LT3837EFE#TRPBF Linear Technology, LT3837EFE#TRPBF Datasheet - Page 16

• Manufacturer Part Number: LT3837EFE#TRPBF
• Description: IC CNTRLR SYNC ISO 16TSSOP
• Manufacturer: Linear Technology
• Type: Flybackr

Specifications of LT3837EFE#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Frequency - Switching: 50kHz ~ 250kHz
• Voltage - Input: 4.5 ~ 20 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Voltage - Output: -
• Power - Output: -

APPLICATIONS INFORMATION
LT3837
Setting Feedback Resistive Divider
Use the equation developed in the Operation section for
the feedback divider.
It is recommended that the Thevenin impedance of the
resistors on the FB Pin is roughly 3k for bias current
cancellation and other reasons.
For the example using primary winding sensing if
ESR = 0.002 and R
So, choose 22.1k.
Current Sense Resistor Considerations
The external current sense resistor is used to control peak
primary switch current, which controls a number of key
converter characteristics including maximum power and
external component ratings. Use a noninductive current
sense resistor (no wire-wound resistors). Mounting the
resistor directly above an unbroken ground plane con-
nected with wide and short traces keeps stray resistance
and inductance low.
The dual sense pins allow for a fully Kelvined connection.
Make sure that SENSE
nect close to the sense resistor to preserve this.
Peak current occurs at 98mV of sense voltage V
the nominal sense resistor is V
peak switch current of 10A requires a nominal sense resistor
of 0.010Ω. Note that the instantaneous peak power in the
sense resistor is 1W, and that it is rated accordingly. The
use of parallel resistors can help achieve low resistance,
low parasitic inductance and increased power capability.
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.
16
R
SENSE
1
=
1 237
SENSE
.
3
SENSE
and maximum V
k
•
and minimum V
⎡
⎢
⎢
⎣
using worst-case conditions, minimum L
⎛
⎜
⎝
(
3 3 10 0 002 0 004
.
PK
DS(ON)
+
= 10.41A . If there is a 5% tolerance
+
and SENSE
•
IN
(
(
. Continuing the example, let us
1 3
= 0.004 then:
/
.
SENSE
)
SENSE
+
= 80mV, then R
–
.
are isolated and con-
/I
PK
SENSE
)
⎞
⎟
⎠
. For example, a
– – .
0 7
= 8.05mΩ.
⎤
⎥
⎥
⎦
SENSE
=
PK
SENSE
22 75
10%
.
. So
P
k
•
,
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:
1. Build a prototype of the desired supply including the
2. Temporarily ground the C
3. Calculate a value for the K1 constant based on V
4. Compute:
If ESR 0 0 002
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
and the measured (differential) effi ciency.
R
K
R
R
1
CMP
CMP
CMP
OUT
=
=
⎛
⎝ ⎜
/ΔI
=
=
= 1
=
V
.
IN
V
K
K
0 417
1 93
OUT
OUT
.
1
.
•
•
•
Eff
Ω
R
R
R
k
= R
CMP
ESR R
SENSE
Ω
S OUT
⎞
⎠ ⎟
SENSE
and R
•
(
=
8 0
S(OUT)
0 002
.
is a good starting point, but empiri-
.
9 88
+
CMP
m
)
(
IN
DS ON
• –
3 3
•
Ω
(
DS ON
.
R N
.
to V
1
Ω +
(
%
was derived in the Operation
1
(
•
•
DC
)
(
CMP
)
1
OUT
0 004
=
=
)
SP
–
.
)
0 417
0 004
0 0 52
•
.
.
pin to disable the load
orN
times (differential) ef-
.
R
Ω
1
)
•
SF
Ω
•
N
22 1
SP
.
=
k
Ω •
R
S S OUT
IN
(
0 33
, V
.
3837fc
OUT
)