Datasheet

LTC3609
15
3609fb
applicaTions inForMaTion
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






Fault Conditions: Current Limit and Foldback
The LTC3609 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
LTC3609 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 LTC3609. 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
C
SEC
1µF
V
OUT2
V
OUT1
C
OUT
C
IN
IN4148
T1
1:N
+
•
•
+
+
3609 F04
V
IN
LTC3609
SGND
26
NC
25
NC
24
V
FB
23
I
ON
22
NC
21
SGND
20
FCB
19
I
TH
18
V
RNG
17
PGOOD
16
SGND
15
GND
PV
IN
1
PV
IN
2
PV
IN
3
PV
IN
4
PV
IN
5
PV
IN
6
PV
IN
7
SW
SW
SGND
SW
8
NC
9
SGND
10
BOOST
11
RUN/SS
12
V
ON
13
SGND
14
PGND
40
PGND
39
PGND
38
PGND
37
PGND
36
PGND
35
PGND
34
SW
33
INTV
CC
32
INTV
CC
31
SV
IN
30
EXTV
CC
29
NC
28
SGND
27
SW
41
SW
42
SW
43
SW
44
SW
45
SW
46
SW
47
PV
IN
48
PV
IN
49
PV
IN
50
PV
IN
51
PV
IN
52
R4
OPTIONAL EXTV
CC
CONNECTION
5V < V
OUT2
< 7V
R3