Datasheet

LTC3766
29
3766fa
For more information www.linear.com/LTC3766
applicaTions inForMaTion
transformer. This causes the gain of the internal current
sense amplifier to be reduced, so that the maximum peak
current voltage is increased to approximately 1V. The
current transformer connections are shown in Figure 12.
voltage and comparing it to the output inductor current.
Figure13 shows an example of a well-calibrated current
sense transformer, where the R
SENSE
voltage has been
scaled by a factor of:
SF =
N
P
R
SENSE
K
CT
N
S
Because of the magnetizing current, the slope of the scaled
R
SENSE
voltage will not exactly match that of the inductor
current. Choose R
CAL
so that the scaled R
SENSE
voltage
and the inductor current are identical at the peak.
D
CT
CURRENT
TRANSFORMER
R
CAL
1k
MAIN
TRANSFORMER
N
S
N
P
PG
V
IN
+
V
IN
–
•
•
R
SENSE
C
FL
I
S
+
V
CC
I
S
–
3766 F12
GND
LTC3766
••
Figure 12. Using a Current Sense Transformer
20A
500ns/DIV
3766 F13
INDUCTOR
CURRENT
2A/DIV
SF • V
RSENSE
2A/DIV
Figure 13. Properly Calibrated Current Transformer
Typically, the current transformer is placed in series with
the power supply feed to the main transformer. This reduces
common mode noise, and is generally convenient for the
PCB layout. Use a small filter capacitor, C
FL
, between 1nF
and 3.3nF, or a time constant for R
SENSE
• C
FL
of less than
75ns, to eliminate high frequency noise. The diode D
CT
is needed to allow the core of the current transformer to
properly reset.
When using a current transformer, set the value of R
SENSE
using:
R
SENSE
=
0.73V
K
CT
I
LIM(AVG)
•
N
P
N
S
where N
P
/N
S
is the turns ratio of the main transformer,
K
CT
is the current gain of the transformer, and I
LIM(AVG)
is
the average current limit desired. For most applications,
a current transformer turns ratio of 1:100 is suitable
(K
CT
= 0.01).
The resistor R
CAL
is added to compensate for the ef-
fects of the magnetizing current in the main and current
sense transformers, both of which cause the voltage on
R
SENSE
to be somewhat higher than expected (2% to
8%). Typically, R
CAL
is in the range of 1.5k to 5k. For the
highest possible accuracy, the value of R
CAL
should be
adjusted to calibrate the current sensing at full load and
nominal input voltage by carefully measuring the R
SENSE
In order to maintain a constant average current while
in current limit, the LTC3766 automatically adjusts the
value of the peak current limit to cancel the effect of the
inductor ripple current. This is accomplished by creating
an internal ramp that mimics the inductor current ripple.
The amplitude of this ramp is determined by the resistor
on the I
PK
pin, which must be set to be proportional to
the output inductor. The LTC3766 establishes a voltage
on the I
PK
pin of (V
SW
– V
S
+
)/15, which is one-fifteenth
of the voltage across the output inductor during the on-
time when SW is high. Therefore, it is imperative that the
SW and V
S
+
pins be connected as shown in Figure 14
or Figure 15 so that the LTC3766 can properly sense the
inductor voltage. If the differential amplifier is not needed,
tie V
S
–
and V
S
+
together to V
OUT
as shown in Figure 14b.
For high V
OUT
applications where the SW node plateau
voltage is greater than 40V, it is necessary to add a resis-
tor divider on both the SW and V
S
+
pins, as shown in
Figure15. This divider will limit the voltage at the SW pin
and also impact the SG reverse overcurrent trip threshold.
See Setting the SG Reverse Overcurrent for details on
selecting the resistor divider on the SW pin.