LT3837
1
DESCRIPTION
Demonstration circuit 1038A-B is a 35 W att Isolated Fly-
back Converter w ith Synchronous Rectification and Pri-
mary-Side Regulation featuring the LT3837.
This circuit w as designed to demonstrate the high levels
of performance, efficiency, and small solution size attain-
able using this part in a flyback pow er supply. It operates
at 200kHz and produces a regulated 5.0V, 7A output
from an input voltage range of 9 to 36V: suitable for
automotive, industrial, and other applications. It has a
footprint area that is less than an eighth-brick. Synchro-
nous rectification helps to attain efficiency exceeding
89%. Isolation voltage is 1500VDC.
Design files for this circuit board are available. Call
the LTC factory.
, LTC, LTM , LT, Burst M ode, OPTI-LOO P, Over-The-Top and PolyPhase are registered
trademarks of Linear Technology Corporation. Adaptive Pow er, C-Load, DirectSense, Easy
Drive, FilterCAD, Hot Sw ap, LinearView , µM odule, M icropow er Sw itcherCAD, M ultimode
Dimming, No Latency , No Latency Delta-Sigma, No R
SENSE
, Operational Filter, PanelProtect,
Pow erPath, Pow erSOT, SmartStart, SoftSpan, Stage Shedding, Sw itcherCAD, ThinSOT,
UltraFast and VLDO are trademarks of Linear Technology Corporation. O ther product names
may be trademarks of the companies that manufacture the products.
PERF ORM A NCE SU M M A RY
Specifications are at TA = 25°C
SYM BOL PARAM ETER CON DITIONS M IN TYP M AX UNITS
V
IN
Input Supply Range
9 36
V
V
OUT
Output Voltage 5.0
V
I
OUT
Output Current Range V
IN
= 9 –36V
0 7
A
F
SW
Sw itching (Clock) Frequency
200 kHz
V
OUT P-P
Output Ripple V
IN
= 18V, I
OUT
= 7A (20M Hz BW ) 40 mV
P–P
I
REG
Output Regulation Line and Load (9-36V, 0-7A)
±1.4 %
P
OUT
/P
IN
Efficiency (see Figure 2) V
IN
=18V, I
OUT
= 7A
89
%
OPERA TING PRINCIPL ES
The LT3837 Synchronous Flyback PW M Controller is
used on the primary and drives a secondary-side
M OSFET through a pulse transformer to provide a syn-
chronous rectified output.
W hen an input voltage is applied, an undervoltage circuit
keeps the LT3837 in its quiescent state w hile a current
source charges Cvcc (C8) to 8.2V. The controller is then
enabled, and start-up commences. The primary circuit
operates from the charge stored in Cvcc until the house-
keeping w inding of T1 starts to support Vcc. W hen a
heavy overload or short-circuit prevents T1 supporting
Vcc, the converter operates in ‘burp-mode’, cutting off
w hen Vcc declines to 7.0V, maintaining low pow er dissi-
pation in the circuit. The LT3837 provides a synchronous
rectifier gate drive signal w hich is passed to the secon-
dary through T2 and subsequently buffered.
Regulation is attained by observing the voltage on the
housekeeping w inding of T1 during the Flyback time, and
Pulse W idth M odulating (PW M ) the Primary Gate drive
(PG) and Synchronous Gate drive (SG). The LT3837 is
programmed to compensate for circuit resistance that is
outside of the control loop.
DEMO
C IR C U IT
A
-
B
Q U IC K S TAR T G U IDE
L T 3 8 3 7
Iso la te d F ly b a c k C o n v e rte r w ith
S y n c h ro n o u s R e c tific a tio n
LT3837
2
Optional LC filter stages on the input and output facilitate low noise.
Q U ICK STA RT PROCEDU RE
Demonstration circuit 1038 is easy to set up to evaluate
the performance of the LT3837. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below :
NOTE.
W hen measuring the output voltage ripple, care must be taken to
avoid a long ground lead on the oscilloscope probe. M easure the output
voltage ripple by touching the probe tip and ground ring directly across the
last ceramic output capacitor as show n in Figure 1.
1.
Set an input pow er supply that is capable of 9V to 36V
to 18V. Then turn off the supply.
2.
Direct an airflow of 200lfm across the unit for sus-
tained operation at full load.
3.
W ith pow er off, connect the supply to the input termi-
nals +Vin and –Vin.
a.
Input voltages low er than 9V can keep the con-
verter from turning on due to the undervoltage
lockout feature of the LT3837.
b.
If efficiency measurements are desired, an amme-
ter capable of measuring 5Adc or a resistor shunt
can be put in series w ith the input supply in order
to measure the DC1038A’s input current.
c.
A voltmeter w ith a capability of measuring at least
36V can be placed across the input terminals in
order to get an accurate input voltage measure-
ment.
4.
Turn on the pow er at the input.
NOTE.
M ake sure that the input voltage never exceeds 36V.
5.
Check for the proper output voltage of 5V. Turn off
the pow er at the input.
6.
Once the proper output voltages are established, con-
nect a variable load capable of sinking 7A at 5V to the
output terminals +Vout and –Vout. Set the current for
0A.
a.
If efficiency measurements are desired, an amme-
ter or a resistor shunt that is capable of handling
10Adc can be put in series w ith the output load in
order to measure the DC1038A’s output current.
b.
A voltmeter w ith a capability of measuring at least
5V can be placed across the output terminals in
order to get an accurate output voltage measure-
ment.
7.
Turn on the pow er at the input.
NOTE.
If there is no output, temporarily disconnect the load to make sure
that the load is not set too high.
8.
Once the proper output voltage is again established,
adjust the load w ithin the operating range and observe
the output voltage regulation, ripple voltage, efficiency
and other desired parameters.
LT3837
3
Figure 1.
Proper M easurem ent Equipm ent Setup
LT3837
4
Efficiency
60
62
64
66
68
70
72
74
76
78
80
82
84
86
88
90
92
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
Current (A)
Efficiency (%)
9VIN
18VIN
36VIN
Figure 2.
Efficiency
Regulation
4.90
4.95
5.00
5.05
5.10
5.15
5.20
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
Current (A)
Output (V)
9VIN
18VIN
36VIN
Figure 3.
Regulation
LT3837
5
Figure 4.
Output Ripple at 18Vin and 7Aout (100M H z)
LT3837
6
Figure 5.
Transient R esponse W aveform at 18Vin and 3.5 - 7Aout
LT3837
7
Figure 6.
Com plete Board Schem atic