Datasheet

LTC3765
13
3765fb
For more information www.linear.com/LTC3765
APPLICATIONS INFORMATION
These two equations result in a wide range of values for
R
NDRV
. For many applications, a 100k resistor will satisfy
these requirements.
The rate of charge of V
CC
from 0V to 8.5V is controlled
by the LTC3765 to be approximately 35µs regardless of
the size of the capacitor connected to the V
CC
pin. The
charging current for this capacitor can be approximated as:
I
C1
=
8.5V
35µs
C
1
The external NMOS should be chosen so that the I
C1
capacitor charging current in the equation above does
not exceed the safe operating area (SOA) of the NMOS.
Excessive values of C1 are unnecessary and should be
avoided. Typically values in the 1µF to 10µF range work
well. A standard 3V threshold NMOS should be used when
possible to better tolerate a high voltage start-up transient;
however, a logic-level NMOS may be used for applications
that require low voltage start-up. Since the NMOS is on
continuously only during the brief start-up period, a small
SOT-23 package can be used.
If an 8.5V to 14.5V supply is available in the system that
can be used to power V
CC
, the linear regulator is not
needed and should be disabled by tying NDRV to V
CC
.
The external supply should be connected to the V
CC
pin
through a series diode if the LTC3766 is configured to
overdrive V
CC
when it begins switching.
Low Input Voltage Start-Up
The minimum value of R
NDRV
is further constrained if low
voltage (V
IN
< 10V) start-up is required. In this application,
the previous equation for the maximum value of R
NDRV
must be satisfied to start the charge pump. Additionally,
the charge pump current flows through R
NDRV
to raise the
NDRV voltage above V
IN
so that the external MOSFET can be
fully enhanced. R
NDRV
therefore needs to be large enough
that the limited charge pump current can raise the NDRV
voltage to this level. Lower threshold logic-level MOSFETs
are preferred for low voltage start-up not only because the
MOSFET requires a lower NDRV voltage above V
IN
, but
also because the charge pump current increases as the
NDRV-V
CC
difference decreases, which is approximately
the MOSFET threshold. For a given threshold voltage,
R
NDRV
should be chosen so that it meets the following
relationship, keeping in mind that the previous equation
for the maximum value of R
NDRV
must also be met.
R
NDRV
>
V
TH(MAX)
5 – V
TH(MAX)






•100k
In this equation, V
TH
is the maximum threshold voltage of
the external MOSFET. Table 1 below shows typical values
of R
NDRV
for common input voltage ranges.
Table 1. Typical R
NDRV
Values
V
IN
RANGE V
TH(MAX)
R
NDRV
RANGE TYPICAL R
NDRV
8V to 36V 2V 70k to 180k 125k
36V to 72V 4V 60k to 1.4M 150k
Setting the Overcurrent Limit
The overcurrent limit for the LTC3765 is principally a safety
feature to protect the converter. The current that flows in
series through the transformer primary winding and the
primary switch is sensed by a resistor (R
SENSE
) connected
between the source of the switch and ground. The voltage
across this resistor is sensed by the I
S
+
and I
S
–
pins. If
the difference between I
S
+
and I
S
–
exceeds 150mV, the
LTC3765 immediately turns off the primary NMOS and, if
SSFLT is not grounded, faults. The overcurrent comparator
is blanked for approximately 200ns after PG goes high to
avoid false trips due to noise.