Datasheet

LTC3805
17
3805fg
Figure 9. Low Power Isolated Telecom Supply with NPN Preregulator
internal soft-start time t
SS(IN)
= 1.8ms. However, in noisy
environments a small C
SS
can be valuable to limit jitter
in the oscillator.
If there is an overcurrent fault detected on the OC pin, the
LTC3805 enters a shutdown mode while a 2µA current
discharges the voltage on the SSFLT pin from 4.75V to
about 0.7V. The fault timeout t
FTO(EXT)
is therefore:
t
FTO(EXT)
= C
SS
4.75V
−
0.7V
2µA
At this point, the LTC3805 attempts a restart.
In the event of a persistent fault, such as a short-circuit
on the converter output, the converter enters a “hiccup”
mode where it continues to try and restart at repetition
rate determined by C
SS
. If the fault is eventually removed
the converter successfully restarts.
applications inForMation
V
IN
OC
COMP
0.6V FB
GND
OPTO
R20
118k
R19
3.01k
C17
1µF
C01
100µF
6.3V
C21
1µF
R12
100k
R14
22.1k
C22
0.47µF
C15
2200pF
250VAC
R9
274Ω
C19
47pF
C20
47nF
R23
221k
C23, 0.1µF
D1
UPS840
D2
BAS516
D4
BAS516
Q1
FDC2512
D4
BAS516
T1
PA1861NL
C18
330pF
V
IN
V
CC
V
CC
NOTE: CAN ALSO USE TRICKLE CHARGER AS SHOWN IN FIGURE 10
R16, 1.33k
R8
8.66k
R5
221k
11
5
4
3
2
1
6
7
8
5
1
4
2
10
8
7
9
9
1
3
2
6
4
5
10
R6
220Ω
R30
51Ω
C16
150pF
200V
D2
PDZ6.8B
6.8V
Q2
MMBTA42
3805 TA02a
C
IN1
2.2µF
100V
C
IN2
2.2µF
100V
V
IN
+
36V TO
72V
V
IN
–
D6
BAT54CWTIG
R11
6.8k
R3
221k
U1
LTC3805EMSE
I
TH
GND
SSFLT
RUN
GATE
OC
I
SENSE
SYNCFS
FB
V
CC
R2
2Ω
R15
2.0k
C02
100µF
6.3V
C03
100µF
6.3V
R
CS
0.068Ω
V
OUT
+
3.3V
3A
V
OUT
–
+V
OUT
ISO2
NECPS2801-1
U2
LT4430ES6 OPT
LOAD CURRENT (A)
0.01
0
EFFICIENCY (%)
POWER LOSS (W)
20
30
40
50
60
70
0.1
1
3805 TA02b
80
90
100
0
1
10
10
10
36V
48V
60V
72V
V
IN
Efficiency and Power Loss
vs Load Current and V
IN
; V
O
= 3.3V
typical application