Datasheet

LT8300
11
8300f
applicaTions inForMaTion
Figure 4. Output Power for 24V Output
Figure 1. Output Power for 3.3V Output Figure 2. Output Power for 5V Output
Figure 3. Output Power for 12V Output
INPUT VOLTAGE (V)
0
OUTPUT POWER (W)
3.5
2.5
1.5
3.0
2.0
1.0
0.5
0
40 8020 60
8300 F01
100
N = 12:1
MAXIMUM
OUTPUT
POWER
N = 8:1
N = 6:1
N = 4:1
INPUT VOLTAGE (V)
0
OUTPUT POWER (W)
3.5
2.5
1.5
3.0
2.0
1.0
0.5
0
40 8020 60
8300 F02
100
MAXIMUM
OUTPUT
POWER
N = 2:1
N = 4:1
N = 6:1
N = 8:1
INPUT VOLTAGE (V)
0
OUTPUT POWER (W)
3.5
2.5
1.5
3.0
2.0
1.0
0.5
0
40 8020 60
8300 F03
100
N = 4:1
MAXIMUM
OUTPUT
POWER
N = 3:1
N = 2:1
N = 1:1
INPUT VOLTAGE (V)
0
OUTPUT POWER (W)
3.5
2.5
1.5
3.0
2.0
1.0
0.5
0
40 8020 60
8300 F04
100
N = 1:1
N = 3:2
N = 1:2
MAXIMUM
OUTPUT
POWER
N = 2:1
Primary Inductance Requirement
The LT8300 obtains output voltage information from the
reflected output voltage on the SW pin. The conduction
of secondary current reflects the output voltage on the
primary SW pin. The sample-and-hold error amplifier needs
a minimum 350ns to settle and sample the reflected output
voltage. In order to ensure proper sampling, the second-
ary winding needs to conduct current for a minimum of
350ns. The following equation gives the minimum value
for primary-side magnetizing inductance:
L
PRI
≥
t
OFF(MIN)
•N
PS
• V
OUT
+ V
F
( )
I
SW(MIN)
t
OFF(MIN)
= Minimum switch-off time = 350ns
I
SW(MIN)
= Minimum switch current limit = 52mA
In addition to the primary inductance requirement for
the minimum switch-off time, the LT8300 has minimum
switch-on time that prevents the chip from turning on
the power switch shorter than approximately 160ns. This
minimum switch-on time is mainly for leading-edge blank-
ing the initial switch turn-on current spike. If the inductor
current exceeds the desired current limit during that time,
oscillation may occur at the output as the current control
loop will lose its ability to regulate. Therefore, the following
equation relating to maximum input voltage must also be
followed in selecting primary-side magnetizing inductance:
L
PRI
≥
t
ON(MIN)
• V
IN(MAX)
I
SW(MIN)
t
ON(MIN)
= Minimum Switch-On Time = 160ns