LTC1909-8 LINER [Linear Technology], LTC1909-8 Datasheet - Page 21

• Manufacturer Part Number: LTC1909-8
• Description: Wide Operating Range,No RSENSE TM Step-Down DC/DC Controller with SMBus Programming
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
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 4 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
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 LTC1909-8, 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
LIMIT(MIN)
SEC(MIN)
I
V
LIMIT
SEC MIN
SEC
(
has risen above its minimum.
below which continuous operation is forced
> I
R
)
V
OUT(MAX)
DS ON
SNS MAX
SEC
. 0 8
(
(
DS(ON)
to the FCB pin sets a minimum voltage
)
V
U
SEC
T
)
1
. The minimum value of current limit
RNG
DS(ON)
of the MOSFETs. The maximum
will droop. An external resistor
R
R
2
U
1
pin. With valley current control,
4
3
I
L
, but not a minimum. A
IN
W
at the highest ambi-
LIMIT
which heats
SEC
U
is nor-
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 LTC1909-8 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.
Minimum Off-time and Dropout Operation
The minimum off-time t
time that the LTC1909-8 is capable of turning on the
bottom MOSFET, tripping the current comparator and
turning the MOSFET back off. This time is generally about
300ns. The minimum off-time limit imposes a maximum
duty cycle of t
cycle is reached, due to a dropping input voltage for
example, then the output will drop out of regulation. The
minimum input voltage to avoid dropout is:
INTV
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC1909-8. The INTV
50mA RMS and must be bypassed to ground with a
minimum of 4.7 F tantalum or other low ESR capacitor.
Good bypassing is necessary to supply the high transient
5V < V
CONNECTION
V
OPTIONAL
Figure 4. Secondary Output Loop and EXTV
IN MIN
SEC
CC
EXTV
(
< 7V
Regulator
CC
)
R4
R3
V
ON
OUT
EXTV
FCB
SGND
/(t
LTC1909-8
CC
ON
t
ON
SENSE
PGND
+ t
OFF(MIN)
SW
V
BG
TG
IN
+
OFF(MIN)
t
t
ON
OFF MIN
(
is the smallest amount of
CC
). If the maximum duty
+
)
pin can supply up to
•
V
C
LTC1909-8
IN
IN
1:N
T1
1N4148
CC
•
Connection
+
+
DS(ON)
C
V
OUT
21
19098 F04
SEC
C
1 F
SEC
19098f
V
lies
OUT