lt3573 Linear Technology Corporation, lt3573 Datasheet - Page 9

• Manufacturer Part Number: lt3573
• Description: Isolated Flyback Converter Without An Opto-coupler
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
ERROR AMPLIFIER—DYNAMIC THEORY
Due to the sampling nature of the feedback loop, there
are several timing signals and other constraints that are
required for proper LT3573 operation.
Minimum Current Limit
The LT3573 obtains output voltage information from the
SW pin when the secondary winding conducts current.
The sampling circuitry needs a minimum amount of time
to sample the output voltage. To guarantee enough time,
a minimum inductance value must be maintained. The
primary side magnetizing inductance must be chosen
above the following value:
t
I
The minimum current limit is higher than that on the Elec-
trical Characteristics table due to the overshoot caused by
the comparator delay.
Leakage Inductance Blanking
When the output switch fi rst turns off, the fl yback pulse
appears. However, it takes a fi nite time until the transformer
primary side voltage waveform approximately represents
the output voltage. This is partly due to the rise time on
the SW node, but more importantly due to the trans-
former leakage inductance. The latter causes a very fast
voltage spike on the primary side of the transformer that
is not directly related to output voltage (some time is also
required for internal settling of the feedback amplifi er
circuitry). The leakage inductance spike is largest when
the power switch current is highest.
In order to maintain immunity to these phenomena, a fi xed
delay is introduced between the switch turn-off command
and the beginning of the sampling. The blanking is internally
set to 150ns. In certain cases, the leakage inductance may
not be settled by the end of the blanking period, but will
not signifi cantly affect output regulation.
MIN
MIN
L
PRI
= minimum off-time, 350ns
= minimum current limit, 250mA
V
OUT
•
I
t
MIN
MIN
•
N
PS
V
OUT
•
N
PS
•
. 1 4
V
µH
Selecting R
The expression for V
tion, can be rearranged to yield the following expression
for R
where,
V
V
N
V
The equation assumes the temperature coeffi cients of
the diode and V
approximation.
Strictly speaking, the above equation defi nes R
an absolute value, but as a ratio of R
tion is, “What is the proper value for R
is that R
is trimmed and specifi ed using this value of R
impedance of R
ditional errors will result. However, a variation in R
several percent is acceptable. This yields a bit of freedom
in selecting standard 1% resistor values to yield nominal
R
Tables 1-4 are useful for selecting the resistor values for
R
R
common winding ratios.
Table 1. Common Resistor Values for 1:1 Transformers
TC
OUT
F
PS
FB
REF
REF
= Ratio of Q1, IC to IE, typically 0.986
V
R
= Switching diode forward voltage
OUT
/R
= 0.55V
= Effective primary-to-secondary turns ratio
3.3
FB
12
15
20
FB
5
= Output voltage
and R
and R
REF
(V)
:
REF
R
ratios.
FB
REF
TC
FB
should be approximately 6.04k. The LT3573
with no equations. The tables provide R
and R
values for common output voltages and
1.00
1.00
1.00
1.00
1.00
N
•
TC
REF
PS
N
PS
are equal, which is a good fi rst-order
varies considerably from 6.04k, ad-
OUT,
REF
V
Resistor Values
R
developed in the Operation sec-
OUT
V
FB
18.7
27.4
64.9
80.6
107
BG
(kΩ)
V
F
REF
R
REF
6.04
6.04
6.04
6.04
6.04
. So the next ques-
REF
(kΩ)
V
TC
?” The answer
LT3573
REF
R
FB
TC
19.1
66.5
80.6
105
28
. If the
not as
(kΩ)
REF
9
3573f
FB
of
,