lt3573 Linear Technology Corporation, lt3573 Datasheet - Page 8

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

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 amplifi er which
derives its feedback information from the fl yback pulse.
Operation is as follows: when the output switch, Q1, turns
off, its collector voltage rises above the V
plitude of this fl yback pulse, i.e., the difference between
it and V
V
I
ESR = Total impedance of secondary circuit
N
The fl yback voltage is then converted to a current by
the action of R
through resistor R
age. This voltage is fed into the fl yback error amplifi er.
The fl yback error amplifi er samples this output voltage
information when the secondary side winding current is
zero. The error amplifi er 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
V
In combination with the previous V
an expression for V
APPLICATIONS INFORMATION
8
SEC
F
PS
FLBK
BG
= Ratio of Q1 I
V
= D1 forward voltage
V
FLBK
= Transformer effective primary-to-secondary turns ratio
FLBK
= Internal bandgap reference
= Transformer secondary current
and V
V
IN
R
FLBK
= (V
FB
, is given as:
BG
V
OUT
BG
FB
may then be expressed as:
C
+ V
R
and Q2. Nearly all of this current fl ows
R
V
R
to I
REF
OUT
REF
BG
REF
REF
FB
F
E
+ I
, in terms of the internal reference,
, typically 0.986
to form a ground-referred volt-
resistor to be nearly equal to the
SEC
or
REF
1
,
BG
• ESR) • N
(R4) and R
. The relationship between
FLBK
PS
expression yields
FB
IN
(R3) resistors
rail. The am-
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 fi rst term in the V
perature dependence, but the diode forward drop has a
signifi cant negative temperature coeffi cient. To compen-
sate for this, a positive temperature coeffi cient current
source is connected to the R
a resistor to ground connected to the T
temperature coeffi cient, the following equation is used:
( V
fi cient
( V
V
The resistor value given by this equation should also be
verifi ed experimentally, and adjusted if necessary to achieve
optimal regulation over temperature.
The revised output voltage is as follows:
TC
R
V
V
TC
F
= 0.55V
OUT
OUT
V
/
TC
T
/ T) = 2mV
F
T
) = Diode’s forward voltage temperature coef-
N
V
V
R
R
R
PS
BG
BG
FB
TC
FB
R
V
•
TC
TC
R
R
R
•
R
REF
REF
FB
V
FB
N
F F
•
OUT
PS
1
1
/
N
R
PS
•
T
equation does not have a tem-
FB
N
PS
•
N
REF
1
1
V
PS
–
T
TC
V
pin. The current is set by
I
T
TC
SEC
or
V
V
(
C
F
ESR
N
R
F
,
pin. To cancel the
PS
FB
I
SEC
)
(
ES
R R )
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