LT3585-0/LT3585-1
LT3585-2/LT3585-3
W U U
APPLICATIO S I FOR ATIO
U
Choosing the Right Device
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
(LT3585-0/LT3585-1/LT3585-2/LT3585-3)
The only difference between the four versions of the
LT3585 series is the peak current level. For the fastest
possible charge time, use the LT3585-3. The LT3585-1
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 LT3585-1
can use a physically smaller transformer. The LT3585-0
and LT3585-2 have a current limit in between that of the
LT3585-1 and the LT3585-3.
to be set is the primary inductance, L . Choose L
PRI
PRI
according to the following formula:
VOUT •200•10–9
LPRI
≥
N•IPK
where V
is the desired output voltage. N is the trans-
OUT
former turns ratio. I is 1.4 (LT3585-0), 0.7 (LT3585-1),
PK
1 (LT3585-2) and 2 (LT3585-3). L needs to be equal
PRI
or larger than this value to ensure that the LT3585 series
has adequate time to respond to the flyback waveform.
All other parameters need to meet or exceed the recom-
mendedlimitsasshowninTable1.Aparticularlyimportant
Transformer Design
TheflybacktransformerisakeyelementforanyLT3585-0/
LT3585-1/LT3585-2/LT3585-3design.Itmustbedesigned
carefullyandcheckedthatitdoesnotcauseexcessivecur-
rentorvoltageonanypinofthepart. Themainparameters
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 LT3585-0/LT3585-1/LT3585-2/LT3585-3
accomplish output voltage detection by monitoring the
flyback waveform on the SW pin. When the SW voltage
parameter is the leakage inductance, L
. When the
LEAK
power switch of the LT3585 series turns off, the leakage
inductance on the primary of the transformer causes a
voltage spike to occur on the SW pin. The height of this
spike must not exceed 50V, even though the absolute
maximum rating of the SW pin is 60V. The 60V absolute
maximum rating is a DC blocking voltage specification,
which assumes that the current in the power NPN is zero.
Figure 4 shows the SW voltage waveform for the circuit
of Figure 8 (LT3585-0). Note that the absolute maximum
rating of the SW pin is not exceeded. Make sure to check
reaches 31.5V higher than the V voltage, the part halts
BAT
powerdelivery.Thus,thechoiceofNsetsthetargetoutput
voltage and changes the amplitude gain of the reflected
voltagefromtheoutputtotheSWpin.ChooseNaccording
to the following equation:
the SW voltage waveform with V
near the target output
OUT
voltage, as this is the worst-case condition for SW volt-
age. Figure 5 shows the various limits on the SW voltage
during switch turn off.
VOUT +2
N=
31.5
where V
is the desired output voltage. The number 2
OUT
in the numerator is used to include the forward voltage
Table 1. Recommended Transformer Parameters
TYPICAL RANGE
TYPICAL RANGE
LT3585-1
TYPICAL RANGE
LT3585-2
TYPICAL RANGE
LT3585-3
PARAMETER
NAME
LT3585-0
UNITS
µH
L
L
Primary Inductance
Primary Leakage Inductance
Secondary/Primary Turns Ratio
>5
>10
200 to 500
8 to 12
>500
>7
>3.5
100 to 300
8 to 12
>500
PRI
100 to 300
8 to 12
>500
200 to 500
8 to 12
>500
nH
LEAK
N
V
Secondary to Primary Isolation
Voltage
V
ISO
I
Primary Saturation Current
Primary Winding Resistance
Secondary Winding Resistance
>1.6
<300
<40
>0.8
<500
<80
>1.0
<400
<60
>2
A
mΩ
Ω
SAT
R
R
<200
<30
PRI
SEC
3585f
11
Linear [ Linear ]
