Datasheet

12
LT3420/LT3420-1
3420fb
APPLICATIO S I FOR ATIO
WUU
U
efficiency of the circuit. In addition, the effective capaci-
tance on the primary is largely dominated by the actual
secondary capacitance. This is simply a result of any
secondary capacitance being multiplied by N
2
when re-
flected to the primary. Since N is generally 10 or higher, a
small capacitance of 10pF on the secondary is 100 times
larger, or 1.0nF, on the primary. This capacitance forms a
resonant circuit with the primary leakage inductance of the
transformer. As such, both the primary leakage induc-
tance and secondary side capacitance should be mini-
mized.
Table 2 shows various predesigned transformers along
with relevant parameters. Contact the individual trans-
former manufacturer for additional information or
customization.
Table 2a. Predesigned Transformers, LT3420
TURNS L SIZE
PART RATIO (µH) LxWxH (mm) VENDOR
SRW10EPC 1:12 24 10.9x10.8x5.2 TDK
-U01H003 [phone]
www.components.tdk.com
6375-T108 1:12 15 10.8x9.5x3.6 Sumida
[phone]
www.sumida.com
SBL-6.4 1:12 17.5 10.3x6.4x5.2 Kijima Musen
852-2489-8266
[e-mail]
Table 2b. Predesigned Transformers, LT3420-1
TURNS L SIZE
PART RATIO (µH) LxWxH (mm) VENDOR
SBL-5.6S-2 1:10 15 5.6x8.5x3.0 Kijima Musen
852-2489-8266
[e-mail]
LDT565630T 1:10.2 14.5 5.8x5.8x3.0 TDK
-002 [phone]
www.components.tdk.com
DIODE SELECTION
The rectifying diode(s) should be low capacitance type
with sufficient reverse voltage and forward current rat-
ings. The peak reverse voltage that the diode(s) will see is
approximately:
V
PK-R
≈ +
()
VNV
OUT BAT
(• )•.165
The peak current of the diode is simply:
I
I
PK-SEC
PK-SEC
=
= −
14
3420
10
3420 1
.
()
.
()
A
N
LT
A
N
LT
For the circuit of Figure 1 with V
BAT
of 3.3V, V
PK-R
is 590V
and I
PK-SEC
is 116mA. Table 3 shows various diodes that
can work with the LT3420/LT3420-1. These are chosen for
low capacitance and high reverse blocking voltage. Use
the appropriate number of diodes to achieve the necessary
reverse breakdown voltage.
Table 3
MAX REVERSE CAPACITANCE
PART VOLTAGE (V) (pF) VENDOR
GSD2004S 2x300 5 Vishay
(Dual diode) [phone]
www.vishay.com
BAS21 250 1.5 Philips Semiconductor
(Single diode) [phone]
www.philips.com
MMBD3004S 2x300 5 Diodes Inc.
[phone]
www.diodes.com