LTC3703EGN Linear Technology, LTC3703EGN Datasheet - Page 11

• Manufacturer Part Number: LTC3703EGN
• Description: IC BUCK/BOOST SYNC ADJ 5A 16SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Step-Up (Boost)r

Specifications of LTC3703EGN

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 93 V
• Current - Output: 5A
• Frequency - Switching: 100kHz ~ 600kHz
• Voltage - Input: 9.3 ~ 100 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

OPERATION
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 volt-
age 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 in-
creased effi ciency at light loads is desired. In this mode,
the bottom MOSFET is turned off when inductor current
reverses to minimize the effi ciency loss due to reverse
current fl ow. As the load is decreased (see Figure 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 begins to skip
m
Figure 5. Comparison of Inductor Current Waveforms for Pulse-Skip Mode and Forced Continuous Operation
amplifi er gains control.
DECREASING
CURRENT
LOAD
(Refer to Functional Diagram)
PULSE-SKIP MODE
MAX
voltages. This
m
amplifi er
cycles to maintain regulation. The frequency drops but
this further improves effi ciency 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 effi cient due to resistive losses, but has the advantage
of better transient response at low currents, constant
frequency operation, and the ability to maintain regula-
tion when sinking current. See Figure 6 for a comparison
of the effect on effi ciency 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 6. Effi ciency 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
3703 F05
IN
V
FORCED CONTINUOUS
PULSE SKIP MODE
IN
= 25V
= 75V
1000
LTC3703
3703 F06
10000
11
3703fb