LT3837EFE-PBF LINER [Linear Technology], LT3837EFE-PBF Datasheet - Page 17

• Manufacturer Part Number: LT3837EFE-PBF
• Description: Isolated No-Opto Synchronous Flyback Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Size R
V
assume that our worst-case conditions yield an I
above nominal so I
on R
105% = 88mV/10.41A and nominal R
Round to the nearest available lower value 8.0mΩ.
Selecting the Load Compensation Resistor
The expression for R
section for primary winding sensing as:
Continuing the example:
This value for R
cal methods are required for producing the best results.
This is because several of the required input variables are
diffi cult to estimate precisely. For instance, the ESR term
above includes that of the transformer secondary, but its
effective ESR value depends on high frequency behavior,
not simply DC winding resistance. Similarly, K1 appears
as a simple ratio of V
fi ciency, but theoretically estimating effi ciency is not a
simple calculation.
The suggested empirical method is as follows:
If ESR 0 0 002
1. Build a prototype of the desired supply including the
2. Temporarily ground the C
SENSE
R
actual secondary components.
compensation function. Measure output voltage while
sweeping output current over the expected range.
Approximate the voltage variation as a straight line,
ΔV
K
R
CMP
1
CMP
SENSE
OUT
=
SENSE
=
and maximum V
⎛
⎝ ⎜
=
=
/ΔI
V
=
.
V
IN
K
0 417
and minimum V
1 93
OUT
OUT
1
.
.
using worst-case conditions, minimum L
•
•
Eff
Ω
R
k
CMP
= R
ESR R
SENSE
Ω
⎞
⎠ ⎟
and R
•
PK
=
8 0
0 002
S(OUT)
.
is a good starting point, but empiri-
.
+
9 88
= 10.41A . If there is a 5% tolerance
CMP
m
IN
(
• –
DS ON
IN
3 3
(
Ω
DS ON
.
to V
1
. Continuing the example, let us
Ω +
(
.
%
was derived in the Operation
(
•
SENSE
DC
)
CMP
(
)
OUT
1
0 004
)
=
=
–
.
)
0 417
0 004
0 0 52
•
.
pin to disable the load
.
= 80mV, then R
times (differential) ef-
R
.
1
Ω
•
)
Ω
SENSE
•
N
22 1
SP
.
=
k
= 8.05mΩ.
Ω •
R
S S OUT
PK
(
SENSE
0 33
.
10%
)
P
•
,
4. Compute:
where R′
resistor, R
in place and R
compensation (from step 2).
Setting Frequency
The switching frequency of the LT3837 is set by an
external capacitor connected between the OSC pin and
ground. Recommended values are between 200pF and
33pF , yielding switching frequencies between 50kHz and
250kHz. Figure 3 shows the nominal relationship between
external capacitance and switching frequency. Place the
capacitor as close as possible to the IC and minimize OSC
trace length and area to minimize stray capacitance and
potential noise pickup.
You can synchronize the oscillator frequency to an external
frequency. This is done with a signal on the SYNC pin. Set
the LT3837 frequency 10% slower than the desired external
frequency using the OSC pin capacitor, then use a pulse on
the SYNC pin of amplitude greater than 2V and with the
desired period. The rising edge of the SYNC signal initiates
an OSC capacitor discharge forcing primary MOSFET off
(PG voltage goes low). If the oscillator frequency is much
different from the sync frequency, problems may occur
3. Calculate a value for the K1 constant based on V
5. Verify this result by connecting a resistor of this value
6. Disconnect the ground short to C
and the measured (differential) effi ciency.
from the R
requisite 0.1μF fi lter capacitor to ground. Measure the
output impedance R
new compensation in place. R
decreased signifi cantly. Fine tuning is accomplished
experimentally by slightly altering R
estimate for R
R
R
CMP
′
CMP
CMP
= 1
=
S(OUT)CMP
K
R
CMP
CMP
is the new value for the load compensation
S(OUT)
•
R
R
S OUT
CMP
SENSE
•
(
pin to ground.
⎛
⎜
⎝
1
is the output impedance with no load
is the output impedance with R
is:
+
)
R
•
S(OUT)
S OUT CMP
R N
R
(
1
S OUT
(
•
)
SP
= ΔV
)
orN
⎞
⎟
⎠
CMP
S(OUT)
OUT
SF
/ΔI
and connect the
CMP
LT3837
OUT
should have
. A revised
with the
IN
17
, V
3837fa
CMP
OUT