LT8300
applicaTions inForMaTion
Minimum Load Requirement
Design Example
The LT8300 samples the isolated output voltage from the
primary-side flyback pulse waveform. The flyback pulse
occursoncetheprimaryswitchturnsoffandthesecondary
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
duringlightloadconditionstoensureaccurateoutputvolt-
age information. The minimum energy delivery creates a
minimum load requirement, which can be approximately
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
V
= 36V, V
= 48V, V
= 72V,
IN(MIN)
OUT
IN(NOM)
= 120mA
IN(MAX)
= 12V, I
OUT
Step 1: Select the Transformer Turns Ratio.
150V − VIN(MAX) − VLEAKAGE
NPS <
VOUT + VF
estimated as:
2
L
•I
• f
PRI
MIN
SW(MIN)
V
= Margin for transformer leakage spike = 30V
I
=
LEAKAGE
LOAD(MIN)
2 • V
OUT
V = Output diode forward voltage = ~0.3V
F
L
PRI
= Transformer primary inductance
Example:
I
f
= Minimum switch current limit = 52mA
SW(MIN)
150V − 72V − 30V
12V + 0.3V
NPS <
= 3.9
= Minimum switching frequency = 7.5kHz
MIN
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,usea6VZenerwithcathodeconnectedtotheoutput.
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
Output Short Protection
V
V
at
(V)
I
at
OUT(MAX)
SW(MAX)
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
N
V
(mA)
DUTY CYCLE (%)
15-25
PS
IN(MAX)
IN(MIN)
1:1
2:1
3:1
84.3
84
96.6
135
168
25-41
108.9
34-51
SincebothN =2andN =3canmeetthe120mAoutput
PS
PS
current requirement, N = 2 is chosen in this example
PS
to allow more margin for transformer leakage inductance
V supply voltage, the switch current may run away and
IN
voltage spike.
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
switchfrequency.Thisoutputshortprotectionpreventsthe
switch current from running away and limits the average
output diode current.
8300f
16
Linear [ Linear ]
