ltc3586 Linear Technology Corporation, ltc3586 Datasheet - Page 28

• Manufacturer Part Number: ltc3586
• Description: High Ef? Ciency Usb Power Manager With Boost, Buck-boost And Dual Bucks
• Manufacturer: Linear Technology Corporation

LTC3586
APPLICATIONS INFORMATION
USB Inrush Limiting
When a USB cable is plugged into a portable product,
the inductance of the cable and the high-Q ceramic input
capacitor form an L-C resonant circuit. If the cable does
not have adequate mutual coupling or if there is not much
impedance in the cable, it is possible for the voltage at
the input of the product to reach as high as twice the
USB voltage (~10V) before it settles out. In fact, due to
the high voltage coefficient of many ceramic capacitors,
a nonlinearity, the voltage may even exceed twice the
USB voltage. To prevent excessive voltage from damag-
ing the LTC3586 during a hot insertion, it is best to have
a low voltage coefficient capacitor at the V
LTC3586. This is achievable by selecting an MLCC capaci-
tor that has a higher voltage rating than that required for
the application. For example, a 16V, X5R, 10μF capacitor
in a 1206 case would be a better choice than a 6.3V, X5R,
10μF capacitor in a smaller 0805 case.
Alternatively, the soft connect circuit (Figure 7) can be
employed. In this circuit, capacitor C1 holds MP1 off
when the cable is first connected. Eventually C1 begins
to charge up to the USB input voltage applying increasing
gate support to MP1. The long time constant of R1 and
C1 prevent the current from building up in the cable too
fast thus dampening out any resonant overshoot.
Battery Charger Stability Considerations
The LTC3586’s battery charger contains both a constant-
voltage and a constant-current control loop. The constant-
voltage loop is stable without any compensation when a
battery is connected with low impedance leads. Excessive
lead length, however, may add enough series inductance
to require a bypass capacitor of at least 1μF from BAT to
GND. Furthermore, when the battery is disconnected, a
28
5V USB
INPUT
USB CABLE
Figure 7. USB Soft Connect Circuit
C1
100nF
Si2333
MP1
R1
40k
C2
10μF
BUS
V
GND
LTC3586
BUS
pin to the
3586 F07
4.7μF capacitor in series with a 0.2Ω to 1Ω resistor from
BAT to GND is required to keep ripple voltage low.
High value, low ESR multilayer ceramic chip capacitors
reduce the constant-voltage loop phase margin, possibly
resulting in instability. Ceramic capacitors up to 22μF may
be used in parallel with a battery, but larger ceramics should
be decoupled with 0.2Ω to 1Ω of series resistance.
In constant-current mode, the PROG pin is in the feed-
back loop rather than the battery voltage. Because of the
additional pole created by any PROG pin capacitance,
capacitance on this pin must be kept to a minimum. With
no additional capacitance on the PROG pin, the battery
charger is stable with program resistor values as high
as 25k. However, additional capacitance on this node
reduces the maximum allowed program resistor. The pole
frequency at the PROG pin should be kept above 100kHz.
Therefore, if the PROG pin has a parasitic capacitance,
C
the maximum resistance value for R
BUCK REGULATOR APPLICATIONS SECTION
Buck Regulator Inductor Selection
Many different sizes and shapes of inductors are avail-
able from numerous manufacturers. Choosing the right
inductor from such a large selection of devices can be
overwhelming, but following a few basic guidelines will
make the selection process much simpler.
The buck converters are designed to work with inductors
in the range of 2.2μH to 10μH. For most applications a
4.7μH inductor is suggested for both buck regulators.
Larger value inductors reduce ripple current which im-
proves output ripple voltage. Lower value inductors result
in higher ripple current and improved transient response
time. To maximize efficiency, choose an inductor with a
low DC resistance. For a 1.2V output, efficiency is reduced
about 2% for 100mΩ series resistance at 400mA load cur-
rent, and about 2% for 300mΩ series resistance at 100mA
load current. Choose an inductor with a DC current rating
PROG
R
PROG
, the following equation should be used to calculate
≤
2
π •
100
kHz C
1
•
PROG
PROG
:
3586f