Datasheet

LT3575
9
3575f
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 LT3575 operation.
Minimum Current Limit
The LT3575 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:
LV
t
I
NV N
µH
V
PRI OUT
MIN
MIN
PS OUT PS
≥=
⎛
⎝
⎜
⎞
•• ••
.088
⎠⎠
⎟
t
MIN
= minimum off-time, 350ns
I
MIN
= minimum current limit, 400mA
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.
Selecting R
FB
and R
REF
Resistor Values
The expression for V
OUT,
developed in the Operation section,
can be rearranged to yield the following expression for R
FB
:
R
RNV V V
V
FB
RE F PS OUT F TC
BG
=
+
()
+
⎡
⎣
⎤
⎦
• α
where,
V
OUT
= Output voltage
V
F
= Switching diode forward voltage
α = 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 coeffi cients of
the diode and V
TC
are equal, which is a good fi rst-order
approximation.
Strictly speaking, the above equation defi nes R
FB
not as
an absolute value, but as a ratio of R
REF
. So, the next
question is, “What is the proper value for R
REF
?” The
answer is that R
REF
should be approximately 6.04k. The
LT3575 is trimmed and specifi ed using this value of R
REF
.
If the impedance of R
REF
varies considerably from 6.04k,
additional 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. The R
FB
resistor given by this equation
should also be verifi ed experimentally, and adjusted if
necessary for best output accuracy.
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