LTC3736-2 Linear Technology, LTC3736-2 Datasheet - Page 21

• Manufacturer Part Number: LTC3736-2
• Description: Synchronous Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
If V
temporary continuous switching operation until V
again above its minimum.
Table 3 summarizes the different states in which the
SYNC/FCB pin can be used
Table 3
SYNC/FCB PIN
0V to 0.5V
0.7V to V
Feedback Resistors
External Clock Signal
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
shown in Figure 11. In a hard short (V
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
change as the supply is reduced down to 2.4V. Also shown
is the effect on V
IN
AUX
decreases below 3V. Figure 12 shows the amount of
FB1
IN
drops below this value, the FCB voltage forces
Figure 10. Auxiliary Output Loop Connection
R6
R5
and D
SYNC/FCB
LTC3736-2
FB2
REF
SW
U
TG
BG
between the output and the I
.
U
V
CONDITION
Forced Continuous Mode
Current Reversal Allowed
Pulse-Skipping Operation Enabled
No Current Reversal Allowed
Regulate an Auxiliary Winding
Enable Phase-Locked Loop
(Synchronize to External CLK)
Pulse-Skipping at Light Loads
No Current Reversal Allowed
IN
1:N
L1
W
OUT
= 0V), the current
+
37362 F10
C
+
OUT
1µF
U
V
V
AUX
OUT
TH
AUX
pin as
is
Minimum On-Time Considerations
Minimum on-time, t
time that the LTC3736-2 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:
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 minimum on-
time for the LTC3736-2 is typically about 200ns. How-
ever, as the peak sense voltage (I
decreases, the minimum on-time gradually increases up
to about 250ns. This is of particular concern in forced
continuous applications with low ripple current at light
t
ON MIN
(
Figure 12. Line Regulation of V
Maximum Sense Voltage for Low Input Supply
)
105
100
<
Figure 11. Foldback Current Limiting
95
90
85
80
75
2.0
f
OSC
V
2.1
OUT
I
1/2 LTC3736-2
TH
•
V
2.2
REF
V
ON(MIN)
IN
2.3
INPUT VOLTAGE (V)
V
MAXIMUM
SENSE VOLTAGE
FB
2.4
, is the smallest amount of
2.5
2.6
R2
R1
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+
2.7
REF
LTC3736-2
L(PEAK)
2.8
and
2.9
37362 F11
37362 F12
D
D
FB1
FB2
3.0
V
OUT
• R
21
DS(ON)
37362fa
)