LTC3555 Linear Technology, LTC3555 Datasheet - Page 28

• Manufacturer Part Number: LTC3555
• Description: High Efficiency USB Power Manager + Triple Step-Down DC/DC
• Manufacturer: Linear Technology

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LTC3555
APPLICATIONS INFORMATION
By using a bias resistor, R
the hot and cold trip points can be moved in either direc-
tion. The temperature span will change somewhat due to
the non-linear behavior of the thermistor. The following
equations can be used to easily calculate a new value for
the bias resistor:
where r
desired hot and cold trip points. Note that these equations
are linked. Therefore, only one of the two trip points can
be chosen, the other is determined by the default ratios
designed in the IC. Consider an example where a 60°C
hot trip point is desired.
From the Vishay Curve 1 R-T characteristics, r
at 60°C. Using the above equation, R
46.4k. With this value of R
16°C. Notice that the span is now 44°C rather than the previ-
ous 40°C. This is due to the decrease in “temperature gain”
of the thermistor as absolute temperature increases.
The upper and lower temperature trip points can be inde-
pendently programmed by using an additional bias resistor
as shown in Figure 5b. The following formulas can be used
to compute the values of R
For example, to set the trip points to 0°C and 45°C with
a Vishay Curve 1 thermistor choose:
the nearest 1% value is 105k:
the nearest 1% value is 12.7k. The fi nal solution is shown
in Figure 5b and results in an upper trip point of 45°C and
a lower trip point of 0°C.
28
R1 = 0.536 • 105k – 0.4368 • 100k = 12.6k
R
R
R
R
R
NOM
NOM
NOM
1 0 536
NOM
=
HOT
=
=
=
.
=
0 536
r
r
3 266 0 4368
3 25
COLD
COLD
r
and r
HOT
.
.
.
•
2 714
R
.
2 714
NOM
–
•
.
COLD
– .
•
R
r
R
HOT
25
25
–
are the resistance ratios at the
r
HOT
NOM
•
NOM
R
NOM
25
•
, different in value from R25,
•
100
, the cold trip point is about
R R 25
and R1:
k
=
NOM
104 2
should be set to
.
k
HOT
is 0.2488
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 coeffi cient of many ceramic capacitors,
a nonlinearity, the voltage may even exceed twice the
USB voltage. To prevent excessive voltage from damag-
ing the LTC3555 during a hot insertion, it is best to have
a low voltage coeffi cient capacitor at the V
LTC3555. 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 following soft connect circuit (Figure 6)
can be employed. In this circuit, capacitor C1 holds MP1
off when the cable is fi rst 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.
Printed Circuit Board Layout Considerations
In order to be able to deliver maximum current under
all conditions, it is critical that the Exposed Pad on the
backside of the LTC3555 package be soldered to the PC
board ground. Failure to make thermal contact between
the Exposed Pad on the backside of the package and the
copper board will result in higher thermal resistances.
5V USB
INPUT
USB CABLE
Figure 6. USB Soft Connect Circuit
C1
100nF
Si2333
MP1
R1
40k
C2
10μF
BUS
V
GND
LTC3555
BUS
pin to the
3555 F06
3555f