Datasheet

LT3574
15
3574f
applications inForMation
Overdriving the BIAS Pin with a Third Winding
The LT3574 provides excellent output voltage regulation
without the need for an opto-coupler, or third winding, but
for some applications with higher input voltages (>20V),
it may be desirable to add an additional winding (often
called a third winding) to improve the system efficiency.
For proper operation of the LT3574, if a winding is used as
a supply for the BIAS pin, ensure that the BIAS pin voltage
is at least 3.15V and always less than the input voltage.
For a typical 24V
IN
application, overdriving the BIAS pin
will improve the efficiency gain 4% to 5%.
Loop Compensation
The LT3574 is compensated using an external resistor-
capacitor network on the VC 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.
Design Example
The following example illustrates the converter design
process using LT3574.
Given the input voltage of 20V to 28V, the required output
is 5V, 0.5A.
V
IN(MIN)
= 20V, V
IN(MAX)
= 28V, V
OUT
= 5V, V
F
= 0.5V
and I
OUT
= 0.5A
1. Select the transformer turns ratio to accommodate
the output.
The output voltage is reflected to the primary side by a
factor of turns ratio N. The switch voltage stress V
SW
is
expressed as:
N
N
N
V V N V V V
P
S
SW MAX IN OUT F
=
= + + <
( )
( ) 50
or rearranged to:
N
V
V V
IN MAX
OUT F
<
−
+
50
( )
( )
On the other hand, the primary-side current is multiplied by
the same factor of N. The converter output capability is:
I D NI
D
N V V
V N
OUT MAX PK
OUT F
IN
( )
. • ( ) •
( )
= −
=
+
+
0 8 1
1
2
(( )V V
OUT F
+
The transformer turns ratio is selected such that the con-
verter has adequate current capability and a switch stress
below 50V. Table 6 shows the switch voltage stress and
output current capability at different transformer turns
ratio.
Table 6. Switch Voltage Stress and Output Current Capability vs
Turns Ratio
N
V
SW(MAX)
AT V
IN(MAX)
(V)
I
OUT(MAX)
AT V
IN(MIN)
(A)
DUTY CYCLE
(%)
1:1 33.5 0.34 16~22
2:1 39 0.57 28~35
3:1 44.5 0.73 37~45
4:1 50 0.84 44~52