Datasheet

LTC3766
38
3766fa
For more information www.linear.com/LTC3766
applicaTions inForMaTion
At maximum V
IN
, there may be considerable power dis-
sipation in the linear regulator pass device Q1. This power
can be calculated using
P
Q1
= (V
BIAS
– V
CC
)I
VCC
In applications where the peak charge and high voltage
linear regulator must operate continuously, transistor
Q1 must be capable of dissipating this power without
excessive temperature rise. In such applications, use a
transistor with a suitable package (SOT89) and connect
the thermal tab of the transistor to an adequately large
island of copper on the PCB.
High Efficiency Secondary-Side Bias Techniques
A high-efficiency alternative to using a peak-charge circuit
to generate secondary-side bias is to connect a buck output
to the transformer secondary. This buck output is normally
combined with a peak charge circuit as shown in Figures23
and 24. The bias voltage from this buck output can be fed
directly into the V
AUX
pin. This arrangement combines
the quick start-up and flexibility of a peak charge circuit
with the higher operating efficiency of a buck bias supply.
For Figure 23, the output voltage of the buck bias supply
is given by:
V
BUCK
= V
OUT
1+
N
AUX
N
S
⎛
⎝
⎜
⎞
⎠
⎟
– 0.5
For Figure 24 the output is given by:
V
BUCK
= V
OUT
N
AUX
N
S
– 0.5
For a buck bias supply, inductor L
BK
must be rated to
carry the required V
CC
bias current and should have an
inductance value that will provide continuous current
operation at one-fourth of the required bias current load
or less. Choose and inductor L
BK
to according to:
L
BK
>
V
CC
I
CC
f
SW
A value of 1mH for L
BK
is adequate for most applications.
The output voltage of the buck bias supply (V
BUCK
) should
be set to optimize efficiency during normal operation.
This will typically require a somewhat higher number of
auxiliary turns than is ideal for a peak charge output. As
a result, the buck supply and the peak charge circuit are
sometimes driven from separate auxiliary windings. Also,
note that the output voltage of the peak charge circuit will
increase somewhat when the V
AUX
bypass regulator is
activated and the high voltage linear regulator is disabled.
Care must be taken not to exceed the maximum voltage
rating on the V
IN
pin of the LTC3766.
Figure 23. Buck Bias Supply for Low V
OUT
Applications
Figure 24. Buck Bias Supply for High V
OUT
Applications
V
IN
V
AUX
REGSD
R
PK
D
PK
N
S
N
AUX
SW
N
P
C
VCC
3766 F23
NDRV
LTC3766
OPTION TO
LIMIT Q1 POWER
MAIN
XFMR
V
BIAS
= 6V TO 32V
V
CC
D
BK
L
BK
D
BK
C
PK
C
RSD
C
BK
V
BUCK
Q1
•
•
•
V
IN
V
AUX
REGSD
R
PK
D
PK
N
S
N
AUX
SW
N
P
C
VCC
3766 F24
NDRV
LTC3766
OPTION TO
LIMIT Q1 POWER
MAIN
XFMR
V
BIAS
= 6V TO 32V
V
CC
D
BK
L
BK
D
BK
C
PK
C
RSD
C
BK
V
BUCK
Q1
•
•
•