Datasheet

LTC3766
48
3766fa
For more information www.linear.com/LTC3766
applicaTions inForMaTion
Volt-Second Clamp
When used in applications with the LTC3765, direct flux
limit will guarantee that no saturation occurs on the main
transformer. Consequently, there is no need to use a volt-
second clamp in applications that have the direct flux
limit feature. In applications where the LTC3766 is used
standalone, however, the volt-second clamp can be used
as a failsafe to prevent excessive volt-seconds from being
applied to the main transformer during the PWM on-time.
Figure 35 illustrates the use of the volt-second clamp. As
shown in Figure 35, the SW voltage is used to monitor
the voltage applied to the main transformer. During the
PWM on-time, the C
VS
capacitor is charged by the SW
node through the R
VS
resistor.
For capacitor C
VS
, use a 5% or better NPO-type ceramic
capacitor, since accuracy is important. Typically a value
of 1nF is suitable. Likewise, use a 1% resistor for R
VS
.
In high output voltage applications where the SW node
must be divided down, use the circuit of Figure 36 to set
the volt-second clamp.
SW
SW
MAIN
XFMR
R
VS
N
S
N
P
C
VS
3766 F35
LTC3766
V
SEC
••
Figure 35. Using the Volt-Second Clamp
SW
SW
MAIN
XFMR
R
VS
N
S
N
P
C
VS
3766 F36
LTC3766
V
SEC
R2
R1
••
Figure 36. Volt-Second Clamp in High V
OUT
Application
SG
R
VIN
(FOR V
IN
> 30V)
V
IN
V
CC
V
AUX
NDRV
PT
+
V
OUT
REGSD
LV BIAS
SUPPLY
MODE
PG
SWP
SWB
MAIN
XFMR
SW
V
IN
C
RST
C
RSD
C
VAUX
LTC3766
FG
C
VIN
5V TO 15V
Q
P
C
VCC
3766 F37
••
Figure 37. Nonisolated Resonant-Reset Application
The PWM on-time is terminated when a pre-determined
threshold is reached. This will limit the applied volt-second
product to:
(V • S)
LIM
= 0.605R
VS
C
VS
The above equation is accurate even when the peak volt-
age on the SW node is relatively low and the charging is
nonlinear, such as in low V
OUT
applications. This is possible
because the LTC3766 senses the voltage on the SW pin
and adjusts the internal volt-second comparator reference
so that constant volt-seconds are maintained regardless
of the voltage on SW. Consequently, it is important that
the LTC3766 SW pin be connected to the secondary SW
node for proper sensing of this voltage to occur.
The volt-second limit should normally be set approximately
10% above the operational volt-second requirement. To
accomplish this, calculate R
VS
using:
R
VS
= 1.10
V
OUT
0.605f
SW
C
VS
In this case, assuming R
VS
>> R1||R2, R
VS
can be calcu-
lated using:
R
VS
= 1.10
V
OUT
0.605f
SW
C
VS
R2
R1+ R2
⎛
⎝
⎜
⎞
⎠
⎟
Nonisolated Applications
In addition to being used with the LTC3765 in isolated
applications, the LTC3766 can also be used standalone to
make a nonisolated resonant-reset forward converter as
shown in Figure 37. In this application, the primary-side
MOSFET is driven directly by the PT
+
pin, and the MODE