LTC3703IGN-5 Linear Technology, LTC3703IGN-5 Datasheet - Page 11

• Manufacturer Part Number: LTC3703IGN-5
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,CMOS,SSOP,16PIN,PLASTIC
• Manufacturer: Linear Technology

OPERATIO
For maximum protection, the LTC3703 current limit con-
sists of a steady-state limit circuit and an instantaneous
limit circuit. The steady-state limit circuit is a g
that pulls a current from the RUN/SS pin proportional to
the difference between the SW and I
current begins to discharge the capacitor at RUN/SS,
reducing the duty cycle and controlling the output voltage
until the current regulates at the limit. Depending on the
size of the capacitor, it may take many cycles to discharge
the RUN/SS voltage enough to properly regulate the
output current. This is where the instantaneous limit
circuit comes into play. The instantaneous limit circuit is
a cycle-by-cycle comparator which monitors the bottom
MOSFET’s drain voltage and keeps the top MOSFET from
turning on whenever the drain voltage is 50mV above the
programmed max drain voltage. Thus the cycle-by-cycle
comparator will keep the inductor current under control
until the g
Pulse Skip Mode
The LTC3703 can operate in one of two modes selectable
with the MODE/SYNC pin—Pulse Skip Mode or forced
continuous mode. Pulse Skip Mode is selected when
increased efficiency at light loads is desired. In this mode,
the bottom MOSFET is turned off when inductor current
reverses to minimize the efficiency loss due to reverse
current flow. As the load is decreased (see Figure 6), the
duty cycle is reduced to maintain regulation until its
minimum on-time (~200ns) is reached. When the load
decreases below this point, the LTC3703 begins to skip
m
Figure 6. Comparison of Inductor Current Waveforms for Pulse Skip Mode and Forced Continuous Operation
amplifier gains control.
U
DECREASING
CURRENT
LOAD
PULSE SKIP MODE
MAX
voltages. This
m
amplifier
cycles to maintain regulation. The frequency drops but
this further improves efficiency by minimizing gate charge
losses. In forced continuous mode, the bottom MOSFET is
always on when the top MOSFET is off, allowing the
inductor current to reverse at low currents. This mode is
less efficient due to resistive losses, but has the advantage
of better transient response at low currents, constant
frequency operation, and the ability to maintain regulation
when sinking current. See Figure 5 for a comparison of the
effect on efficiency at light loads for each mode. The
MODE/SYNC threshold is 0.8V ±7.5%, allowing the MODE/
SYNC to act as a feedback pin for regulating a second
winding. If the feedback voltage drops below 0.8V, the
LTC3703 reverts to continuous operation to maintain
regulation in the secondary supply.
Figure 5. Efficiency in Pulse Skip/Forced Continuous Modes
FORCED CONTINUOUS
100
90
80
70
60
50
40
30
20
10
0
10
V
V
IN
IN
= 25V
= 75V
100
LOAD (mA)
V
IN
V
FORCED CONTINUOUS
PULSE SKIP MODE
IN
= 25V
3703 F06
= 75V
1000
LTC3703
3703 F05
10000
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