LTC3703

11

3703f

OPERATIO

For maximum protection, the LTC3703 current limit con-

sists of a steady-state limit circuit and an instantaneous

that pulls a current from the RUN/SS pin proportional to

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

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

Figure 5. Efficiency in Pulse Skip/Forced Continuous Modes

LOAD (mA)

10

100

90

80

70

60

50

40

30

20

10

0

100

1000

10000

3703 F05

FORCED CONTINUOUS

PULSE SKIP MODE

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%,allowingtheMODE/

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 6. Comparison of Inductor Current Waveforms for Pulse Skip Mode and Forced Continuous Operation

PULSE SKIP MODE

FORCED CONTINUOUS

DECREASING

LOAD

CURRENT

3703 F06