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LT8302ES8E#PBF

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LT8302
10
8302fc
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operaTion
Boundary conduction mode returns the secondary current
to zero every cycle, so parasitic resistive voltage drops
do not cause load regulation errors. Boundary conduc
-
tion mode also allows the use of smaller transformers
compared to continuous conduction mode and does not
exhibit subharmonic oscillation.
Discontinuous Conduction Mode Operation
As the load gets lighter, boundary conduction mode in
-
creases the
switching frequency and decreases the switch
peak current at
the same ratio. Running at a higher switching
frequency up to several MHz increases switching and gate
charge losses. To avoid this scenario, the LT8302 has an
additional internal oscillator, which clamps the maximum
switching frequency to be less than 380kHz. Once the
switching frequency hits the internal frequency clamp,
the part starts to delay the switch turn-on and operates
in discontinuous conduction mode.
Low Ripple Burst Mode Operation
Unlike traditional flyback converters, the LT8302 has to
turn on and off at least for a minimum amount of time
and with a minimum frequency to allow accurate sampling
of the output voltage. The inherent minimum switch cur
-
rent limit and minimum switch-off time are necessary to
guarantee the correct operation of specific applications.
As the load gets very light
, the LT8302 starts to fold back
the switching frequency
while keeping the minimum switch
current limit. So the load current is able to decrease while
still allowing minimum switch-off time for the sample-and-
hold error amplifier. Meanwhile, the part switches between
sleep mode and active mode, thereby reducing the effec
-
tive quiescent current to improve light load efficiency. In
this condition, the LT8302 runs in low ripple Burst Mode
operation. The typical 12kHz minimum switching frequency
determines how often the output voltage is sampled and
also the minimum load requirement.
LT8302
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Output Voltage
The R
FB
and R
REF
resistors as depicted in the Block Diagram
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
IN
supply. The
amplitude of the flyback pulse, i.e., the difference between
the SW pin voltage and V
IN
supply, is given as:
V
FLBK
= (V
OUT
+ V
F
+ I
SEC
• ESR) • N
PS
V
F
= Output diode 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, I
RFB
,
by the R
FB
resistor and the flyback pulse sense circuit
(M2 and M3). This current, I
RFB
, also flows through the
R
REF
resistor to generate a ground-referred voltage. The
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
SEC
• ESR) term in the V
FLBK
equation can be
assumed to be zero.
The internal reference voltage, V
REF
, 1.00V, feeds to the
noninverting input of the sample-and-hold error ampli-
fier. The
relatively high gain in the overall loop causes the
voltage at the
R
REF
pin to be nearly equal to the internal
reference voltage V
REF
. The resulting relationship between
V
FLBK
and V
REF
can be expressed as:
V
FLBK
R
FB
⎛
⎝
⎜
⎞
⎠
⎟
•R
REF
= V
REF
or
V
FLBK
= V
REF
•
R
FB
R
REF
⎛
⎝
⎜
⎞
⎠
⎟
V
REF
= Internal reference voltage 1.00V
Combination with the previous V
FLBK
equation yields an
equation for V
OUT
, in terms of the R
FB
and R
REF
resistors,
transformer turns ratio, and diode forward voltage:
V
OUT
= V
REF
•
R
FB
R
REF
⎛
⎝
⎜
⎞
⎠
⎟
•
1
N
PS
⎛
⎝
⎜
⎞
⎠
⎟
– V
F
Output Temperature Compensation
The first term in the V
OUT
equation does not have tempera-
ture dependence, but the
output diode forward voltage, V
F
,
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.
