LTC4110EUHF LINER [Linear Technology], LTC4110EUHF Datasheet - Page 42

• Manufacturer Part Number: LTC4110EUHF
• Description: Battery Backup System Manager
• Manufacturer: LINER [Linear Technology]

LTC4110
APPLICATIONS INFORMATION
Example:
SNUBBER DESIGN
The values given in the applications schematics have been
found to work quite well for this 12V-1A application. Care
should be taken in selecting other values for your applica-
tion since effi ciency may be impacted by a poor choice.
For a detailed look at snubber design, Application Note
19 is very helpful.
COMPONENT SELECTION
Current Sense Resistors
The LTC4110 uses up to three sense resistors—one of
them optional. In general, current sense resistors should
have a low temperature coeffi cient and suffi cient power
dissipation capability to avoid self-heating. Tolerance
depends on system accuracy requirements.
R
highest value between I
turns ratio. Use one the following equations to calculate
I
ever is higher.
Plug in the higher value of the above two results as
I
P
42
RSNS(FET)
R(SNSFET)
SNS(FET)
R(SNSFET)
V
I
P
I
I
I
I
Q
R(SNSFETCHG)
CHG
R(SNSFETCAL)
CAL
DCIN
D
= 3mA
= 144mW
•
•
:
= 12V, f
The power rating of R
depending on which value, I
and solve for power:
= I
1+
1+
R(SNSFET)
N• V
N• V
=
V
=
V
OSC
BAT
BAT
DCIN
DCIN
2
= 300kHz, Q
CHG
• R
N•E
E
SNS(FET)
or I
N• V
2
• V
SNS(FET)
V
CAL
2
DCIN
BAT
DCIN
• V
and the transformer
G1
BAT
CHG
+ 1
is defi ned by the
= Q
+ 1
or I
G2
CAL
= 15nC,
which-
R
current the system load draws. See Figure 11.
P
Find a sense resistor who’s power rating is greater than
P
R
highest current value between I
the battery will work. Plug in the higher of the two into
I
P
Use a sense resistor with a power rating greater than
P
FLYBACK MOSFET SELECTION
The LTC4110 uses two low side N-channel switching
MOSFETs in its fl yback converter. These MOSFETs have
dual roles. An any given time, only one MOSFET is the
primary switch while the other acts as a synchronous recti-
fi er on the secondary to improve effi ciency. The individual
MOSFETs’ roles depend on whether the battery is being
charged or calibrated. Each MOSFET specifi cation must
account for both roles.
The MOSFET voltage ratings in a fl yback design must deal
with other factors beyond
current is established in the primary leakage inductance
(L
turns off, the energy stored in L
causes the switch voltage to fl y up, starting from the input
voltage on up to the breakdown of the MOSFET if the volt-
age is not clamped. Thus, the snubber design is critical
in dealing with this voltage spike and can infl uence the
MOSFET voltage selection value. From a MOSFET point
of view, the minimum voltage must be greater than the
snubber clamp voltage V
zener clamp dissipation rises rapidly thus encouraging
higher MOSFET voltages.
the N-channel MOSFETs sees is:
BAT(MAX)
CL
R(CL)
R(CL)
SNS(BAT)
R(SNSBAT)
SNS(BAT)
L )
V
V
:
equal to peak primary current (I
R
CHG FET
CAL FET
CL
= I
(
(
power rating is a function of the maximum forward
:
MAX
and solve:
R
= I
SNS(BAT)
)
)
2
=
=
MAX
• R
V
V
BAT
DCIN
2
CL
• R
power rating is a function of the
+ •
+
SNS(BAT)
N V
V
SNUB
V
The maximum DC voltage that
BAT
N
IN
DCIN
.
. If V
During switch “on” time, a
L
CHG
, (Energy = I
SNUB
PRI
or I
). When the switch
itself is too low,
CAL
PRI
with which
2
•
L
4110fa
L
/2)