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LTC3813 Datasheet, PDF (17/32 Pages) Linear Technology – 100V Current Mode Synchronous Step-Up Controller
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LTC3813
APPLICATIONS INFORMATION
With these resistor values, the frequency will remain
relatively constant at:
f = 1+ R1/ R2 (Hz)
ROFF (76pF)
for the range of 0.45VIN to 1.55 • VIN, and will be propor-
tional to VIN outside of this range.
Changes in the load current magnitude will also cause
a frequency shift. Parasitic resistance in the MOSFET
switches and inductor reduce the effective voltage across
the inductance, resulting in increased duty cycle as the
load current increases. By shortening the off-time slightly
as current increases, constant-frequency operation can be
maintained. This is accomplished with a resistor connected
from the ITH pin to the IOFF pin to increase the IOFF current
slightly as VITH increases. The values required will depend
on the parasitic resistances in the specific application. A
good starting point is to feed about 10% of the ROFF cur-
rent with RITH as shown in Figure 6.
VIN
R1
R2
VOFF
LTC3813
3813 F05
Figure 5. VOFF Connection to Keep the Operating
Frequency Constant as the Input Supply Varies
ROFF
VOUT
IOFF
1000pF
RITH
LTC3813
ITH
RITH =
10ROFF
VOUT
3813 F06
Figure 6. Correcting Frequency Shift with Load Current Changes
Minimum On-Time and Dropout Operation
The minimum on-time tON(MIN) is the smallest amount of
time that the LTC3813 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
MOSFET back off. This time is generally about 350ns. The
minimum on-time limit imposes a minimum duty cycle
of tON(MIN)/(tON(MIN) + tOFF). If the minimum duty cycle is
reached, due to a rising input voltage, for example, then
the output will rise out of regulation. The maximum input
voltage to avoid dropout is:
VIN(MAX)
=
VOUT
tOFF
tON(MIN) +
tOFF
A plot of maximum duty cycle vs switching frequency is
shown in Figure 7.
2.0
1.5
DROPOUT
REGION
1.0
0.5
0
0
0.25
0.50
0.75
1.0
VIN/VOUT
3813 F07
Figure 7. Maximum Duty Cycle vs Switching Frequency
Inductor Selection
An inductor should be chosen that can carry the maximum
input DC current which occurs at the minimum input volt-
age. The peak-to-peak ripple current is set by the inductance
and a good starting point is to choose a ripple current of
at least 40% of its maximum value:
IL
=
40%
•
IO(MAX)
1 DMAX
3813fb
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