LTC4110
42
4110fb
Example:
V
DCIN
= 12V, f
OSC
= 300kHz, Q
G1
= Q
G2
= 15nC,
I
Q
= 3mA
P
D
= 144mW
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
SNS(FET)
:
The power rating of R
SNS(FET)
is defi ned by the
highest value between I
CHG
or I
CAL
and the transformer
turns ratio. Use one the following equations to calculate
I
RSNS(FET)
depending on which value, I
CHG
or I
CAL
which-
ever is higher.
I
R(SNSFETCHG)
=
I
CHG
•
1
+
V
BAT
N
•
V
DCIN
N
•
V
BAT
E
2
•
V
DCIN
+
1
I
R(SNSFETCAL)
=
I
CAL
•
1
+
V
BAT
N
•
V
DCIN
N
•
E
2
•
V
BAT
V
DCIN
+
1
Plug in the higher value of the above two results as
I
R(SNSFET)
and solve for power:
P
R(SNSFET)
= I
R(SNSFET)
2
•
R
SNS(FET)
R
CL
:
R
CL
power rating is a function of the maximum forward
current the system load draws. See Figure 11.
P
R(CL)
= I
MAX
2
•
R
CL
Find a sense resistor who’s power rating is greater than
P
R(CL)
R
SNS(BAT)
:
R
SNS(BAT)
power rating is a function of the
highest current value between I
CHG
or I
CAL
with which
the battery will work. Plug in the higher of the two into
I
BAT(MAX)
and solve:
P
R(SNSBAT)
= I
MAX
2
•
R
SNS(BAT)
Use a sense resistor with a power rating greater than
P
SNS(BAT)
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
V
IN
.
During switch “on” time, a
current is established in the primary leakage inductance
(L
L)
equal to peak primary current (I
PRI
). When the switch
turns off, the energy stored in L
L
, (Energy = I
PRI
2
•
L
L
/2)
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
SNUB
. If V
SNUB
itself
is too low,
zener clamp dissipation rises rapidly thus encouraging
higher MOSFET voltages.
The maximum DC voltage that
the N-channel MOSFETs sees is:
V
V
V
N
CHG FET
DCIN
BAT
(
)
=
+
V
V
N V
CAL FET
BAT
DCIN
(
)
=
+ •
APPLICATIONS INFORMATION
