LTC3608(RevC) Просмотр технического описания (PDF) - Linear Technology
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LTC3608 Datasheet PDF : 26 Pages
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LTC3608
Applications Information
Because the voltage at the ION pin is about 0.7V, the cur-
rent into this pin is not exactly inversely proportional to
VIN, especially in applications with lower input voltages.
To correct for this error, an additional resistor, RON2,
connected from the ION pin to the 5V INTVCC supply will
further stabilize the frequency.
RON2
=
5V
0.7V
RON
Changes in the load current magnitude will also cause
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 lengthening the on-time slightly
as current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the ITH pin to the VON pin and VOUT. The values
required will depend on the parasitic resistances in the
specific application. A good starting point is to feed about
25% of the voltage change at the ITH pin to the VON pin
as shown in Figure 2a. Place capacitance on the VON pin
to filter out the ITH variations at the switching frequency.
The resistor load on ITH reduces the DC gain of the error
amp and degrades load regulation, which can be avoided
by using the PNP emitter follower of Figure 2b.
1000
VOUT = 1.5V
VOUT = 3.3V
VOUT = 2.5V
12
100
100
1000
RON (kΩ)
10000
3608 F01a
Figure 1a. Switching Frequency vs RON (VON = 0V)
1000
VOUT = 3.3V
VOUT = 12V
VOUT = 5V
100
100
1000
RON (kΩ)
10000
3608 F01b
Figure 1b. Switching Frequency vs RON (VON = INTVCC)
3608fc
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