Table of Contents
Advertisement
applicaTions inForMaTion
Output Voltage
The R
and R
resistors as depicted in the Block Diagram
FB
REF
are external resistors used to program the output voltage.
The LT8302 operates similar to traditional current mode
switchers, except in the use of a unique flyback pulse
sense circuit and a sample-and-hold error amplifier, which
sample and therefore regulate the isolated output voltage
from the flyback pulse.
Operation is as follows: when the power switch M1 turns
off, the SW pin voltage rises above the V
amplitude of the flyback pulse, i.e., the difference between
the SW pin voltage and V
V
= (V
+ V
+ I
FLBK
OUT
F
SEC
V
= Output diode forward voltage
F
I
= Transformer secondary current
SEC
ESR = Total impedance of secondary circuit
N
= Transformer effective primary-to-secondary
PS
turns ratio
The flyback voltage is then converted to a current, I
by the R
resistor and the flyback pulse sense circuit
FB
(M2 and M3). This current, I
R
resistor to generate a ground-referred voltage. The
REF
resulting voltage feeds to the inverting input of the sample-
and-hold error amplifier. Since the sample-and-hold error
amplifier samples the voltage when the secondary current
is zero, the (I
• ESR) term in the V
SEC
assumed to be zero.
The internal reference voltage, V
noninverting input of the sample-and-hold error ampli-
fier. The relatively high gain in the overall loop causes the
voltage at the R
pin to be nearly equal to the internal
REF
reference voltage V
. The resulting relationship between
REF
V
and V
can be expressed as:
FLBK
REF
⎛
⎞
V
FLBK
•R
= V
⎝ ⎜
⎠ ⎟
REF
REF
R
FB
⎛
⎞
R
FB
V
= V
•
⎝ ⎜
⎠ ⎟
FLBK
REF
R
REF
V
= Internal reference voltage 1.00V
REF
supply. The
IN
supply, is given as:
IN
• ESR) • N
PS
, also flows through the
RFB
equation can be
FLBK
, 1.00V, feeds to the
REF
or
For more information
Combination with the previous V
equation for V
OUT
transformer turns ratio, and diode forward voltage:
V
= V
•
OUT
REF
Output Temperature Compensation
The first term in the V
ture dependence, but the output diode forward voltage, V
has a significant negative temperature coefficient (–1mV/°C
to –2mV/°C). Such a negative temperature coefficient pro-
duces approximately 200mV to 300mV voltage variation
on the output voltage across temperature.
For higher voltage outputs, such as 12V and 24V, the
output diode temperature coefficient has a negligible ef-
fect on the output voltage regulation. For lower voltage
outputs, such as 3.3V and 5V, however, the output diode
temperature coefficient does count for an extra 2% to 5%
output voltage regulation.
,
RFB
The LT8302 junction temperature usually tracks the output
diode junction temperature to the first order. To compensate
the negative temperature coefficient of the output diode,
a resistor, R
, connected between the TC and R
TC
generates a proportional-to-absolute-temperature (PTAT)
current. The PTAT current is zero at 25°C, flows into the
R
pin at hot temperature, and flows out of the R
REF
at cold temperature. With the R
output voltage equation is revised as follows:
R
V
= V
•
OUT
REF
R
(
)
T –TO
•
TO=Room temperature 25°
(
)
= Output diode forward voltage
V
/ T
F
temperature coefficient
(
)
= 3.35mV/ C
V
/ T
TC
www.linear.com/LT8302
equation yields an
FLBK
, in terms of the R
and R
FB
⎛
⎞
⎛
⎞
R
1
FB
•
– V
⎝ ⎜
⎠ ⎟
⎝ ⎜
⎠ ⎟
F
R
N
REF
PS
equation does not have tempera-
OUT
resistor in place, the
TC
1
( )
FB
•
– V
TO
– V
F
N
REF
PS
R
1
(
FB
•
– V
/ T
F
R
N
TC
PS
C
°
LT8302
resistors,
REF
,
F
pins
REF
pin
REF
(
)
/ T
•
TC
(
)
)
• T–TO
8302fb
11
Advertisement
Table of Contents
Need help?
Do you have a question about the LT8302 and is the answer not in the manual?