LT3485-0/LT3485-1/
LT3485-2/LT3485-3
10
34850123fb
APPLICATIO S I FOR ATIO
WUU
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Choosing the Right Device
(LT3485-0/LT3485-1/LT3485-2/LT3485-3)
The only difference between the four versions of the
LT3485 is the peak current level. For the fastest possible
charge time, use the LT3485-3. The LT3485-1 has the
lowest peak current capability, and is designed for
applications that need a more limited drain on the
batteries. Due to the lower peak current, the LT3485-1 can
use a physically smaller transformer. The LT3485-0 and
LT3485-2 have a current limit in between that of the
LT3485-3 and the LT3485-1.
Transformer Design
The flyback transformer is a key element for any
LT3485-0/LT3485-1/LT3485-2/LT3485-3 design. It must
be designed carefully and checked that it does not cause
excessive current or voltage on any pin of the part. The
main parameters that need to be designed are shown in
Table 1.
The first transformer parameter that needs to be set is the
turns ratio N. The LT3485-0/LT3485-1/LT3485-2/LT3485-
3 accomplish output voltage detection by monitoring the
flyback waveform on the SW pin. When the SW voltage
reaches 31.5V higher than the V
BAT
voltage, the part will
halt power delivery. Thus, the choice of N sets the target
output voltage as it changes the amplitude of the reflected
voltage from the output to the SW pin. Choose N accord-
ing to the following equation:
N
V
OUT
=
+ 2
31 5.
where V
OUT
is the desired output voltage. The number 2
in the numerator is used to include the effect of the voltage
drop across the output diode(s).
Thus for a 320V output, N should be 322/31.5 or 10.2. For
a 300V output, choose N equal to 302/31.5 or 9.6.
The next parameter that needs to be set is the primary
inductance, L
PRI
. Choose L
PRI
according to the following
formula:
L
V
NI
PRI
OUT
PK
≥
−
••
•
200 10
9
where V
OUT
is the desired output voltage. N is the trans-
former turns ratio. I
PK
is 1.4 (LT3485-0), 0.7 (LT3485-1),
1 (LT3485-2) and 2 (LT3485-3).
L
PRI
needs to be equal or larger than this value to ensure
that the LT3485-0/LT3485-1/LT3485-2/LT3485-3 has ad-
equate time to respond to the flyback waveform.
All other parameters need to meet or exceed the recom-
mended limits as shown in Table 1. A particularly impor-
tant parameter is the leakage inductance, L
LEAK
. When the
power switch of the LT3485-0/LT3485-1/LT3485-2/
LT3485-3 turns off, the leakage inductance on the pri-
mary of the transformer causes a voltage spike to occur
on the SW pin. The height of this spike must not exceed
40V, even though the absolute maximum rating of the SW
Pin is 50V. The 50V absolute maximum rating is a DC
blocking voltage specification, which assumes that the
current in the power NPN is zero. Figure 3 shows the SW
voltage waveform for the circuit of Figure 8 (LT3485-0).
Table 1. Recommended Transformer Parameters
TYPICAL RANGE TYPICAL RANGE TYPICAL RANGE TYPICAL RANGE
PARAMETER NAME LT3485-0 LT3485-1 LT3485-2 LT3485-3 UNITS
L
PRI
Primary Inductance >5 >10 >7 >3.5 µH
L
LEAK
Primary Leakage Inductance 100 to 300 200 to 500 200 to 500 100 to 300 nH
N Secondary: Primary Turns Ratio 8 to 12 8 to 12 8 to 12 8 to 12
V
ISO
Secondary to Primary Isolation Voltage >500 >500 >500 >500 V
I
SAT
Primary Saturation Current >1.6 >0.8 >1.0 >2 A
R
PRI
Primary Winding Resistance <300 <500 <400 <200 mΩ
R
SEC
Secondary Winding Resistance <40 <80 <60 <30 Ω
