Datasheet

LT3748
19
3748fa
APPLICATIONS INFORMATION
selected MOSFET switch at the expected V
IN
and INTV
CC
voltages and multiply that charge required with each
turn-on event by the maximum operating frequency. The
maximum operating frequency in a given application can
be approximated from the primary transformer inductance,
the windings ratio (N
PS
), the nominal output voltage and
the maximum input voltage. Unless the part is limited by
minimum on- or off-times, this maximum frequency will
occur when the part is regulating in boundary mode at the
minimum peak switch current, and can be derived from:
f
SW(MAX)
≈
V
IN(MAX)
•V
OUT
+ V
F(DIODE)
()
•N
PS
L
PRI
•I
LIM(MIN)
•V
OUT
+ V
F(DIODE)
•N
PS
+ V
IN(MAX)
()
With the maximum INTV
CC
current calculated, the expected
dropout when V
IN
drops below 7V can be extracted from
the curves in the Typical Performance Characteristics sec-
tion. The LT3748 is tested as low as V
IN
= 5V but the hard
limit on minimum V
IN
operation is the INTV
CC
regulator
dropout and the 3.6V under voltage lockout. Figure 12
illustrates an example where operation with V
IN
= 5V
and I
INTVCC
= 20mA might be fully functional at room
temperature, but when the dropout for the same current
exceeds 1.4V and trips the UVLO at higher temperatures
the LT3748 will stop switching.
Overdriving INTV
CC
with a Third Winding
The LT3748 provides excellent output voltage regulation
without the need for an opto-coupler or third winding,
but for some applications with input voltages greater than
20V, an additional winding may improve overall system
efficiency. The third winding should be designed to out-
put a voltage between 7.2V and 20V. For a typical 48V
IN
,
10W application, overdriving the INTV
CC
pin may improve
efficiency by several percent at maximum load and as
much as 30% at light loads.
Loop Compensation
The LT3748 is compensated using an external resistor-
capacitor network on the V
C
pin. Typical values are in the
range of R
C
= 50k and C
C
= 1nF (see the numerous sche-
matics in the Typical Applications section for other possible
values). If too large of an R
C
value is used, the part will be
more susceptible to high frequency noise and jitter. If too
small of an R
C
value is used, the transient performance will
suffer. The value choice for C
C
is somewhat the inverse
of the R
C
choice: if too small a C
C
value is used, the loop
may be unstable and if too large a C
C
value is used, the
transient performance will also suffer. Transient response
plays an important role for any DC/DC converter.
LT3748
3.6V < BIAS < 20V,
V
IN
> BIAS
5V TO 100V
INTV
CC
V
IN
3748 F09
EXTERNAL SUPPLY
OR THIRD WINDING
LDO
LT3748
(V
IN
– DROPOUT) TO 7V
5V TO 100V
INTV
CC
V
IN
LDO
LT3748
5V TO 20V
INTV
CC
V
IN
OPTIONAL
LDO
Figure 11. INTV
CC
Pin Configurations
TEMPERATURE (oC)
–50
0
INTV
CC
DROPOUT (V)
0.5
1.0
1.5
2.0
050
100
150
3748 F12
2.5
3.0
–25 25
75
125
V
IN
= 5V
INTV
CC
UVLO = 3.6V
I
INTVCC
= 20mA
Figure 12. INTV
CC
Current at Low V
IN
Can Cause the LT3748 to
Stop Switching Due to INTV
CC
Undervoltage Lockout