LT3511
11
3511fc
APPLICATIONS INFORMATION
Turns Ratio
Note that when using an R
FB
/R
REF
resistor ratio to set
output voltage, the user has relative freedom in selecting
a transformer turns ratio to suit a given application. In
contrast, the use of simple ratios of small integers, e.g.,
1:1, 2:1, 3:2, provides more freedom in setting total turns
and mutual inductance.
Typically, choose the transformer turns to maximize avail-
able output power. For low output voltages (3.3V or 5V), a
N:1 turns ratio can be used with multiple primary windings
relative to the secondary to maximize the transformer’s
current gain (and output power). However, remember that
the SW pin sees a voltage that is equal to the maximum
input supply voltage plus the output voltage multiplied by
the turns ratio. In addition, leakage inductance will cause
a voltage spike (V
LEAKAGE
) on top of this reflected voltage.
This total quantity needs to remain below the absolute
maximum rating of the SW pin to prevent breakdown of
the internal power switch. Together these conditions place
an upper limit on the turns ratio, N, for a given application.
Choose a turns ratio low enough to ensure:
N <
150V – V
IN(MAX)
–V
LEAKAGE
V
OUT
+ V
F
For larger N:1 values, choose a transformer with a larger
physical size to deliver additional current. In addition,
choose a large enough inductance value to ensure that
the off-time is long enough to measure the output voltage.
For lower output power levels, choose a 1:1 or 1:N trans-
former for the absolute smallest transformer size. A 1:N
transformer will minimize the magnetizing inductance
(and minimize size), but will also limit the available output
power. A higher 1:N turns ratio makes it possible to have
very high output voltages without exceeding the breakdown
voltage of the internal power switch.
The turns ratio is an important element in the isolated
feedback scheme. Make sure the transformer manufacturer
guarantees turns ratio accuracy within ±1%.
Saturation Current
The current in the transformer windings should not ex-
ceed its rated saturation current. Energy injected once the
core is saturated will not be transferred to the secondary
and will instead be dissipated in the core. Information on
saturation current should be provided by the transformer
manufacturers. Table 1 lists the saturation current of the
transformers designed for use with the LT3511.
Primary Inductance Requirements
The LT3511 obtains output voltage information from the
reflected output voltage on the switch pin. The conduction
of secondary winding current reflects the output voltage
on the primary. The sampling circuitry needs a minimum
of 400ns to settle and sample the reflected output voltage.
In order to ensure proper sampling, the secondary winding
needs to conduct current for a minimum of 400ns. The
following equation gives the minimum value for primary-
side magnetizing inductance:
L
PRI
≥
t
OFF(MIN)
•N
PS
•V
OUT
+ V
F
()
I
PEAK(MIN)
t
OFF(MIN)
= 400ns
I
PEAK(MIN)
= 55mA
In addition to the primary inductance requirement for
sampling time, the LT3511 has internal circuit constraints
that prevent the switch from staying on for less than 100ns.
If the inductor current exceeds the desired current limit
during that time, oscillation may occur at the output as
the current control loop will lose its ability to regulate.
The following equation, based on maximum input voltage,
must also be followed in selecting primary-side magnetiz-
ing inductance:
L
PRI
≥
t
ON(MIN)
•V
IN(MAX)
I
PEAK(MIN)
t
ON(MIN)
= 100ns
I
PEAK(MIN)
= 55mA
