100V Micropower No-Opto Isolated Flyback Converter in 5-Lead TSOT-23

design features
100V Micropower No-Opto Isolated Flyback Converter in
5-Lead TSOT-23
Min Chen
The non-synchronous flyback topology is widely used
in isolated power supplies ranging from sub-watt power
levels to tens of watts. Linear’s no-opto isolated flyback
family dramatically simplifies isolated power supply design
with proprietary primary-side sensing, which requires
no opto-coupler or transformer third winding for output
regulation. The new LT8300, the first micropower part in
this family, significantly improves light load efficiency and
reduces no-load input standby current to about 200µA.
The LT8300 operates from an input
voltage range of 6V to 100V and delivers up to 2W of isolated output power.
The 150V integrated DMOS power switch
eliminates the need for a snubber in most
applications. By sampling the isolated
output voltage directly from the primaryside flyback waveform, the LT8300 requires
no opto-coupler or transformer third
winding for regulation. The output voltage is set with a single external resistor.
Internal loop compensation and soft start
further reduce external component count.
Boundary mode operation at heavy load
enables the use of small magnetics and
produces excellent load regulation. Low
ripple Burst Mode operation maintains
high efficiency at light load while minimizing output voltage ripple. All these features
are packed in a 5-lead TSOT-23 package
(Figure 1) with high voltage pin spacing
conforming to IPC-2221 requirement.
Figure 2. The LT8300 isolated
flyback converter solution size
is less than 1 inch by ½ inch in a
standard demo board DC1825A.
Figure 1. The LT8300 is available in a 5-lead TSOT-23
package with high voltage pin spacing between pins
4 and 5.
PERFORMANCE AND SIMPLICITY
A complete isolated flyback solution
fits into an area less than 1 by ½ inch,
as shown in Figure 2. Figure 3 shows a
typical LT8300 application, generating a
5V isolated output from a 36V-to-72V input.
The solution only requires five external
components (input capacitor, output
capacitor, transformer, feedback resistor and output diode) and two optional
undervoltage lockout resistors.
Although the LT8300 simplifies isolated
flyback converter design, it delivers
superior performance. Figure 4 shows
the power efficiency (85% peak) of the
5V application in Figure 3. Figure 5 shows
the load and line regulation (±0.5%) of
the 5V application in Figure 3. Figures
6 and 7 show its 50m A-to-250m A load
step transient and 1m A resistive load
start-up waveforms, respectively.
October 2012 : LT Journal of Analog Innovation | 27
By sampling the isolated output voltage directly from the
primary-side flyback waveform, the LT8300 requires no
opto-coupler or transformer third winding for regulation.
The output voltage is set with a single external resistor.
VIN
36V TO 72V
2.2µF
•
300µH
VIN
1M
VOUT–
RFB
GND
VIN = 36V
5.15
70
VIN = 72V
60
OUTPUT VOLTAGE (V)
80
VIN = 48V
50
40
30
20
5.10
5.05
5.00
4.95
4.90
VIN = 36V
VIN = 48V
VIN = 72V
4.85
10
0
50
100
150
200
LOAD CURRENT (mA)
250
Figure 4. Power efficiency of the 5V application in
Figure 3
300
4.80
ILPRI
100mA/DIV
VSW
50V/DIV
VSW
50V/DIV
VOUT
500mV/DIV
VOUT
5V/DIV
500µs/DIV
Figure 6. 50mA-to-250mA load step transient
waveforms of the 5V application in Figure 3
28 | October 2012 : LT Journal of Analog Innovation
0
50
100
150
200
LOAD CURRENT (mA)
250
300
Figure 5. Output load and line regulation of the 5V
application in Figure 3
IOUT
100mA/DIV
The output voltage in a typical LT8300
application can be expressed as
The first term in the VOUT equation does
not have temperature dependence, but
the output diode forward voltage VF has
a significant negative temperature coefficient (–1mV/°C to –2mV/°C). Such a negative
temperature coefficient produces approximately 200mV to 300mV voltage variation on the output across temperature.
