Datasheet

LTC3300-1
30
33001fb
For more information www.linear.com/LTC3300-1
APPLICATIONS INFORMATION
External Sense Resistor Selection
The external current sense resistors for both primary
and secondary windings set the peak balancing current
according to the following formulas:
R
SENSE|PRIMARY
=
50mV
I
PEAK _PRI
R
SENSE|SECONDARY
=
50mV
I
PEAK _SEC
Balancer Synchronization
Due to the stacked configuration of the individual synchro-
nous flyback power circuits and the interleaved nature of
the gate drivers, it is possible at higher balance currents
for adjacent and/or penadjacent balancers within a group
of six to sync up. The synchronization will typically be to
the highest frequency of any active individual balancer and
can result in a slightly lower balance current in the other
affected balancer(s). This error will typically be very small
provided that the individual cells are not significantly out
of balance voltage-wise and due to the matched I
PEAK
/
I
ZERO
’s and matched power circuits. Balancer synchro-
nization can be reduced by lowpass filtering the primary
and/or secondar
y current sense signals with a simple RC
network as shown in Figure 10. A good starting point for
the RC time constant is one-tenth of the on-time of the
associated switch (primary or secondary). In the case
of I
PEAK
sensing, phase lag associated with the lowpass
filter will result in a slightly lower voltage seen by the
LTC3300-1 compared to the true sense resistor voltage.
This error can be compensated for by selecting the R value
to add back this same drop using the typical current value
of 20µA out of the LTC3300-1 current sense pins at the
comparator trip point.
Setting Appropriate Max On-Times
The primary and secondary winding volt-second clamps
are intended to be used as a current runaway protection
feature and not as a substitute means of current control
replacing the sense resistors. In order to not interfere with
normal I
PEAK
/I
ZERO
operation, the maximum on times must
be set longer than the time required to ramp to I
PEAK
(or
I
ZERO
) for the minimum cell voltage seen in the application:
t
ON(MAX)|PRIMARY
> L
PRI
• I
PEAK_PRI
/V
CELL(MIN)
t
ON(MAX)|SECONDARY
> L
PRI
• I
PEAK_SEC
• T/(S • V
CELL(MIN)
)
These can be further increased by 20% to account for
manufacturing tolerance in the transformer winding
inductance and by 10% to account for I
PEAK
variation.
LTC3300-1
n = 2 TO 6
20µA
R
C
R
SNS
33001 F10
G1P/GnP/G1S/GnS
I1P/InP/I1S/InS
V
–
/Cn – 1/V
–
/V
–
Figure 10. Using an RC Network to Filter Current Sense Inputs to
the LTC3300-1