LT8300
16
8300f
Minimum Load Requirement
The LT8300 samples the isolated output voltage from the
primary-side flyback pulse waveform. The flyback pulse
occurs once the primary switch turns off and the secondary
winding conducts current. In order to sample the output
voltage, the LT8300 has to turn on and off at least for a
minimum amount of time and with a minimum frequency.
The LT8300 delivers a minimum amount of energy even
during light load conditions to ensure accurate output volt-
age information. The minimum energy delivery creates a
minimum load requirement, which can be approximately
estimated as:
I
LOAD(MIN)
=
L
PRI
• I
SW(MIN)
2
• f
MIN
2 • V
OUT
L
PRI
= Transformer primary inductance
I
SW(MIN)
= Minimum switch current limit = 52mA
f
MIN
= Minimum switching frequency = 7.5kHz
The LT8300 typically needs less than 0.5% of its full output
power as minimum load. Alternatively, a Zener diode with its
breakdown of 20% higher than the output voltage can serve
as a minimum load if pre-loading is not acceptable. For a 5V
output, use a 6V Zener with cathode connected to the output.
Output Short Protection
When the output is heavily overloaded or shorted, the
reflected SW pin waveform rings longer than the internal
blanking time. After the 350ns minimum switch-off time,
the excessive ring falsely trigger the boundary mode
detector and turn the power switch back on again before
the secondary current falls to zero. Under this condition,
the LT8300 runs into continuous conduction mode at
750kHz maximum switching frequency. Depending on the
V
IN
supply voltage, the switch current may run away and
exceed 260mA maximum current limit. Once the switch
current hits 520mA over current limit, a soft-start cycle
initiates and throttles back both switch current limit and
switch frequency. This output short protection prevents the
switch current from running away and limits the average
output diode current.
applicaTions inForMaTion
Design Example
Use the following design example as a guide to design
applications for the LT8300. The design example involves
designing a 12V output with a 120mA load current and an
input range from 36V to 72V.
V
IN(MIN)
= 36V, V
IN(NOM)
= 48V, V
IN(MAX)
= 72V,
V
OUT
= 12V, I
OUT
= 120mA
Step 1: Select the Transformer Turns Ratio.
N
PS
<
150V
V
IN(MAX)
V
LEAKAGE
V
OUT
+ V
F
V
LEAKAGE
= Margin for transformer leakage spike = 30V
V
F
= Output diode forward voltage = ~0.3V
Example:
N
PS
<
150V
72V
30V
12V
0.3V
= 3.9
The choice of transformer turns ratio is critical in deter-
mining output current capability of the converter. Table 4
shows the switch voltage stress and output current capa-
bility at different transformer turns ratio.
Table 4. Switch Voltage Stress and Output Current Capability
vs Turns Ratio
N
PS
V
SW(MAX)
at
V
IN(MAX)
(V)
I
OUT(MAX)
at
V
IN(MIN)
(mA) DUTY CYCLE (%)
1:1 84.3 84 15-25
2:1 96.6 135 25-41
3:1 108.9 168 34-51
Since both N
PS
= 2 and N
PS
= 3 can meet the 120mA output
current requirement, N
PS
= 2 is chosen in this example
to allow more margin for transformer leakage inductance
voltage spike.
