Datasheet

LTC3611
15
3611fd
LTC3611
SGND
48
SGND
47
SGND
46
SGND
45
EXTV
CC
44
V
FB
43
SGND
42
I
ON
41
R4
SGND
40
FCB
39
I
TH
38
V
RNG
37
PGOOD
36
V
ON
35
SGND
34
SGND
SGND
PGND
33
PV
IN
17
PV
IN
18
PV
IN
19
PV
IN
20
PV
IN
21
PV
IN
22
PV
IN
23
PV
IN
24
PV
IN
25
SW
SW
SW
26
NC
27
SGND
28
BOOST
29
RUN/SS
30
SGND
31
SGND
32
PGND
64
PGND
63
PGND
62
PGND
61
PGND
60
PGND
59
PGND
58
PGND
57
PGND
56
SW
55
INTV
CC
54
INTV
CC
53
SV
IN
52
SV
IN
51
SGND
50
SGND
49
PGND
1
PGND
2
PGND
3
SW
4
SW
5
SW
6
SW
7
SW
8
SW
9
SW
10
SW
11
PV
IN
V
IN
12
PV
IN
13
PV
IN
14
PV
IN
15
PV
IN
16
3611 F04
C
SEC
1µF
V
OUT2
V
OUT1
C
OUT
C
IN
IN4148
OPTIONAL EXTV
CC
CONNECTION
5V < V
OUT2
< 7V
T1
1:N
R3
+
+
•
•
+
SGND
with changes in V
IN
. Tying the FCB pin below the 0.6V
threshold forces continuous synchronous operation, al-
lowing current to reverse at light loads and maintaining
high frequency operation.
In addition to providing a logic input to force continuous
operation, the FCB pin provides a means to maintain a
flyback winding output when the primary is operating
in discontinuous mode. The secondary output V
OUT2
is
normally set as shown in Figure 4 by the turns ratio N
of the transformer. However, if the controller goes into
discontinuous mode and halts switching due to a light
primary load current, then V
OUT2
will droop. An external
resistor divider from V
OUT2
to the FCB pin sets a minimum
voltage V
OUT2(MIN)
below which continuous operation is
forced until V
OUT2
has risen above its minimum:
V
OUT2(MIN)
= 0.6V 1+
R4
R3
⎛
⎝
⎜
⎞
⎠
⎟
ApplicAtions inForMAtion
Fault Conditions: Current Limit and Foldback
The LTC3611 has a current mode controller which inher-
ently limits the cycle-by-cycle inductor current not only
in steady state operation but also in transient. To further
limit current in the event of a short circuit to ground, the
LTC3611 includes foldback current limiting. If the output
falls by more than 25%, then the maximum sense voltage is
progressively lowered to about one sixth of its full value.
INTV
CC
Regulator and EXTV
CC
Connection
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC3611. The INTV
CC
pin can supply up to 50mA
RMS and must be bypassed to ground with a minimum of
4.7μF tantalum or ceramic capacitor. Good bypassing is
necessary to supply the high transient currents required
by the MOSFET gate drivers.
Figure 4. Secondary Output Loop and EXTV
CC
Connection