Datasheet

LTC3766
39
3766fa
For more information www.linear.com/LTC3766
applicaTions inForMaTion
The buck bias winding can also be used standalone
without the peak charge supply, as shown in Figure 25.
This is sometimes done in applications where the peak
charge circuit is impractical, such when the V
IN
voltage
has a wide range.
When using the buck bias supply standalone, particular
care must be taken to ensure that the bias output comes
up more quickly than the main output, and that there is
adequate bias voltage immediately after control hand-
off. This is made more difficult by the presence of some
load on the V
CC
pin during start-up whereas there may
be no load on the main output. In general, a clean start-
up with a standalone buck bias supply can be achieved
by observing the following guidelines: 1) set the turns
ratio of the auxiliary winding so that the operating V
AUX
will be at least 3V above the rising V
CC
UVLO voltage,
2) use a smaller value for L
BK
, typically one-half of that
calculated in the above equation, but always large enough
for continuous current in L
BK
during normal operation
3) use the high-voltage linear regulator to minimize the
load on V
CC
during start-up, 4) use the RUN pin to monitor
the bias voltage and set the start-up voltage to 2V above
the rising V
CC
UVLO voltage with a hysteresis of 1.5V, 5)
use a shorter soft-start time, less than 10ms if possible,
6) use a small V
CC
capacitor (typically C
VCC
= 0.22μF) and
a capacitor C
BK
given by:
C
BK
=
20 Q
GPRI
f
SW
+ 3mA
( )
N
PT
+ 18mA
⎡
⎣
⎤
⎦
f
SW
V
HYST
where V
HYST
is the hysteresis set by the RUN pin (1.5V).
Note that this value for C
BK
is as small as possible so that
V
BUCK
rises quickly, but large enough to support the bias
current until control is handed off to the secondary and
the duty cycle increases. Once control is handed off, both
the buck supply and the main converter will be operating
in continuous current mode, so their outputs will track.
Another high efficiency option for generating bias is to make
use of an inductor overwinding, as shown in Figure26.
This supply is created by adding a second winding on the
main output inductor.
During the on-time of the synchronous MOSFET, the V
OUT
voltage is scaled and coupled through diode D
OW
to capaci-
tor C
OW
, so that the resulting bias voltage is:
V
OW
= V
OUT
N
L2
N
L1
– 0.5
This is similar to the buck supply in that it is highly efficient
and fairly well regulated. However, it is simpler in that it
does not require the use of an additional inductor to gener
-
ate the bias voltage. Another advantage of this technique
is that the bias voltage always tracks V
OUT
, so there is
no concern about the bias voltage potentially lagging the
output voltage during start-up. Like the buck bias supply,
the inductor overwinding can be used either stand alone
(as shown in Figure 26) or together with a peak charge
bias supply. Use a schottky diode D
OW
with a peak surge
current rating of 5A or higher. Capacitor C
OW
should be a
ceramic capacitor with a value of 2.2μF or greater.
V
IN
V
BUCK
RUN
R
R2
D
BK
D
BK
L
BK
C
VCC
NDRV
LTC3766
V
CC
R
R1
C
BK
Q1
3766 F25
N
P
N
AUX
MAIN
XFMR
••
Figure 25. Using the Buck Bias Supply Standalone
V
IN
V
AUX
NDRV
LTC3766
V
CC
C
VCC
3766 F26
C
OW
D
OW
N
L2
SW
MAIN
XFMR
N
L1
N
S
N
P
V
OW
C
OUT
V
OUT
•
•
Figure 26. Inductor Overwinding Bias Supply