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
TC
, connected between the TC and R
REF
pins
generates a proportional-to-absolute-temperature (PTAT)
current. The PTAT current is zero at 25°C, flows into the
R
REF
pin at hot temperature, and flows out of the R
REF
pin
at cold temperature. With the R
TC
resistor in place, the
output voltage equation is revised as follows:
V
OUT
= V
REF
•
R
FB
R
REF
•
1
N
PS
– V
F
TO
( )
– V
TC
/ T
( )
•
T –TO
( )
•
R
FB
R
TC
•
1
N
PS
– V
F
/ T
( )
• T–TO
( )
TO=Room temperature 25°
°
C
V
F
/ T
( )
= Output diode forward voltage
temperature coefficient
V
TC
/ T
( )
= 3.35mV/ C
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To cancel the output diode temperature coefficient, the
following two equations should be satisfied:
V
OUT
= V
REF
•
R
FB
R
REF
•
1
N
PS
– V
F
TO
( )
V
TC
/ T
( )
•
R
FB
R
TC
•
1
N
PS
= – V
F
/ T
( )
Selecting Actual R
REF
, R
FB
, R
TC
Resistor Values
The LT8302 uses a unique sampling scheme to regulate
the isolated output voltage. Due to the sampling nature,
the scheme contains repeatable delays and error sources,
which will affect the output voltage and force a re-evaluation
of the R
FB
and R
TC
resistor values. Therefore, a simple
2-step sequential process is recommended for selecting
resistor values.
Rearrangement of the expression for V
OUT
in the previous
sections yields the starting value for R
FB
:
R
FB
=
R
REF
•N
PS
• V
OUT
+ V
F
TO
( )
( )
V
REF
V
OUT
= Output voltage
V
F
(TO) = Output diode forward voltage at 25°C = ~0.3V
N
PS
= Transformer effective primary-to-secondary
turns ratio
The equation shows that the R
FB
resistor value is indepen-
dent of the R
TC
resistor value. Any R
TC
resistor connected
between the TC and R
REF
pins has no effect on the output
voltage setting at 25°C because the TC pin voltage is equal
to the R
REF
regulation voltage at 25°C.
The R
REF
resistor value should be approximately 10k
because the LT8302 is trimmed and specified using this
value. If the R
REF
resistor value varies considerably from
10k, additional errors will result. However, a variation in
R
REF
up to 10% is acceptable. This yields a bit of freedom
in selecting standard 1% resistor values to yield nominal
R
FB
/R
REF
ratios.
First, build and power up the application with the starting
R
REF
, R
FB
values (no R
TC
resistor yet) and other compo-
nents connected, and measure
the regulated output volt-
age, V
OUT(MEAS)
. The new R
FB
value can be adjusted to:
R
FB(NEW)
=
V
OUT
V
OUT(MEAS)
•R
FB
Second, with a new R
FB
resistor value selected, the output
diode temperature coefficient in the application can be
tested to determine the R
TC
value. Still without the R
TC
resistor, the V
OUT
should be measured over temperature
at a desired target output load. It is very important for
this evaluation that uniform temperature be applied to
both the output diode and the LT8302. If freeze spray or
a heat gun is used, there can be a significant mismatch
in temperature between the two devices that causes sig
-
nificant error. Attempting to
extrapolate the data from a
diode data sheet is another option if there is no method
to apply uniform heating or cooling such as an oven. With
at least two data points spreading across the operating
temperature range, the output diode temperature coef
-
ficient can be determined by:
– δV
F
/δT
( )
=
V
OUT
T1
( )
– V
OUT
T2
( )
T1– T2
Using the measured output diode temperature coefficient,
an exact R
TC
value can be selected with the following
equation:
R
TC
=
δV
TC
/δT
( )
– δV
F
/δT
( )
•
R
FB
N
PS
⎛
⎝
⎜
⎞
⎠
⎟
Once the R
REF
, R
FB
, and R
TC
values are selected, the regula-
tion accuracy from board
to board for a given application
will be very consistent, typically under ±5% when includ-
ing device
variation of all the components in the system
(assuming resistor tolerances
and transformer windings
matching within ±1%). However, if the transformer or
the output diode is changed, or the layout is dramatically
altered, there may be some change in V
OUT
.
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LT8302ES8E#PBF

Mfr. #:
Buy LT8302ES8E#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 42VIN Micropower No-Opto Isolated Flyback Converter with 65V/4.5A Switch
Lifecycle:
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