Datasheet

LTC3300-1
31
33001fb
For more information www.linear.com/LTC3300-1
APPLICATIONS INFORMATION
External FET Selection
In addition to being rated to handle the peak balancing
current, external NMOS transistors for both primary and
secondary windings must be rated with a drain-to-source
breakdown such that for the primary MOSFET:
V
DS(BREAKDOWN)|MIN
> V
CELL
+
V
STACK
+ V
DIODE
T
= V
CELL
1+
S
T
⎛
⎝
⎜
⎞
⎠
⎟
+
V
DIODE
T
and for the secondary MOSFET:
V
DS(BREAKDOWN)|MIN
>
V
STACK
+
T V
CELL
+
V
DIODE
( )
= V
CELL
S+ T
( )
+ T V
DIODE
where S is the number of cells in the secondary winding
stack and 1:T is the transformer turns ratio from primary
to secondary. For example, if there are 12 Li-Ion cells in
the secondary stack and using a turns ratio of 1:2, the
primary FETs would have to be rated for greater than 4.2V
(1 + 6) + 0.5 = 29.9V and the secondary FETs would have
to be rated for greater than 4.2V (12 + 2) + 2V = 60.8V.
Good design practice recommends increasing this voltage
rating by at least 20% to account for higher voltages present
due to leakage inductance ringing. See Table 7 for a list of
FETs that are recommended for use with the LTC3300-1.
Table 7
PART NUMBER MANUFACTURER I
DS(MAX)
V
DS(MAX)
SiR882DP Vishay 60A 100V
SiS892DN Vishay 25A 100V
IPD70N10S3-12 Infineon 70A 100V
IPB35N10S3L-26 Infineon 35A 100V
RJK1051DPB Renesas 60A 100V
RJK1054DPB Renesas 92A 100V
Transformer Selection
The LTC3300-1 is optimized to work with simple 2-wind-
ing transformers with a primary winding inductance of
between 1 and 20 microhenries, a 1:2 turns ratio (primar
y
to secondar
y), and the secondary winding paralleling up
to 12 cells. If a larger number of cells in the secondary
stack is desired for more efficient balancing, a transformer
with a higher turns ratio can be selected. For example, a
1:10 transformer would be optimized for up to 60 cells in
the secondary stack. In this case the external FETs would
need to be rated for a higher voltage (see above). In all
cases the saturation current of the transformer must be
selected to be higher than the peak currents seen in the
application.
See Table 8 for a list of transformers that are recommended
for use with the LTC3300-1.
Table 8
PART NUMBER MANUFACTURER
TURNS
RATIO*
PRIMARY
INDUCTANCE I
SAT
750312504 (SMT) Würth Electronics 1:1 3.5µH 10A
750312677 (THT) Würth Electronics 1:1 3.5µH 10A
MA5421-AL Coilcraft 1:1 3.4µH 10A
CTX02-18892-R Coiltronics 1:1 3.4µH 10A
XF0036-EP13S XFMRS Inc 1:1 3µH 10A
LOO-321 BH Electronics 1:1 3.4µH 10A
DHCP-X79-1001 TOKO 1:1 3.4µH 10A
C128057LF GCI 1:1 3.4µH 10A
T10857-1 Inter Tech 1:1 3.4µH 10A
*All transformers listed in the table are 8-pin components and can be
configured with turns ratios of 1:1, 1:2, 2:1, or 2:2.
Snubber Design
Careful attention must be paid to any transient ringing
seen at the drain voltages of the primary and secondary
winding FETs in application. The peak of the ringing should
not approach and must not exceed the breakdown voltage
rating of the FETs chosen. Minimizing leakage inductance
present in the application and utilizing good board layout
techniques can help mitigate the amount of ringing. In
some applications, it may be necessary to place a series
resistor + capacitor snubber network in parallel with each
winding of the transformer. This network will typically
lower efficiency by a few percent, but will keep the FETs
in a safer operating region. Determining values for R and
C usually requires some trial-and-error optimization in the
application. For the transformers shown in Table 8, good
starting point values for the snubber network are 330Ω
in series with 100pF.