LTC4110
36
4110fb
to monitor the primary current in both sides with a single
R
SNS(FET)
resistor, both transformer windings must be
connected prior to R
SNS(FET)
. Since the secondary phase
is always 180 degrees out of phase with the primary, the
following current waveform in Figure 14 is the result.
APPLICATIONS INFORMATION
I
PRI
SECONDARY CURRENT
PRIMARY CURRENT
I
PRI
N
0
4110 F14
Figure 14. R
SNS(FET)
Current Waveform
in calibration mode input current is regulated, not the
output current.
The LTC4110’s I
SENSE
pin has a limited usable positive
voltage range for V
SNS(FET)
. The range must be between
30mV and 150mV peak in both charge and calibration
modes when operating at full current. The negative portion
of the waveform is also monitored but has a dynamic trip
level that tracks the actual primary current. The trip level
has a gain factor of –3. If the secondary current trips the
negative level, the fl yback goes into current limit.
These limits have the following implications:
• The ratio of peak current between I
PRI(CHG)
and I
PRI(CAL)
cannot be greater than 5-to-1 as seen by R
SNS(FET)
.
•
The transformer turns ratio will approximately reduce
the maximum available DC current ratio between I
CHG
to I
CAL
by a factor of 1/N. The additional variables
being ripple current and effi ciency.
•
You cannot use a transformer with a turns ratio
greater than 3.
•
Because effi ciency is always less than 100%, you
never have to worry about peak secondary current
causing a false short circuit trip within the turns ratio
limit of 3 or less.
As a design starting point, use the lowest value between
I
PRI(CHG)
and I
PRI(CAL)
for I
PRI
, let V
SNS(FET)
be set to 50mV
for good effi ciency and solve for R
SNS(FET)
.
R
V
I
SNS FET
SNS FET
PRI
(
)
(
)
=
With an initial value of R
SNS(FET)
identifi ed, solve for
V
SNS(FET)
using the highest value between I
PRI(CHG)
or
I
PRI(CAL)
and see if the calculated value of V
SNS(FET)
falls
below the upper limits. If it is too high, you may have to
drop the value of R
SNS(FET).
If you cannot meet the V
SNS(FET)
upper or lower limits and/or ratio limits, you may have to
back off on one
of the I
CHG
and I
CAL
DC current parameters
to compensate.
Once within all the limits, optimize R
SNS(FET)
for maximum
effi ciency by using very low value of R
SNS(FET)
and/or fi nd
a popular R
SNS(FET)
value. The tradeoff of using lower
values of R
SNS(FET)
is increased waveform jitter due to
higher switching noise sensitivity issues.
In terms of current sensing, the primary current portion
of the above waveform is monitored for peak current (DC
+ AC) at any time in any mode. It does not monitor the
batteries’ DC current. The LTC4110 uses leading edge
blanking to mask out noise to make the application of this
part simple to use. The secondary portion of the above
waveform is monitored for negative peak current to sense
for short circuit.
The value of ripple current,
Δ
I, is a direct function of the
transformer inductance. See transformer section for more
information about transformer ripple current.
You must calculate the I
PRI
for both charge current mode
and calibration current mode. The equation for calculating
the I
PRI
for charge mode is as follows:
I
PRI(CHG)
=
I
CHG
E
•
V
BAT
V
DCIN
+
N
+
V
BAT
• V
DCIN
2 • f • L
PRI
• V
BAT
+
N • V
DCIN
(
)
(1)
I
PRI
for the Calibration mode is as follows:
I
PRI(CAL)
=
I
CAL
•
V
BAT
N • V
DCIN
+
1
+
V
BAT
• V
DCIN
2 • f • (N
2
• L
PRI
) • V
DCIN
+
V
BAT
N
(2)
The value of E is the fl yback effi ciency. Use 80% (0.8) as
the value since the fl yback uses synchronous rectifi ca-
tion. E is not used for the calibration equation because
