Datasheet

LT3751
15
3751fc
applicaTions inForMaTion
The LT3751 charger controller can be optimized for either
capacitor charging only or low noise regulation applica-
tions. Several equations are provided to aid in the design
process.
Safety Warning
Large capacitors charged to high voltage can deliver a lethal
amount of energy if handled improperly. It is particularly
important to observe appropriate safety measures when
designing the LT3751 into applications. First, create a
discharge circuit that allows the designer to safely dis-
charge the output capacitor. Second, adequately space
high voltage nodes from adjacent traces to satisfy printed
circuit board voltage breakdown requirements.
Selecting Operating Mode
Tie the FB pin to GND to operate the LT3751 as a capacitor
charger. In this mode, the LT3751 charges the output at
peak primary current in boundary mode operation. This
constitutes maximum power delivery and yields the fast-
est charge times. Power delivery is halted once the output
reaches the desired output voltage set by the RV
OUT
and
RBG pins.
Tie a resistor divider from the FB pin to V
OUT
and GND
to operate the LT3751 as a low noise voltage regulator
(refer to Low Noise regulation section for proper design
procedures). The LT3751 operates as a
voltage regulator
using both peak current and duty cycle modulation to
vary output current during different loading conditions.
Selecting Component Parameters
Most designs start with the initial selection of V
TRANS
,
V
OUT
, C
OUT
, and either charge time, t
CHARGE
, (capacitor
charger) or P
OUT,MAX
(regulator). These design inputs
are then used to select the transformer ratio, N, the peak
primary current, I
PK
, and the primary inductance, L
PRI
.
Figure 7 can be used as a rough guide for maximum power
output for a given V
TRANS
and I
PK
.
Selecting Transformer Turns Ratio
The transformer ratio, N, should be selected based on
the input and output voltages. Smaller N values equate
to faster charge times and larger available output power.
Note that drastically reducing N below the V
OUT
/V
TRANS
ratio will increase the flyback voltage on the drain of the
NMOS and increase the current through the output diode.
The ratio, N, should not be drastically increased either, due
to the increased capacitance, N
2
• C
SEC
, reflected to the
primary. A good choice is to select N equal to V
OUT
/V
TRANS
.
N≤
V
OUT
V
TRANS
Choosing Capacitor Charger I
PK
When operating the LT3751 as capacitor charger, choose
I
PK
based on the required capacitor charge time, t
CHARGE
,
and the initial design inputs.
I
PK
=
2 • N • V
TRANS
+ V
OUT
( )
• C
OUT
• V
OUT
Efficiency • V
TRANS
• t
CHARGE
− t
d
( )
The converter efficiency varies over the output voltage
range. The I
PK
equation is based on the average effi-
ciency over the entire charging period. Several factors can
cause the charge time to increase. Efficiency is the most
dominant factor and is mainly affected by the transformer
winding resistance, core losses, leakage inductance, and
transistor R
DS
. Most applications have overall efficiencies
above 70%.
Figure 7. Maximum Power Output
PEAK PRIMARY CURRENT (A)
V
TRANS
(V)
3751 F07
100
10
20
30
40
50
60
70
80
90
0
1 10 100
P = 20 WATTS
P = 50 WATTS
P = 100 WATTS