ltc3711 Linear Technology Corporation, ltc3711 Datasheet - Page 14

• Manufacturer Part Number: ltc3711
• Description: 5-bit Adjustable, Wide Operating Range, No Rsense? Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3711
to approximately V
to store about 100 times the gate charge required by the
top MOSFET. In most applications a 0.1 F to 0.47 F X5R
or X7R dielectric capacitor is adequate.
Discontinuous Mode Operation and FCB Pin
The FCB pin determines whether the bottom MOSFET
remains on when current reverses in the inductor. Tying
this pin above its 0.8V threshold enables discontinuous
operation where the bottom MOSFET turns off when
inductor current reverses. The load current at which
current reverses and discontinuous operation begins de-
pends on the amplitude of the inductor ripple current and
will vary with changes in V
0.8V threshold forces continuous synchronous operation,
allowing 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
mally set as shown in Figure 6 by the turns ratio N of the
transformer. However, if the controller goes into discon-
tinuous mode and halts switching due to a light primary
load current, then V
divider from V
V
until V
14
5V < V
OUT2(MIN)
CONNECTION
V
Figure 6. Secondary Output Loop and EXTV
OPTIONAL
OUT MIN
OUT2
EXTV
OUT2
2
< 7V
(
CC
below which continuous operation is forced
has risen above its minimum.
R4
R3
)
OUT2
EXTV
FCB
SGND
0 8 1
.
IN
LTC3711
to the FCB pin sets a minimum voltage
U
CC
OUT2
V
+ INTV
SENSE
PGND
SW
V
BG
TG
will droop. An external resistor
IN
U
+
IN
R
R
CC
. Tying the FCB pin below the
4
3
. The boost capacitor needs
+
W
•
C
V
IN
IN
1:N
T1
1N4148
CC
•
Connection
+
SEC
U
+
C
OUT
is nor-
3711 F06
C
1 F
OUT2
V
V
OUT2
OUT
Fault Conditions: Current Limit and Foldback
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage. In
the LTC3711, the maximum sense voltage is controlled by
the voltage on the V
the maximum sense voltage and the sense resistance
determine the maximum allowed inductor valley current.
The corresponding output current limit is:
The current limit value should be checked to ensure that
I
generally occurs with the largest V
ent temperature, conditions that cause the largest power
loss in the converter. Note that it is important to check for
self-consistency between the assumed MOSFET junction
temperature and the resulting value of I
the MOSFET switches.
Caution should be used when setting the current limit
based upon the R
current limit is determined by the minimum MOSFET on-
resistance. Data sheets typically specify nominal and
maximum values for R
reasonable assumption is that the minimum R
the same amount below the typical value as the maximum
lies above it. Consult the MOSFET manufacturer for further
guidelines.
To further limit current in the event of a short circuit to
ground, the LTC3711 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.
LIMIT(MIN)
I
LIMIT
> I
R
V
OUT(MAX)
SNS MAX
DS ON
(
(
DS(ON)
)
RNG
T
)
. The minimum value of current limit
DS(ON)
of the MOSFETs. The maximum
pin. With valley current control,
2
1
I
L
, but not a minimum. A
IN
at the highest ambi-
LIMIT
which heats
DS(ON)
lies
3711f