Datasheet

LT3573
9
3573fd
For more information www.linear.com/LT3573
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:
L
PRI
≥ V
OUT
•
t
MIN
I
MIN
•N
PS
= V
OUT
•N
PS
•
1.4µH
V






t
MIN
= minimum off-time, 350ns
I
MIN
= minimum current limit, 250mA
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 first turns off, the flyback pulse
appears. However
, it takes a finite 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 primar
y-side of the transformer that
is not directly related to output voltage (some time is also
required for internal settling of the feedback amplifier
cir
cuitry). The leakage inductance spike is largest when
the power switch current is highest.
In order to maintain immunity to these phenomena, a fixed
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 significantly affect output regulation.
Selecting R
FB
and R
REF
Resistor Values
The expression for V
OUT,
developed in the Operation sec-
tion, can be rearranged to yield the following expression
for R
FB
:
R
FB
=
R
REF
•N
PS
V
OUT
+ V
F
( )
a+ V
TC




V
BG
where,
V
OUT
= Output voltage
V
F
= Switching diode forward voltage
a = Ratio of Q1, IC to IE, typically 0.986
N
PS
= Effective primary-to-secondary turns ratio
V
TC
= 0.55V
The equation assumes the temperature coefficients of
the diode and V
TC
are equal, which is a good first-order
approximation.
Strictly speaking, the above equation defines R
FB
not as an
absolute value, but as a ratio of R
REF
. So, the next ques-
tion is, “What is the proper value for R
REF
?” The answer
is that R
REF
should be approximately 6.04k. The LT3573
is trimmed and specified using this value of R
REF
. If the
impedance of R
REF
varies considerably from 6.04k, ad-
ditional errors will result. However, a variation in R
REF
of
several percent is acceptable. This yields a bit of freedom
in selecting standard 1% resistor values to yield nominal
R
FB
/R
REF
ratios.
Tables 1-4 are useful for selecting the resistor values for
R
REF
and R
FB
with no equations. The tables provide R
FB
,
R
REF
and R
TC
values for common output voltages and
common winding ratios.
Table 1. Common Resistor Values for 1:1 Transformers
V
OUT
(V) N
PS
R
FB
(kΩ) R
REF
(kΩ) R
TC
(kΩ)
3.3 1.00 18.7 6.04 19.1
5 1.00 27.4 6.04 28
12 1.00 64.9 6.04 66.5
15 1.00 80.6 6.04 80.6
20 1.00 107 6.04 105