LTC3611_15 Datasheet, PDF (12/26 Pages) Linear Technology – 10A, 32V Monolithic Synchronous Step-Down DC/DC Converter

LTC3611
Applications Information
R_ON. Figures 1a and 1b show how R_ON relates to switching
frequency for several common output voltages.
Because the voltage at the I_ON pin is about 0.7V, the cur-
rent into this pin is not exactly inversely proportional to
V_IN, especially in applications with lower input voltages.
To correct for this error, an additional resistor, R_ON2,
connected from the I_ON pin to the 5V INTV_CC supply will
further stabilize the frequency.
R_ON2
=
5V
0.7V
R_ON
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 I_TH pin to the V_ON pin and V_OUT. 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 I_TH pin to the V_ON pin
as shown in Figure 2a. Place capacitance on the V_ON pin
to filter out the I_TH variations at the switching frequency.
The resistor load on I_TH 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
V_OUT = 1.5V
V_OUT = 3.3V
V_OUT = 2.5V
12
100
100
1000
R_ON (kΩ)
10000
3611 F01a
Figure 1a. Switching Frequency vs R_ON (V_ON = 0V)
1000
V_OUT = 3.3V
V_OUT = 12V
V_OUT = 5V
100
100
1000
R_ON (kΩ)
10000
3611 F01b
Figure 1b. Switching Frequency vs R_ON (V_ON = INTV_CC)
3611fd