Datasheet

LTC3766
37
3766fa
For more information www.linear.com/LTC3766
applicaTions inForMaTion
When calculating t
OUT
, use a value for V
HO
that corre-
sponds to the target output voltage for control hand-off,
typi
cally one-half the normal regulation level or less. If t
OUT
is less than t
BIAS
, then the LTC3765 soft-start capacitor
value should be increased. Note that these equations are
approximations and the actual times will vary somewhat
with circuit parameters.
Peak Charge Bias Configurations
When the peak voltage on the SW node is in the range of
7V to 32V, the peak charge can be taken directly from the
SW node as shown in Figure 20. In practice, this condi
-
tion only holds when the output voltage of the converter
is approximately 5V
.
In most applications, it is necessar
y to add an additional
auxiliary winding on the secondary for use in generating an
adequate bias voltage. For low V
OUT
applications (V
OUT
<
5V), this winding can be configured as shown in Figure21
to provide a higher voltage for bias generation.
This configuration is advantageous because it achieves
a higher voltage on the transformer secondary with a
minimum number of additional turns. The number of turns
on the auxiliary winding for this configuration should be
approximately:
N
AUX
≈N
S
V
B(MIN)
D
MAX
V
OUT
–1
⎛
⎝
⎜
⎞
⎠
⎟
where D
MAX
is the maximum operating duty cycle (typically
0.65 to 0.70) and V
B(MIN)
is either 7V for low voltage or
10V for high voltage drive mode operation. These values
for V
B(MIN)
are approximately 2V higher than the UVLO
levels on the LTC3766 to allow for drops in the peak charge
circuit. As an example, for V
OUT
= 1.5V, D
MAX
= 0.65V and
N
S
= 1 turn, use N
AUX
= 2 turns, assuming low voltage
drive mode.
For high V
OUT
applications (V
OUT
> 6V), this winding can
be configured as shown in Figure 22 to provide a reduced
voltage for generating bias. In this case, choose an auxiliary
winding with turns
N
AUX
≈N
S
V
B(MIN)
D
MAX
V
OUT
⎛
⎝
⎜
⎞
⎠
⎟
V
IN
R
PK
D
PK
SW
N
S
N
P
C
VCC
3766 F20
NDRV
LTC3766
MAIN
XFMR
V
BIAS
= 6V TO 32V
V
CC
C
PK
Q1
••
Figure 20. Peak Charge Directly from SW for V
OUT
≈ 5V
V
IN
R
PK
D
PK
N
S
N
AUX
SW
N
P
C
VCC
3766 F21
NDRV
LTC3766
MAIN
XFMR
V
BIAS
= 6V TO 32V
V
CC
C
PK
Q1
•
•
•
Figure 21. Peak Charge for Low V
OUT
Applications
V
IN
R
PK
D
PK
N
S
N
AUX
SW
N
P
C
VCC
3766 F22
NDRV
LTC3766
MAIN
XFMR
V
BIAS
= 6V TO 32V
V
CC
C
PK
Q1
•
•
•
Figure 22. Peak Charge for High V
OUT
Applications