LTC3555-X_15 Datasheet, PDF (26/32 Pages) Linear Technology – High Efficiency USB Power Manager Triple Step-Down DC/DC

LTC3555/LTC3555-X
APPLICATIONS INFORMATION
electrical characteristics. Inductors that are very thin or
have a very small volume typically have much higher core
and DCR losses, and will not give the best efficiency. The
choice of which style inductor to use often depends more
on the price vs size, performance and any radiated EMI
requirements than on what the LTC3555 family requires
to operate.
The inductor value also has an effect on forced Burst
Mode and Burst Mode operations. Lower inductor values
will cause the Burst and forced Burst Mode switching
frequencies to increase.
Table 7 shows several inductors that work well with the
LTC3555 family’s general purpose regulators. These in-
ductors offer a good compromise in current rating, DCR
and physical size. Consult each manufacturer for detailed
information on their entire selection of inductors.
Table 7. Recommended Inductors
INDUCTOR L
TYPE
(µH)
DE2818C 4.7
3.3
D312C
4.7
3.3
2.2
DE2812C 4.7
3.3
2.0
MAX
I_DC (A)
1.25
1.45
0.79
0.90
1.14
1.2
1.4
1.8
MAX
DCR (Ω)
0.072
0.053
0.24
0.20
0.14
0.13*
0.10*
0.067*
SIZE in mm
(L × W × H) MANUFACTURER
3.0 × 2.8 × 1.8 Toko
3.0 × 2.8 × 1.8 www.toko.com
3.6 × 3.6 × 1.2
3.6 × 3.6 × 1.2
3.6 × 3.6 × 1.2
3.0 × 2.8 × 1.2
3.0 × 2.8 × 1.2
3.0 × 2.8 × 1.2
CDRH3D16 4.7 0.9
3.3 1.1
2.2 1.2
CDRH2D11 4.7 0.5
3.3 0.6
2.2 0.78
CLS4D09 4.7 0.75
0.11
0.085
0.072
0.17
0.123
0.098
0.19
4 × 4 × 1.8 Sumida
4 × 4 × 1.8 www.sumida.
4 × 4 × 1.8 com
3.2 × 3.2 × 1.2
3.2 × 3.2 × 1.2
3.2 × 3.2 × 1.2
4.9 × 4.9 × 1
SD3118
SD3112
SD12
SD10
4.7 1.3 0.162 3.1 × 3.1 × 1.8 Cooper
3.3 1.59 0.113 3.1 × 3.1 × 1.8 www.cooperet.
2.2 2.0 0.074 3.1 × 3.1 × 1.8 com
4.7 0.8 0.246 3.1 × 3.1 × 1.2
3.3 0.97 0.165 3.1 × 3.1 × 1.2
2.2 1.12 0.14 3.1 × 3.1 × 1.2
4.7 1.29 0.117* 5.2 × 5.2 × 1.2
3.3 1.42 0.104* 5.2 × 5.2 × 1.2
2.2 1.80 0.075* 5.2 × 5.2 × 1.2
4.7 1.08 0.153* 5.2 × 5.2 × 1.0
3.3 1.31 0.108* 5.2 × 5.2 × 1.0
2.2 1.65 0.091* 5.2 × 5.2 × 1.0
LPS3015 4.7 1.1
3.3 1.3
2.2 1.5
0.2 3.0 × 3.0 × 1.5 Coil Craft
0.13 3.0 × 3.0 × 1.5 www.coilcraft.
0.11 3.0 × 3.0 × 1.5 com
*Typical DCR
General Purpose Switching Regulator Input/Output
26
Capacitor Selection
Low ESR (equivalent series resistance) MLCC capacitors
should be used at both switching regulator outputs as well
as at each switching regulator input supply (V_INX). Only X5R
or X7R ceramic capacitors should be used because they
retain their capacitance over wider voltage and temperature
ranges than other ceramic types. A 10μF output capaci-
tor is sufficient for most applications. For good transient
response and stability the output capacitor should retain
at least 4μF of capacitance over operating temperature
and bias voltage. Each switching regulator input supply
should be bypassed with a 1μF capacitor. Consult with
capacitor manufacturers for detailed information on their
selection and specifications of ceramic capacitors. Many
manufacturers now offer very thin (<1mm tall) ceramic
capacitors ideal for use in height-restricted designs. Table
8 shows a list of several ceramic capacitor manufacturers.
Table 8. Recommended Ceramic Capacitor Manufacturers
AVX
www.avxcorp.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay Siliconix
www.vishay.com
TDK
www.tdk.com
Over-Programming the Battery Charger
The USB high power specification allows for up to 2.5W to
be drawn from the USB port (5V × 500mA). The PowerPath
switching regulator transforms the voltage at V_BUS to just
above the voltage at BAT with high efficiency, while limiting
power to less than the amount programmed at CLPROG.
In some cases the battery charger may be programmed
(with the PROG pin) to deliver the maximum safe charging
current without regard to the USB specifications. If there
is insufficient current available to charge the battery at the
programmed rate, the PowerPath regulator will reduce
charge current until the system load on V_OUT is satisfied
and the V_BUS current limit is satisfied. Programming the
battery charger for more current than is available will not
cause the average input current limit to be violated. It will
merely allow the battery charger to make use of all available
power to charge the battery as quickly as possible, and
with minimal power dissipation within the battery charger.
3555fd