Datasheet

LT1170/LT1171/LT1172
11
117012fg
OPERATION
With t
S
= 0.2µs, f
S
= 150kHz, V
C
= 1.5V, and R3 = 2k, offset
voltage shift is ≈ 3.8mV. This is not particularly bother-
some, but note that high offsets could result if R3 were
reduced to a much lower value. Also, the synchronizing
transistor must sink higher currents with low values of
R3, so larger drives may have to be used. The transistor
must be capable of pulling the V
C
pin to within 200mV of
ground to ensure synchronizing.
Synchronizing with Bipolar Transistor Synchronizing with MOS Transistor
1170/1/2 OP01
C2
39pF
R1
3k
R2
2.2k
LT1170
GND
V
IN
V
C
C1
R3
2N2369
FROM 5V
LOGIC
1170/1/2 OP02
D1
1N4158
R2
2.2k
LT1170
GND
V
IN
V
C
C1
R3
FROM 5V
LOGIC
C2
100pF
D2
1N4158
* SILICONIX OR EQUIVALENT
VN2222*
TYPICAL APPLICATIONS
Flyback Converter
1170/1/2 TA03
D1
C1
2000µF
C4*
100µF
C2
0.15µF
R1
3.74k
R2
1.24k
V
IN
20V TO 30V
R3
1.5k
*REQUIRED IF INPUT LEADS ≥ 2"
LT1170
V
IN
V
SW
FB
V
C
OPTIONAL
FILTER
L2
5µH
C4
100µF
V
OUT
5V
6A
V
SNUB
CLAMP TURN-ON
SPIKE
PRIMARY FLYBACK VOLTAGE =
LT1170 SWITCH VOLTAGE
AREA “a” = AREA “b” TO MAINTAIN
ZERO DC VOLTS ACROSS PRIMARY
SECONDARY VOLTAGE
AREA “c” = AREA “d” TO MAINTAIN
ZERO DC VOLTS ACROSS SECONDARY
PRIMARY CURRENT
SECONDARY CURRENT
LT1170 SWITCH CURRENT
SNUBBER DIODE CURRENT
V
OUT
+ Vf
N
0V
V
IN
a
b
0V
c
d
V
OUT
+ V
f
N • V
IN
$I
I
PRI
0
I
PRI
/N
I
PRI
I
PRI
t =
(I
PRI
)(L
L
)
V
SNUB
0
0
0
1
N* = 1/3
N*
D3
25V
1W
D2
MUR110
GND
+
+