200mΩ
72mΩ
input pin and