Datasheet

LT3748
9
3748fa
Pseudo-DC Theory of Operation
The R
REF
and R
FB
resistors as depicted in the Block Diagram
are external resistors used to program the output voltage.
The LT3748 operates much the same way as traditional
current mode switchers with the exception of the unique
error amplifier which derives its feedback information
from the flyback pulse.
Operation is as follows: when the NMOS output switch
turns off, its drain voltage rises above V
IN
. The amplitude
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
= D
OUT
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 converted to a current by R
FB
and
Q2. Nearly all of this current flows through resistor R
REF
to
form a ground-referred voltage. This voltage is fed into the
flyback error amplifier. The flyback error amplifier samples
this output voltage information when the secondary-side
winding current reaches zero. The error amplifier uses a
bandgap voltage, 1.223V, 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:
V
FLBK
R
FB
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
=
V
BG
R
REF
or
V
FLBK
= V
BG
R
FB
R
REF
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
V
BG
= Internal bandgap reference
Combining 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
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.
Temperature Compensation
The first term in the V
OUT
equation does not have a tem-
perature dependence, but the diode forward drop, V
F
, has a
significant negative temperature coefficient. To compensate
for this, a positive temperature coefficient current source
is internally connected to the R
REF
pin. The current is set
by resistor R
TC
to ground connected between the TC pin
and ground. To cancel the temperature coefficient, the
following equation is used:
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 coef-
ficient
(δV
TC
/δ
T
) = 1.85mV/°C
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
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
− V
F
−
V
TC
R
TC
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
•
R
FB
N
PS
–I
SEC
(ESR)
APPLICATIONS INFORMATION