Datasheet
LT3751
17
3751fc
Figure 9. Maximum Switching Frequency
PEAK PRIMARY CURRENT (A)
L
PRI
/WATT (µH/WATT)
3751 F09
10.000
0.010
0.100
1.000
0.001
1 10 100
f
MAX
= 50kHz
f
MAX
= 100kHz
f
MAX
= 200kHz
applicaTions inForMaTion
Table 1. Recommended Transformers
MANUFACTURER PART NUMBER SIZE L × W × H (mm) MAXIMUM I
PRI
(A) L
PRI
(µH) TURNS RATIO (PRI:SEC)
Coilcraft
www.coilcraft.com
DA2033-AL
DA2034-AL
GA3459-BL
GA3460-BL
HA4060-AL
HA3994-AL
17.4 × 24.1 × 10.2
20.6 × 30 × 11.3
32.65 × 26.75 × 14
32.65 × 26.75 × 14
34.29 × 26.75 × 14
34.29 × 28.75 × 14
5
10
20
50
2
5
10
10
5
2.5
300
7.5
1:10
1:10
1:10
1:10
1:3
2:1:3:3*
Würth Elektronik/Midcom
www.we-online.com
750032051
750032052
750310349
750310355
28.7 × 22 × 11.4
28.7 × 22 × 11.4
36.5 × 42 × 23
36.5 × 42 × 23
5
10
20
50
10
10
5
2.5
1:10
1:10
1:10
1:10
Sumida
www.sumida.com
C8117
C8119
PS07-299
PS07-300
23 × 18.6 × 10.8
32.2 × 27 × 14
32.5 × 26.5 × 13.5
32.5 × 26.5 × 13.5
5
10
20
50
10
10
5
2.5
1:10
1:10
1:10
1:10
TDK
www.tdk.com
DCT15EFD-U44S003
DCT20EFD-U32S003
DCT25EFD-U27S005
22.5 × 16.5 × 8.5
30 × 22 × 12
27.5 × 33 × 15.5
5
10
20
10
10
5
1:10
1:10
1:10
*Transformer has three secondaries where the ratio is designated as PRI:SEC1:SEC2:SEC3
RV
TRANS
, RV
OUT
and R
DCM
Selection
RV
TRANS
sets the common-mode reference voltage for
both the DCM comparator and
V
OUT
comparator. Select
RV
TRANS
from Table 2 based on the transformer supply
voltage range, V
TRANS
, and the maximum trip voltage,
ΔV
DRAIN
(V
DRAIN
-V
TRANS
).
The RV
TRANS
pin is connected to an internal 40µA current
source. Pin current increases as the pin voltage is taken
higher than the internal 60V Zener clamp. The LT3751 can
operate from V
TRANS
greater than the 60V internal Zener
clamps by limiting the RV
TRANS
pin current to 250µA.
Operating V
TRANS
above 200V requires the use of resis-
tor dividers. Tw o applications are presented that operate
Table 2. Suggested RV
TRANS
, RV
OUT
, and R
DCM
Values
V
TRANS
Range
(V)
∆V
DRAIN
RANGE
(V)
RV
TRANS
(kΩ)
RV
OUT
(kΩ)
R
DCM
(kΩ)
4.75 to 55 0 to 5 5.11 5.11 2.32
4.75 to 60
2.5 to 50 25.5 25.5 11.5
5 to 80 40.2 40.2 18.2
8 to 80 8 to 160 80.6 80.6 36.5
80 to 200 2mA • RV
OUT
V
TRANS
−
55V
0.25
V
TRANS
−
55V
0.25
0.86 • RV
TRANS
>200 Resistor Divider Dependent Use Resistor Divider Use Resistor Divider Use Resistor Divider