Datasheet

LT3799-1
15
37991f
Loop Compensation
The current output feedback loop is an integrator con-
figuration with the compensation capacitor between the
negative input and output of the operational amplifier.
This is a one-pole system therefore a zero is not needed
in the compensation. For offline applications with PFC,
the crossover should be set an order of magnitude lower
than the line frequency of 120Hz or 100Hz. In a typical
application, the compensation capacitor is 0.1μF.
In non-PFC applications, the crossover frequency may
be increased to improve transient performance. The
desired crossover frequency needs to be set an order
of magnitude below the switching frequency for optimal
performance.
MOSFET and Diode Selection
With a strong 1.9A gate driver, the LT3799-1 can effectively
drive most high voltage MOSFETs. A low Q
g
MOSFET is
recommended to maximize efficiency. In most applications,
the R
DS(ON)
should be chosen to limit the temperature rise
of the MOSFET. The drain of the MOSFET is stressed to
V
OUT
• N
PS
+ V
IN
during the time the MOSFET is off and
the secondary diode is conducting current. But in most
applications, the leakage inductance voltage spike exceeds
this voltage. The voltage of this stress is determined
by the switch voltage clamp. Always check the switch
waveform with an oscilloscope to make sure the leakage
inductance voltage spike is below the breakdown voltage
of the MOSFET. A transient voltage suppressor and diode
are slower than the leakage inductance voltage spike,
therefore causing a higher voltage than calculated.
The secondary diode stress may be as much as
V
OUT
+ 2 • V
IN
/N
PS
due to the anode of the diode ringing
with the secondary leakage inductance. An RC snubber
in parallel with the diode eliminates this ringing, so that
the reverse voltage stress is limited to V
OUT
+ V
IN
/N
PS
.
With a high N
PS
and output current greater than 3A, the
I
RMS
through the diode can become very high and a low
forward drop Schottky is recommended.
OPERATION
Table 1. Predesigned Transformers—Typical Specifications, Unless Otherwise Noted
TRANSFORMER
PART NUMBER
SIZE
(L × W × H)
L
PRI
(μH)
N
PSA
(N
P
:N
S
:N
A
)
R
PRI
(mΩ)
R
SEC
(mΩ) MANUFACTURER
TARGET
APPLICATION
(V
OUT
/I
OUT
)
JA4429
21.1mm × 21.1mm × 17.3mm
400 1:0.24:0.24 252 126 Coilcraft 22V/1A
7508110210
15.75mm × 15mm × 18.5mm
2000 6.67:1:1.67 5100 165 Würth Elektronik 10V/0.4A
750813002
15.75mm × 15mm × 18.5mm
2000 20:1.0:5.0 6100 25 Würth Elektronik 3.8V/1.1A
750811330
43.2mm × 39.6mm × 30.5mm
300 6:1.0:1.0 150 25 Würth Elektronik 18V/5A
750813144
16.5mm × 18mm × 18mm
600 4:1:0.71 2400 420 Würth Elektronik 28V/0.5A
750813134
16.5mm × 18mm × 18mm
600 8:1:1.28 1850 105 Würth Elektronik 14V/1A
750811291
31mm × 31mm × 25mm
400 1:1:0.24 550 1230 Würth Elektronik 85V/0.4A
750813390
43.18mm × 39.6mm × 30.48mm
100 1:1:0.22 150 688 Würth Elektronik 90V/1A
750811290
31mm × 31mm × 25mm
460 1:1:0.17 600 560 Würth Elektronik 125V/0.32A
X-11181-002
23.5mm × 21.4mm × 9.5mm
500 72:16:10 1000 80 Premo 30V/0.5A