LM3555TLE Datasheet, PDF (27/33 Pages) Texas Instruments – LM3555 Synchronous Boost Converter with 500 mA High Side LED Driver and Dual-Mode Control Interface

LM3555
www.ti.com
SNVS594E – JANUARY 2009 – REVISED NOVEMBER 2011
INDUCTOR SELECTION
The LM3555 is designed to use a 2.2 µH inductor. When the device is boosting (V_OUT > V_IN) the inductor is one
of the biggest sources of efficiency loss in the circuit. Therefore, choosing an inductor with the lowest possible
series resistance is important. Additionally, the saturation rating of the inductor should be greater than the
maximum operating peak current of the LM3555. This prevents excess efficiency loss that can occur with
inductors that operate in saturation and prevents over heating of the inductor and possible damage. For proper
inductor operation and circuit performance ensure that the inductor saturation and the peak current limit setting of
the LM3555 (1.25A, 1.5A, 1.75A or 2A) is greater than I_PEAK. I_PEAK can be calculated by:
I_LOAD V_OUT
I_PEAK = _K u _VIN + 'I_L
where
( ) ^VIN ^{u V}OUT ^{- V}IN
'I_L = _{2 u fSW u L u VOUT}
Manufacturer
Toko
Coilcraft
TDK
Table 1. Recommended Inductors
Part#
FDSE312-2R2M
LPS4012-222ML
VLF4014ST-2R2M1R9
L / I_SAT
2.2 µH / 2.3A
2.2 µH / 2.3A
2.2 µH / 2.0A
CAPACITOR SELECTION
The LM3555 requires 2 external capacitors for proper operation (C_IN = 10 µF recommended (4.7 µF min.) and
C_OUT = 10 µF ). An additional 0.1 µF input capacitor placed right next to the VIN pin is recommended. Surface-
mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very
low equivalent series resistance (ESR <20 mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum
electrolytic capacitors are not recommended for use with the LM3555 due to their high ESR, as compared to
ceramic capacitors.
For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with
the LM3555. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over
temperature (X7R: ±15% over −55°C to 125°C; X5R: ±15% over −55°C to 85°C).
Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM3555.
Capacitors with these temperature characteristics typically have wide capacitance tolerance (+80%, -20%) and
vary significantly over temperature (Y5V: +22%, -82% over −30°C to +85°C range; Z5U: +22%, -56% over +10°C
to +85°C range). Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a capacitance of only
0.1 µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum
capacitance requirements of the LM3555.
The recommended voltage rating for the input capacitor is 10V (min = 6.3V). The recommended output
capacitor voltage rating is 16V (min = 10V). The recommended value takes into account the DC bias
capacitance losses, while the minimum rating takes into account the OVP trip levels.
LAYOUT CONSIDERATIONS
The micro SMD is a chip-scale package with good thermal properties. For more detailed instructions on handling
and mounting micro SMD packages, please refer to Texas Instruments Application Note AN-1112.
The high switching frequencies and large peak currents make the PCB layout a critical part of the design. The
proceeding steps must be followed to ensure stable operation and proper current source regulation.
1. Connect the inductor as close as possible to the SW pin. This reduces the inductance and resistance of the
switching node which minimizes ringing and excess voltage drops.
2. Connect the return terminals of the input capacitor and the output capacitor as close as possible to the two
ground pins (PGND and SGND) and through low impedance traces.
3. Bypass V_IN with a 10 µF ceramic capacitor and an additional 0.1 µF ceramic capacitor. Connect the positive
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