LT3573EMSEPBF LINER [Linear Technology], LT3573EMSEPBF Datasheet - Page 8

• Manufacturer Part Number: LT3573EMSEPBF
• Description: Isolated Flyback Converter without an Opto-Coupler
• Manufacturer: LINER [Linear Technology]

LT3573
ERROR AMPLIFIER—PSEUDO DC THEORY
In the Block Diagram, the R
can be found. They are external resistors used to program
the output voltage. The LT3573 operates much the same
way as traditional current mode switchers, the major
difference being a different type of error amplifier which
derives its feedback information from the flyback pulse.
Operation is as follows: when the output switch, Q1, turns
off, its collector voltage rises above the V
plitude of this flyback pulse, i.e., the difference between
it and V
The flyback voltage is then converted to a current by
the action of R
through resistor R
age. This voltage is fed into the flyback error amplifier.
The flyback error amplifier samples this output voltage
information when the secondary side winding current is
zero. The error amplifier uses a bandgap voltage, 1.23V,
as the reference voltage.
The relatively high gain in the overall loop will then cause
the voltage at the R
bandgap reference voltage V
V
applications inForMation

FLBK
V
V
I
ESR = Total impedance of secondary circuit
N
turns ratio
a = Ratio of Q1 I
V
a
V
SEC
FLBK
F
BG
PS
FLBK



= D1 forward voltage
and V
V
= Internal bandgap reference
= Transformer secondary current
IN
R
= Transformer effective primary‑to‑secondary
FLBK
= (V
FB
, is given as:
=
BG
V
OUT

 =

BG
FB
may then be expressed as:


 
+ V
R
and Q2. Nearly all of this current flows
R
V
R
REF
REF
BG
C
REF
REF
FB
F
to I
+ I
to form a ground‑referred volt‑
resistor to be nearly equal to the



E
SEC

 
or
, typically ≈ 0.986
REF
a
1
,
BG
• ESR) • N

 
(R4) and R
. The relationship between
PS
FB
IN
(R3) resistors
rail. The am‑
In combination with the previous V
an expression for V
programming resistors, transformer turns ratio and diode
forward voltage drop:
Additionally, it includes the effect of nonzero secondary
output impedance (ESR). This term can be assumed to
be zero in boundary control mode. More details will be
discussed in the next section.
Temperature Compensation
The first term in the V
perature dependence, but the diode forward drop has a
significant negative temperature coefficient. To compen‑
sate for this, a positive temperature coefficient current
source is connected to the R
a resistor to ground connected to the T
temperature coefficient, the following equation is used:
The resistor value given by this equation should also be
verified experimentally, and adjusted if necessary to achieve
optimal regulation over temperature.
The revised output voltage is as follows:
(dV
coefficient
(dV
V
R
V
V
d
d
TC
OUT
OUT
V
TC
T
F
TC
F
= 0.55V
/d
=
= −
/dT) = 2mV
=
=
T
−
N
) = Diode’s forward voltage temperature
V
V
−
R
R
R
PS
BG
BG
FB


 
TC
FB
R
V






•
TC
TC
R
R
R
•
R
d
REF
OUT
REF
FB

 •

V
FB
N
F F
OUT
PS
1
1
, in terms of the internal reference,



/
N






d
R



PS
•
T
a
equation does not have a tem‑
FB
N
PS
a
d
•
N
REF
1
1
d
V
d
PS
–
T
TC
d
V
a
pin. The current is set by
I
T
TC

 −


 −

SEC
FLBK
≈
or
V
V
(
C
F
ESR
N
R
F
,
expression yields
pin. To cancel the
−
PS
FB
I
SEC
)
(
ES
R R )
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