LTC3703-5_15 Datasheet, PDF (11/32 Pages) Linear Technology – 60V Synchronous Switching Regulator Controller

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OPERATIO (Refer to Functional Diagram)
For maximum protection, the LTC3703-5 current limit
consists of a steady-state limit circuit and an instanta-
neous limit circuit. The steady-state limit circuit is a g_m
amplifier that pulls a current from the RUN/SS pin propor-
tional to the difference between the SW and I_MAX voltages.
This 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_m amplifier gains control.
Pulse Skip Mode
The LTC3703-5 can operate in one of two modes select-
able 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 Fig-
ure 5), the duty cycle is reduced to maintain regulation
until its minimum on-time (~200ns) is reached. When the
load decreases below this point, the LTC3703-5 begins to
LTC3703-5
skip 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 4 for a com-
parison of the effect on efficiency at light loads for each
mode. The MODE/SYNC threshold is 0.8V ±7.5%, allow-
ing the MODE/SYNC to act as a feedback pin for regulating
a second winding. If the feedback voltage drops below
0.8V, the LTC3703-5 reverts to continuous operation to
maintain regulation in the secondary supply.
100
V_IN = 12V
90
V_IN = 42V
80
70
60
50
V_IN = 12V
40
V_IN = 42V
30
20
10
0
10
V_OUT = 5V
FORCED CONTINUOUS
PULSE SKIP MODE
100
1000
LOAD (mA)
10000
37035 F04
Figure 4. Efficiency in Pulse Skip/Forced Continuous Modes
PULSE SKIP MODE
FORCED CONTINUOUS
DECREASING
LOAD
CURRENT
37035 F05
Figure 5. Comparison of Inductor Current Waveforms for Pulse Skip Mode and Forced Continuous Operation
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