Datasheet

LT3748
18
3748fb
For more information www.linear.com/LT3748
applications inForMation
Soft-Start
The LT3748 contains an optional soft-start function that
is enabled by connecting an explicit external capacitor
between the SS pin and ground. Internal circuitry prevents
the control voltage at the V
C
pin from exceeding that on
the SS pin.
The soft-start function is engaged whenever power at V
IN
is removed, or as a result of either undervoltage lockout,
overcurrent in the sense resistor or thermal (overtempera
-
ture) shutdown. The SS node is then discharged to roughly
600mV. When this condition is removed, a nominal 5µA
current acts to charge up the SS node towards roughly
2.2V. For example, a 0.1µF soft-start capacitor will place
a 0.05V/ms limit on the turn-on ramp rate at the V
C
node.
ENABLE and Undervoltage Lockout (UVLO)
A resistive divider from V
IN
to the EN/UVLO pin implements
undervoltage lockout (UVLO). The EN/UVLO pin threshold
is set at 1.223V. In addition, the EN/UVLO pin draws 2.4µA
when the voltage at the pin is below 1.223V. This current
provides user programmable hysteresis based on the value
of R1. The effective UVLO thresholds are:
V
IN(UVLO,RISING)
=
1.223V • (R1+ R2)
R2
+ 2.4µA • R1
V
IN(UVLO,FALLING)
=
1.223V • (R1+ R2)
R2
Figure 10 shows the implementation of external shutdown
control while still using the UVLO function. The NMOS
grounds the EN/UVLO pin when turned on, and puts the
LT3748 in shutdown with a quiescent current draw of
less than 1µA.
Minimum Load Requirement
The LT3748 recovers output voltage information using
the flyback pulse that occurs once the external MOSFET
turns off and the secondary winding conducts current. In
order to regulate the output voltage, the LT3748 needs to
sample the flyback pulse. The LT3748 delivers a minimum
amount of energy even during light load conditions to
ensure accurate output voltage information. The minimum
delivery of energy creates a minimum load requirement
on the output of approximately 2% of maximum load.
The minimum operating frequency at minimum load is
approximately 42kHz.
Alternatively, a Zener diode sufficiently rated to handle the
minimum load power can be used to provide a minimum
load without decreasing efficiency in normal operation.
In selecting a Zener diode for this purpose, the Zener
voltage should be high enough that the diode does not
become the load path during transient conditions but the
voltage must still be low enough that the MOSFET and
output voltage ratings are not exceeded
when the Zener
functions as the minimum load.
INT
V
CC
Pin Considerations
The INTV
CC
pin powers the internal circuitry and gate
driver of the LT3748. Three unique configurations exist
for regulation of the INTV
CC
pin as shown in Figure 11. In
the first configuration, the internal LDO drives the INTV
CC
pin internally from the V
IN
supply. In the second configura-
tion, the V
IN
supply directly drives the INTV
CC
pin through
a direct connection bypassing the internal LDO. Use this
optional configuration for voltages lower than 20V. In the
third configuration, an external supply or third winding
drives the INTV
CC
pin. Use this option when a voltage
supply exists lower than the input supply but higher than
the regulated INTV
CC
voltage. Using a lower voltage sup-
ply provides a more efficient source of power for internal
cir
cuitry and reduces power dissipation in the LT3748.
When calculating the minimum input voltage required for
a valid INTV
CC
, or the power dissipated in the LT3748, it is
useful to know how much current will be drawn from the
INTV
CC
LDO during normal operation. The easiest way to
calculate this current is to use the gate charge (Q
G
) for the
selected MOSFET switch at the expected V
IN
and INTV
CC
LT3748
EN/UVLO
GND
R2
R1
V
IN
3748 F10
RUN/STOP
CONTROL
(OPTIONAL)
Figure 10. Undervoltage Lockout (UVLO)
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