21
LTC3736
3736fa
APPLICATIO S I FOR ATIO
W
U
U
U
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
Burst Mode 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,
foldback current limiting can be added to reduce the
current in proportion to the severity of the fault.
Foldback current limiting is implemented by adding di-
odes 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 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
.
Minimum On-Time Considerations
Minimum on-time, t
ON(MIN)
,
is the smallest amount of time
in which the LTC3736 is capable of turning the top
P-channel 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
applications may approach the minimum on-time limit
and care should be taken to ensure that:
t
V
f
V
ON MIN
OUT
OSC
IN
(
)
•
<
If the duty cycle falls below what can be accommodated
by the minimum on-time, the LTC3736 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 minimum on-
time for the LTC3736 is typically about 250ns. However,
as the peak sense voltage (I
L(PEAK)
• R
DS(ON)
) decreases,
the minimum on-time gradually increases up to about
300ns. This is of particular concern in forced continuous
applications with low ripple current at light loads. If forced
+
1/2 LTC3736
V
FB
I
TH
R2
D
FB1
V
OUT
D
FB2
3736 F11
R1
Figure 11. Foldback Current Limiting
INPUT VOLTAGE (V)
75
NORMALIZED VOLTAGE OR CURRENT (%)
85
95
105
80
90
100
2.2
2.4
2.6
2.8
3736 F12
3.0
2.1
2.0
2.3
2.5
2.7
2.9
V
REF
MAXIMUM
SENSE VOLTAGE
Figure 12. Line Regulation of V
REF
and
Maximum Sense Voltage for Low Input Supply
LTC3736
+
+
R6
R5
1
µ
F
V
OUT
V
AUX
C
OUT
L1
1:N
SYNC/FCB
BG
SW
TG
3736 F10
V
IN
Figure 10. Auxiliary Output Loop Connection
