Datasheet

LT3511
8
3511fc
APPLICATIONS INFORMATION
PSEUDO DC THEORY
In the Block Diagram, R
REF
(R4) and R
FB
(R3) are external
resistors used to program the output voltage. The LT3511
operates similar to traditional current mode switchers,
except in the use of a unique 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) • NPS
V
F
= D1 forward voltage
I
SEC
= Transformer secondary current
ESR = Total impedance of secondary circuit
NPS = Transformer effective primary-to-secondary turns
ratio
R
FB
and Q2 convert the flyback voltage into a current. Nearly
all of this current flows through R
REF
to form a ground-
referred voltage. The resulting voltage forms the input
to the flyback error amplifier. The flyback error amplifier
samples the voltage information when the secondary side
winding current is zero. The bandgap voltage, 1.20V, acts
as the reference for the flyback error amplifier.
The relatively high gain in the overall loop will then cause
the voltage at R
REF
to be nearly equal to the bandgap
reference voltage V
BG
. The resulting relationship between
V
FLBK
and V
BG
approximately equals:
V
FLBK
R
FB
⎛
⎝
⎜
⎞
⎠
⎟
=
V
BG
R
REF
or V
FLBK
= V
BG
R
FB
R
REF
⎛
⎝
⎜
⎞
⎠
⎟
V
BG
= Internal bandgap reference
Combination of the preceding expression with earlier
derivation of V
FLBK
results in the following equation:
V
OUT
= V
BG
R
FB
R
REF
⎛
⎝
⎜
⎞
⎠
⎟
1
N
PS
⎛
⎝
⎜
⎞
⎠
⎟
–V
F
–I
SEC
ESR
()
The expression defines V
OUT
in terms of the internal ref-
erence, programming resistors, transformer turns ratio
and diode forward voltage drop. Additionally, it includes
the effect of nonzero secondary output impedance (ESR).
Boundary control mode minimizes the effect of this im-
pedance term.
Temperature Compensation
The first term in the V
OUT
equation does not have tem-
perature dependence, but the diode forward drop has a
significant negative temperature coefficient. A positive
temperature coefficient current source connects to the
R
REF
pin to compensate. A resistor to ground from the
T
C
pin sets the compensation current.
The following equation explains the cancellation of the
temperature coefficient:
δV
F
δT
= –
R
FB
R
TC
•
1
N
PS
•
δV
TC
δT
or,
R
TC
=
–R
FB
N
PS
•
1
δV
F
/ δT
•
δV
TC
δT
≈
R
FB
N
PS
(δV
F
/δ
T
) = Diode’s forward voltage temperature coefficient
(δV
TC
/δ
T
) = 2mV
V
TC
= 0.55V
Experimentally verify the resulting value of R
TC
and adjust as
necessary to achieve optimal regulation over temperature.
The addition of a temperature coefficient current modifies
the expression of output voltage as follows:
V
OUT
= V
BG
R
FB
R
REF
⎛
⎝
⎜
⎞
⎠
⎟
1
N
PS
⎛
⎝
⎜
⎞
⎠
⎟
–V
F
–
V
TC
R
TC
⎛
⎝
⎜
⎞
⎠
⎟
•
R
FB
N
PS
–I
SEC
ESR
()
Output Power
A flyback converter has a complicated relationship be-
tween the input and output current compared to a buck
or a boost. A boost has a relatively constant maximum
input current regardless of input voltage and a buck has a
relatively constant maximum output current regardless of
input voltage. This is due to the continuous nonswitching
behavior of the two currents. A flyback converter has both
discontinuous input and output currents which makes it