Linear Technology Analog Devices LT8301 Datasheet page 10

LT8301
APPLICATIONS INFORMATION
7
MAXIMUM
6
OUTPUT
CURRENT
5
4
3
2
1
0
0 10
INPUT VOLTAGE (V)
Figure 1. Output Power for 3.3V Output
7
MAXIMUM
6
OUTPUT
CURRENT
5
4
3
2
1
0
0 10
INPUT VOLTAGE (V)
Figure 3. Output Power for 12V Output
Primary Inductance Requirement
The LT8301 obtains output voltage information from the
reflected output voltage on the SW pin. The conduction of
secondary current reflects the output voltage on the pri-
mary SW pin. The sample-and-hold error amplifier needs
a minimum 450ns to settle and sample the reflected out-
put voltage. In order to ensure proper sampling, the sec-
ondary winding needs to conduct current for a minimum
of 450ns. The following equation gives the minimum
value for primary-side magnetizing inductance:
t • N
OFF(MIN) PS
L
≥
PRI
I
SW(MIN)
t = Minimum switch-off time = 450ns
OFF(MIN)
I = Minimum switch current limit = 290mA (typ)
SW(MIN)
10
N = 5:1
N = 3:1
N = 2:1
N = 1:1
20 30 40
8301 F01
N = 3:2
N = 1:1
N = 2:3
N = 1:3
20 30 40
8301 F03
( )
• V + V
OUT F
For more information
7
MAXIMUM
6
OUTPUT
CURRENT
5
4
3
2
1
0
0 10
INPUT VOLTAGE (V)
Figure 2. Output Power for 5V Output
7
MAXIMUM
6
OUTPUT
CURRENT
5
4
3
2
1
0
0 10
INPUT VOLTAGE (V)
Figure 4. Output Power for 24V Output
In addition to the primary inductance requirement for
the minimum switch-off time, the LT8301 has minimum
switch-on time that prevents the chip from turning on
the power switch shorter than approximately 170ns.
This minimum switch-on time is mainly for leading-edge
blanking 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 cur-
rent 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 magnetiz-
ing inductance:
t • V
ON(MIN) IN(MAX)
L
≥
PRI
I
SW(MIN)
t = Minimum switch-on time = 170ns
ON(MIN)
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N = 4:1
N = 3:1
N = 2:1
N = 1:1
20 30 40
8301 F02
N = 4:5
N = 1:2
N = 1:3
N = 1:5
20 30 40
8301 F04
Rev. A