5.20
90
POST REGULATOR ELIMINATES
OUTPUT TEMPERATURE VARIATION
R 
VOUT = 100µA •  FB  − VF
 NPS 
210k
100
EFFICIENCY (%)
47µF
SW
40.2k
Figure 3. A 5V/300mA micropower
isolated flyback converter from a
36V-to-72V input
19µH
•
LT8300
EN/UVLO
0
VOUT+
5V
1mA TO 300mA
4:1
500µs/DIV
Figure 7. 1mA resistive load start-up waveforms of
the 5V application in Figure 3
For relatively high voltage outputs, say
12V and 24V, the output diode temperature coefficient has a negligible effect on
the output voltage regulation. But for
lower voltage outputs, such as 3.3V and
5V, the output diode temperature coefficient contributes an additional 2%
to 5% output voltage regulation.
For designs requiring tight output voltage
regulation across temperature, a micropower low dropout linear regulator can be
added to post-regulate the LT8300 output.
The LT8300 should be programmed slightly
higher than the sum of the regulation
voltage and the LDO’s dropout voltage.
Figure 8 shows the LT8300 combined
with a LT3009-3.3 post-regulator to
generate a 3.3V/20m A isolated output from an 18V-to-32V input. The
no-load input standby current is less
than 250µ A as shown in Figure 9,
which conforms to DEF-STAN61-5.
design features
The LT8300 greatly simplifies the design of
isolated flyback converters, improves light load
efficiency and reduces no-load input standby
current when compared to traditional schemes.
400
VIN
18V TO 32V
1µF
VOUT+
3.3V
0mA TO 20mA
OUT
LT3009-3.3
150µH
•
LT8300
EN/UVLO
IN
•
150µH
VIN
1M
D1
Z1
1µF
SHDN
GND
300
1µF
IVIN (µA)
L1
1:1
VOUT–
SW
42.2k D1: DIODES INC. SBR0560S1-7
L1: DRQ73-151-R
Z1: CENTRAL CMDZ4L7
93.1k
RFB
GND
200
100
0
Figure 8. A 3.3V/20mA micropower isolated converter from an 18V-to-32V input conforming to DEF-STAN61-5
VARIOUS INPUT-REFERRED POWER
SUPPLIES
VOUT+
10V
50mA
The LT8300 greatly simplifies the design
of isolated flyback converters, improves
light load efficiency and reduces no-load
input standby current when compared
VOUT–
VIN
15V TO 80V
VOUT+
10V
100mA
1µF
Z1
L1
330µH
VIN
LT8300
EN/UVLO
D1
1M
SW
4.7µF
30
32
Z1
L1
330µH
VIN
LT8300
EN/UVLO
102k
118k
D1
SW
102k
118k
RFB
RFB
GND
L1: COILTRONICS DR73-331-R
D1: DIODES INC. SBR1U150SA
Z1: CENTRAL CMDZ12L
28
VOUT–
1µF
1M
24
26
VIN (V)
Figure 11. A VIN to (VIN – 10V) micropower converter
VIN
15V TO 80V
4.7µF
22
to traditional schemes. The high level
of integration and the use of boundary
and low ripple burst modes results in
a simple to use, low component count,
and high efficiency solution for isolated
power supplies, as well as various special nonisolated power supplies. n
CONCLUSION
Figure 10. A VIN to (VIN + 10V) micropower converter
20
Figure 9. No-load input standby current of the 3.3V
application in Figure 8
micropower converter. In both of these
converters, the LT8300’s unique feedback
sensing scheme is used to easily develop
an output voltage that tracks VIN .
In addition to isolated power supplies, the
LT8300 can be used in various nonisolated
applications. Two interesting applications
are input-referred positive and negative
power supplies often used for special gate
drivers. Figure 10 shows a simple VIN to
(VIN +10V) micropower converter, and
Figure 11 shows a simple VIN to (VIN – 10V)
18
GND
L1: COILTRONICS DR73-331-R
D1: DIODES INC. SBR1U150SA
Z1: CENTRAL CMDZ12L
October 2012 : LT Journal of Analog Innovation | 29
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