LM3687 Datasheet, PDF (23/35 Pages) National Semiconductor (TI) – Step-Down DC-DC Converter with Integrated Low Dropout Regulator and Startup Mode

LM3687
www.ti.com
SNVS473A – DECEMBER 2007 – REVISED JULY 2008
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 efficiency. 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 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 shielded inductor,
in the event that noise from low-cost bobbin models is unacceptable.
Model
NR3015T2R2M
LPS3015-222ML
DO3314-222MX
Table 1. Suggested Inductors and their Suppliers
Vendor
Taiyo Yuden
Coilcraft
Coilcraft
Dimensions LxWxH (mm)
3.0 x 3.0 x 1.5
3.0 x 3.0 x 1.5
3.3 x 3.3 x 1.4
DCR (max)
72mΩ
110mΩ
200mΩ
EXTERNAL CAPACITORS
As is common with most regulators, the LM3687 requires external capacitors to ensure stable operation. The
LM3687 is specifically designed for portable applications requiring minimum board space and the smallest size
components. These capacitors must be correctly selected for good performance.
INPUT CAPACITOR SELECTION
V_BATT
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_BATT pin of the device. A larger value may be used for 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 including 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 current. Select a capacitor with sufficient ripple current
rating. The input current ripple can be calculated as:
I_RMS = I_OUTMAX x
¨§ ¸· ^{VOUT x}
1-
V_OUT
r²
+
© ¹ ^VBATT
V_BATT 12
(V_BATT - V_OUT) x V_OUT
r=
L x f x I_OUTMAX x V_BATT
The worst case is when V_BATT = 2 x V_OUT
(2)
V_{IN_LIN}
If the linear regulator is used as post regulation no additional capacitor is needed at V_{IN_LIN} as the output filter
capacitor of the DC-DC converter is close by and therefore sufficient.
In case of independent use, a 1.0µF ceramic capacitor is recommended at V_{IN_LIN} if no other filter capacitor is
present in the V_{IN_LIN} supply path. This capacitor must be located a distance of not more than 1 cm from the
V_{IN_LIN} input pin and returned to a clean analogue ground. Any good quality ceramic, 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 manufacturer to have a surge current rating sufficient for the application.
Copyright © 2007–2008, Texas Instruments Incorporated
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