LTC3765_15 Datasheet, PDF (13/24 Pages) Linear Technology – Active Clamp Forward Controller and Gate Driver

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
C1
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
>



5
V_{TH(MAX )}
– 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)
8V to 36V
2V
36V to 72V
4V
R_NDRV RANGE
70k to 180k
60k to 1.4M
TYPICAL R_NDRV
125k
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.
For more information www.linear.com/LTC3765
3765fb
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