Datasheet

LT3575
8
3575f
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 LT3575 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
IN
rail. The
amplitude of this fl yback 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 fl yback voltage is then converted to a current by
the action of R
FB
and Q2. Nearly all of this current fl ows
through resistor R
REF
to form a ground-referred voltage.
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
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:
α
V
R
V
R
or
VV
R
R
FL BK
FB
BG
RE F
FLBK BG
FB
RE F
⎛
⎝
⎜
⎞
⎠
⎟
=
=
⎛
,
⎝⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
1
α
α = 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:
VV
R
RN
VI ES
OUT BG
FB
RE F PS
FSEC
=
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
−−
1
α
(RR)
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
OUT
equation does not have a tem-
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
REF
pin. The current is set by
a resistor to ground connected to the TC pin. To cancel the
temperature coeffi cient, the following equation is used:
δ
δ
δ
δ
δ
V
T
R
RN
V
T
or
R
R
NV
FFB
TC PS
TC
TC
FB
PS
=−
=
−
•• ,
•
1
1
FF
TC FB
PS
T
V
T
R
N/
•
δ
δ
δ
≈
(δV
F
/δ
T
) = Diode’s forward voltage temperature
coeffi cient
(δV
TC
/δT) = 2mV
V
TC
= 0.55V
The resistor value given by this equation should also be
verifi ed experimentally, and adjusted if necessary to achieve
optimal regulation overtemperature.
The revised output voltage is as follows:
VV
R
RN
V
V
R
OUT BG
FB
RE F PS
F
TC
TC
=
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
−
−
⎛
1
α
⎝⎝
⎜
⎞
⎠
⎟
•–()
R
N
IESR
FB
PS
SEC
α
APPLICATIONS INFORMATION