LT1103 Datasheet, PDF (27/32 Pages) Linear Technology – Offline Switching Regulator

TYPICAL APPLICATIONS
Setting Oscillator Frequency
LT1103/LT1105
OSC
C_OSC
LT1103 TA05
CHOOSE 20kHz ≤ f_OSC ≤ 200kHz
( )( ) ( )( ) ^{COSC =}
SF ₌
f_OSC
I
∆V f_OSC
=
100µA
2.5V f_OSC
DC ≅ 0.66 ⇒ 66%
LT1103/LT1105
Setting Overvoltage Lockout
OVLO_TH
R2
LT1105
OVLO
R1
LT1103 TA09
CHOOSE OVLO_TH
LET R1 = 5k
( ) ^{R2 =}
OVLO_TH
2.5V
–1
R1
Decreasing Oscillator
Maximum Duty Cycle
5V
LT1105
OSC
C_OSC
R
I1
LT1103 TA07
CHOOSE 0 ≤ DC ≤ 0.66
(6 – 9DC)
SOLVE FOR X ⇒ X =
2
0 ≤X≤3
⇒ I1 = X • I = X • 100µA
⇒R=
1.75V
I1
( )( ) ^{( ) COSC =}
100µA
2.5V f_OSC
•
1+
3X – 2X²
9
Increasing Oscillator
Maximum Duty Cycle
Synchronizing Oscillator Frequency
to an External Clock
LT1103/LT1105
OSC
C_OSC
R I1
LT1103 TA06
CHOOSE 0.66 ≤ DC ≤ 1.0
(9DC – 6)
SOLVE FOR X ⇒ X =
2
0 ≤ X ≤ 1.5
⇒ I1 = X • I = X • 100µA
⇒R=
3.25V
I1
( )( ) ^{( ) COSC =}
100µA
2.5V f_OSC
•
1–
3X + 2X²
9
LT1103/LT1105
500ns
5V
OSC
1µF
0V
1:0.5
C_OSC
LT1103 TA08
ISOLATION
BOUNDARY
27