Datasheet

LT3573
8
3573fd
For more information www.linear.com/LT3573
ERROR AMPLIFIER—PSEUDO DC THEORY
In the Block Diagram, the R
REF
(R4) and R
FB
(R3) resistors
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
IN
rail. The am-
plitude of this flyback pulse, i.e., the difference between
it and V
IN
, is given as:
V
FLBK
= (V
OUT
+ V
F
+ I
SEC
• ESR) • N
PS
V
F
= D1 forward voltage
I
SEC
= Transformer secondary current
ESR = Total impedance of secondary circuit
N
PS
= Transformer effective primary-to-secondary
turns ratio
The flyback voltage is then converted to a current by
the action of R
FB
and Q2. Nearly all of this current flows
through resistor R
REF
to form a ground-referred volt-
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
REF
resistor to be nearly equal to the
bandgap reference voltage V
BG
. The relationship between
V
FLBK
and V
BG
may then be expressed as:
a
V
FLBK
R
FB






=
V
BG
R
REF
or,
V
FLBK
= V
BG
R
FB
R
REF






1
a






a = Ratio of Q1 I
C
to I
E
, typically ≈ 0.986
V
BG
= Internal bandgap reference
In combination with the previous V
FLBK
expression yields
an expression for V
OUT
, in terms of the internal reference,
programming resistors, transformer turns ratio and diode
forward voltage drop:
V
OUT
= V
BG
R
FB
R
REF






1
a N
PS






− V
F
−I
SEC
(ESR)
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
OUT
equation does not have a tem-
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
REF
pin. The current is set by
a resistor to ground connected to the T
C
pin. To cancel the
temperature coefficient, the following equation is used:
dV
F
dT
= −
R
FB
R
TC
•
1
N
PS
•
dV
TC
dT
or,
R
TC
=
−R
FB
N
PS
•
1
dV
F
/dT
•
dV
TC
dT
≈
R
FB
N
PS
(dV
F
/d
T
) = Diode’s forward voltage temperature
coefficient
(dV
TC
/dT) = 2mV
V
TC
= 0.55V
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:
V
OUT
= V
BG
R
FB
R
REF






1
N
PS
a






− V
F
−
V
TC
R
TC






•
R
FB
N
PS
a
–I
SEC
(ESR)
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