Datasheet

LTC3736-2
21
37362fb
APPLICATIONS INFORMATION
If V
AUX
drops below this value, the FCB voltage forces
temporary continuous switching operation until V
AUX
is
again above its minimum.
Table 3 summarizes the different states in which the
SYNC/FCB pin can be used.
Table 3
SYNC/FCB PIN CONDITION
0V to 0.5V Forced Continuous Mode
Current Reversal Allowed
0.7V to V
IN
Pulse-Skipping Operation Enabled
No Current Reversal Allowed
Feedback Resistors Regulate an Auxiliary Winding
External Clock Signal Enable Phase-Locked Loop
(Synchronize to External CLK)
Pulse-Skipping at Light Loads
No Current Reversal Allowed
Fault Condition: Short-Circuit and Current Limit
To prevent excessive heating of the bottom MOSFET, fold-
back current limiting can be added to reduce the current
in proportion to the severity of the fault.
Foldback current limiting is implemented by adding
diodes D
FB1
and D
FB2
between the output and the I
TH
pin as shown in Figure 11. In a hard short (V
OUT
= 0V),
the current will be reduced to approximately 50% of the
maximum output current.
Low Supply Operation
Although the LTC3736-2 can function down to below
2.4V, the maximum allowable output current is reduced
as V
IN
decreases below 3V. Figure 12 shows the amount
of change as the supply is reduced down to 2.4V. Also
shown is the effect on V
REF
.
Figures 10. Auxiliary Output Loop Connection
LTC3736-2
+
+
R6
R5
1μF
V
OUT
V
AUX
C
OUT
L1
1:N
SYNC/FCB
BG
SW
TG
37362 F10
V
IN
Figures 11. Foldback Current Limiting
Figures 12. Line Regulation of V
REF
and
Maximum Sense Voltage for Low Input Supply
+
1/2 LTC3736-2
V
FB
I
TH
R2
D
FB1
V
OUT
D
FB2
37362 F11
R1
INPUT VOLTAGE (V)
75
NORMALIZED VOLTAGE OR CURRENT (%)
85
95
105
80
90
100
2.2 2.4 2.6 2.8
37362 F12
3.02.12.0 2.3 2.5 2.7 2.9
V
REF
MAXIMUM
SENSE VOLTAGE
Minimum On-Time Considerations
Minimum on-time, t
ON(MIN)
, is the smallest amount of time
that the LTC3736-2 is capable of turning the top P-chan-
nel MOSFET on and then off. It is determined by internal
timing delays and the gate charge required to turn on the
top MOSFET. Low duty cycle and high frequency applica-
tions may approach the minimum on-time limit and care
should be taken to ensure that:
t
V
fV
ON MIN
OUT
OSC IN
()
•
<
If the duty cycle falls below what can be accommodated
by the minimum on-time, the LTC3736-2 will begin to
skip cycles (unless forced continuous mode is selected).
The output voltage will continue to be regulated, but the
ripple current and ripple voltage will increase. The mini-
mum on-time for the LTC3736-2 is typically about 200ns.
However, as the peak sense voltage (I
L(PEAK)
• R
DS(ON)
)
decreases, the minimum on-time gradually increases up
to about 250ns. This is of particular concern in forced