LTC3300-1 Datasheet, PDF (16/44 Pages) Linear Technology – High Effciency Bidirectional Multicell Battery Balancer

LTC3300-1
OPERATION
Cell Discharging (Synchronous)
When discharging is enabled for a given cell, the primary
side switch is turned on and current ramps in the primary
winding of the transformer until the programmed peak
current (I_{PEAK_PRI}) is detected at the In P pin. The primary
side switch is then turned off, and the stored energy in
the transformer is transferred to the secondary-side cells
causing current to flow in the secondary winding of the
transformer. The secondary-side synchronous switch
is turned on to minimize power loss during the transfer
period until the secondary current drops to zero (detected
at In S). Once the secondary current reaches zero, the
secondary switch turns off and the primary-side switch
is turned back on thus repeating the cycle. In this manner,
charge is transferred from the cell being discharged to all
of the cells connected between the top and bottom of the
secondary side—thereby charging the adjacent cells. In the
example of Figure 2, the secondary-side connects across
12 cells including the cell being discharged.
I_{PEAK_PRI} is programmed using the following equation:
I_PEAK
_
PRI
=
50mV
R_{SNS _PRI}
Cell discharge current (primary side) and secondary-side
charge recovery current are determined to first order by
the following equations:
I_DISCHARGE
=
I_{PEAK _ PRI}
2


S
S
+
T


I_SECONDARY
=
I_{PEAK _ PRI}
2


S
1
+
T


η_DISCHARGE
(detected at the In S pin), the secondary switch is turned
off and current then flows in the primary side thus charging
the selected cell from the entire stack of secondary cells. As
with the discharging case, the primary-side synchronous
switch is turned on to minimize power loss during the cell
charging phase. Once the primary current drops to zero,
the primary switch is turned off and the secondary-side
switch is turned back on thus repeating the cycle.
I_{PEAK_SEC} is programmed using the following equation:
I_PEAK
_
SEC
=
50mV
R_{SNS _ SEC}
Cell charge current and corresponding secondary-side
discharge current are determined to first order by the
following equations:
I_CHARGE
=
I_{PEAK _}
2
SEC


ST
S+T


η_CHARGE
I_SECONDARY
=
I_{PEAK _}
2
SEC


S
T
+
T


where S is the number of secondary cells in the stack, 1:T
is the transformer turns ratio from primary to secondary,
and η_CHARGE is the transfer efficiency from secondary-side
stack discharge to the primary-side cell.
Each balancer’s charge transfer “frequency” and duty
factor depend on a number of factors including I_{PEAK_PRI},
I_{PEAK_SEC}, transformer winding inductances, turns ratio,
cell voltage and the number of secondary-side cells.
The frequency of switching seen at the gate driver outputs
is given by:
where S is the number of secondary-side cells, 1:T is the
transformer turns ratio from primary to secondary, and
η_DISCHARGE is the transfer efficiency from primary cell
discharge to the secondary side stack.
Cell Charging
When charging is enabled for a given cell, the secondary-
side switch for the enabled cell is turned on and current
flows from the secondary-side cells through the trans-
former. Once I_{PEAK_SEC} is reached in the secondary side
f_DISCHARGE
=
S
S+
T
•
L_PRI
V_CELL
•I_PEAK
_ PRI
f_CHARGE
=
S
S+
T
•
L_PRI
V_CELL
•I_{PEAK _}
SEC
•
T
where L_PRI is the primary winding inductance.
Figure 3 shows a fully populated LTC3300-1 application
employing all six balancers.
33001f
16
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