Datasheet

LTC3766
19
3766fa
For more information www.linear.com/LTC3766
applicaTions inForMaTion
When used with a bias supply that is between 5V and 10V,
the V
CC
pin can be directly connected to the bias supply as
shown in Figure 4a. Note that the V
IN
and NDRV pins must
also be connected to the bias supply for proper operation
of internal circuitry. When a bias supply between 6V and
15V is available, the V
AUX
bypass linear regulator can
be used standalone as shown in Figure 4b. In this case,
proper start-up is assured by connecting the NDRV pin
to V
CC
. Since there is no external pass device on NDRV,
however, the effective UVLO levels will be dictated by
the V
AUX
switchover thresholds instead of the V
CC
UVLO
thresholds. Rather than relying on the V
AUX
thresholds, the
start-up and shutdown levels are normally set by using the
RUN pin to monitor the bias supply voltage as shown in
Figure4b. See the RUN Pin Operation section for details.
For applications where the available bias supply is greater
than 30V, the LTC3766 also contains a current-limited 30V
clamp on the V
IN
pin. This clamp can sink up to 3.5mA
to allow the V
IN
pin to be used as a shunt regulator. This
is especially useful in nonisolated applications where the
LTC3766 is used standalone. See the Nonisolated Applica
-
tions section for more information.
RUN Pin Operation
Normal operation is enabled when the voltage on RUN
rises above its 1.22V threshold. As shown in Figure 5,
the RUN pin can be used with an external resistor divider
to enable the LTC3766 operation based on a sensed volt
-
age V
X
. In self-starting applications, V
X
is normally either
the converter output voltage (V
OUT
) or a bias voltage. In
nonisolated applications, V
X
is normally the converter
input voltage (V
IN
). See Nonisolated Applications for more
information on the use of the RUN pin in nonisolated
applications.
RUN
R1
R2
LTC3766
V
X
GND
3766 F05
Figure 5. Using the RUN Pin to Determine Start-Up
V
IN
LTC3766
V
CC
3766 F04a
C
VCC
NDRV
LV BIAS
SUPPLY
5V TO 10V
V
IN
V
AUX
LTC3766
V
CC
C
VCC
3766 F04b
C
VAUX
R1
NDRV
RUN
LV BIAS
SUPPLY
6V TO 15V
R2
Figure 4a. No Linear Regulator Used
Figure 4b. V
AUX
Regulator Used Standalone
A 3µA current is pulled into the RUN pin when it is below
its threshold that, when combined with the value chosen
for R1, increases the hysteresis beyond the internal amount
of 4%. When used in this manner, the values for R1 and
R2 can be calculated from the desired rising and falling
V
X
thresholds by the following equations:
R1=
V
X(RISING)
–1.043• V
X(FALLING)
3µA
R2=
1.17 •R1
V
X(FALLING)
–1.17
In self-starting applications where the LTC3765 performs
an open-loop soft-start, the voltage V
X
can be tied to V
OUT
of the converter (V
X
= V
OUT
) to inhibit the LTC3766 start-
up until the output voltage is above a given level. This
sets the exact output voltage at which soft-start control
is handed off from primary to secondary. This hand-off
output voltage should be set high enough so as to avoid
pulse-skipping operation when the LTC3766 initially takes